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Light, colour and human response
Light, colour and human response
15
D.L. Loe Fellow of the Society of Light and Lighting (FSLL), Chartered Institution of Building Services Engineers (CIBSE), London, United Kingdom
15.1 Introduction Light is a fundamental element of human life. It allows us to accomplish visual tasks from the simple to the complex, and can have a bearing on health and wellbeing through its intensity distribution and colour. It can also create psychological human responses that can enhance man’s relationship with the built environment. Much of the latter is still not properly understood and is an area that needs further investigation. The following explores the limitations and hypothesises of electric lighting and colour specification based on human response by drawing from the historical development of the subject as well as from past research with the aim of furthering improvements in design. Recent developments in technology could enable better systems of measurement of light and colour and hence specification. Light sources are also changing, making new lighting solutions possible for the benefit of the occupants together with improved energy efficiency. Along with light and illumination, colour is something we tend to take for granted until we are faced with an anomaly – something appears not quite right or that an error occurs because of how we describe and measure colour which ends up with a mistaken choice of colour or colour combinations or even the choice of a product in terms of its attractiveness or the freshness of natural produce. The colour fidelity, as perceived of an object, a material or a surface, will depend primarily on the interaction of the inherent colour/s of the object i.e. its spectral reflectance or transmission. It will also depend on the colour of the light that illuminates it, that is its spectral emission in terms of wavelength. Both of these elements are usually constant but human colour response is more complicated and has degrees of adjustment largely due to visual adaptation to both colour and brightness but stray outside the limits then errors of judgement can occur. Bearing this in mind, it is perhaps strange, with the benefit of hindsight, that so much of light metrology is only based on the spectral distribution of the centre of our visual field – the fovea which has the greatest density of visual receptors which enables critical sight. The following discusses these issues and how some of the apparent anomalies might be overcome. It also discusses how we deal with light and colour in appearance design.
15.2 Light and vision Light can be described as the transmitter of information from the world around us to the brain. It allows us to carry out tasks ranging from the complex, such as threading Colour Design. http://dx.doi.org/10.1016/B978-0-08-101270-3.00015-1 Copyright © 2017 Elsevier Ltd. All rights reserved.
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a needle, to the more general, for example, allowing us to move easily through a crowded retail outlet. It allows us to discriminate between different features of the three-dimensional scene by luminance and colour differences. It is also an element that can induce a sense of wellbeing as with a sunlit room which has areas of light and shade which can be described as ‘light’ and ‘cheerful’ or one as ‘gloomy’ and depressing when a scene is illuminated relatively uniformly with low levels of illumination – as one might describe the illumination from an overcast sky on a dull day. Light can also be described as electromagnetic radiation ranging approximately from 380 to 780 nm the spectral range of the human eye. Ultraviolet radiation ranges from around 100 to 400 nm, and although it is invisible to the eye, it can be a source of concern because it can damage certain materials. It can also interact with fluorescent materials which can provide special visual effects either decorative or functional, or as in retro-reflective warning signs on a motorway. The human eye has an optical system which focuses the light information on to the inside surface of the retina at the back of the eye, which has a network of light receptors. The eye has a conical field of view of approximately 100° wide, with the most sensitive area a cone of approximately 2° wide which is described as the fovea. This is the area of the retina with the highest density of light sensors thus allowing critical sight. But the spectral response of the fovea is limited to a relatively narrow colour distribution centred on 550 nm and what is described by the CIE as the Vλ photopic spectral distribution and is the standard spectral response by which light and illumination is measured. This is surprising since human vision usually spans the full colour range and not as that described by the Vλ distribution (see Fig. 15.1). There are, however, some people with colour defective vision to which this does not apply.
Relative sensitivity of the human eye: the Vλ curve 400 nm
Gamma radiation 10
Xrays
100 1000 picometres, pm 1
500
Ultraviolet
600
Visible
Microwave Radar
1 10
Infra-red
700
100 1000 nanometres, nm
10 100 1000 micrometres, µm, 1
RADIO TV
MW 1
10
10
100 1000 metres, m
1000 100 millimetres, mm
Fig. 15.1 Electromagnetic spectrum described by wavelength in metres together with the human visual spectrum and CIE Photopic Vλ spectral distribution. (Courtesy of Peter Tregenza, with permission from Taylor & Francis Group.)
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This observation, together with some research evidence, has led me to suggest that there are three main areas of the retina (Loe, 2016). The fovea with critical vision and limited colour response which I have labelled as the Micro field which embraces a conical field of view of approximately 10° wide and a secondary, annular field of view around the Micro field to a total of 40° wide which I have labelled as the Macro field (Fig. 15.2). The dimension of the Macro field was indicated in research into lighting for appearance (Loe et al., 1994). It can also be demonstrated by placing a pencil (approximately 6 mm diameter) with a dark colour outer skin vertically and touching the eyebrow. Then by fixating on the pencil and thus masking the fovea, it is possible to see an out of focus but coloured image. This suggests that there are two images, or sources of information, from which the brain selects information and combines them, or not, depending on the requirement. The third field is the peripheral field of view beyond the Macro field which, as far as we know, only responds to sudden movement and has a very limited colour response. A further effect reported in 1941is the Kruithof effect (Kruithof, 1941), in that the colour of the light and the task illuminance are linked. This being that when the task illuminance is low, say around 50 lux and below, people prefer a warm appearance light. Logically this would seem likely particularly if the reverse is considered i.e. that cool appearance illumination is likely to be unacceptable at low levels of illuminance which tends to indicate a sense of gloom and foreboding. However, later works carried out by Boyce and Cuttle (1990), where colour discrimination visual tasks were used to assess the
Number of rods or cones per square millimeter x 104
18 Blind spot
16 14
Rods
Rods
12 10 8 6 4 2
Cones
Cones
0 70° 60° 50° 40° 30° 20° 10° 0° Temporal
10° 20° 30° 40° 50° 60° 70° 80° Nasal
Perimetric angle in degrees
Peripheral
Mac.
Mac.
Peripheral
Mic.
Fig. 15.2 The distribution of rod and cone photoreceptors across the retina with suggested zones for the micro (foveal), macro (para-foveal) and peripheral fields of view. The divisions between the zones are approximate. (Adapted from Pirenne, M.H., 1967. Vision and the Eye, second ed. Chapman and Hall, London.)
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effect which presumably would have required foveal vision, the effect was not confirmed. More recently in work by Chen et al. (2016), where work was carried out to study the relationship between the light colour appearance and illuminance i.e. the correlated colour temperature (CCT) and task illuminance suggested that the Kruithof effect may exist. To some extent this supports the suggestion that there are two prime areas of the retina, as described by the Micro and Macro fields that operate differently in terms of light colour appearance depending on the primary task in hand. This will also suggest that the eye/brain combination recognises different colours across the spectrum using both fields of view. And to base illumination only on foveal vision, except for task lighting, where the primary concern is a difference in luminance i.e. as in black type on white paper seems unsound. A further effect, although only loosely understood and receiving little attention in lighting design, is that of adaptation, either in terms of light but also of colour. Human vision includes an automatic process which adjusts to the amount of light entering the eye, presumably relative to the average luminance of the visual field or something similar. It operates similar to an automatic exposure camera which adjusts the size of the aperture and length of the exposure relative to the average luminance of the field. This means that any areas of luminance close to the average luminance will be properly displayed but areas with a luminance much higher than the average will be over exposed and those much lower will be under exposed. Ralph Hopkinson was one of the earlier researchers of this (Hopkinson and Collins, 1970). Fig. 15.3 shows his hypothesis which led to his extensive work on discomfort glare. An example of this is shown in an art gallery when a white sculpture is displayed against a light background and a black sculpture also against a light background. In the first example the form of the sculpture is clear but in the second example only the silhouette is visible (Fig. 15.4A and B). A similar effect 104
Object luminance (cd/m2)
Glare 102
100 Discrimination 10–2 Black shadow 10–4 10–4
102 10–2 100 2 Adaptation luminance (cd/m )
104
Fig. 15.3 A schematic illustration of the range of object luminances within which discrimination is possible for different adaptation luminances. The boundaries are approximate. (Adapted from Boyce, P.R., 2014. Human Factors in Lighting, third ed. CRC Press, Boca Raton, FL, figure 2.11, with permission from Taylor & Francis Group.)
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(A)
(B) Fig. 15.4 (A) Light coloured sculpture against a light coloured background shows the form of the sculpture – visibility is enhanced. (B) Dark coloured sculpture against a light coloured background only shows the profile of the sculpture – visibility is limited. (Guggenheim Museum New York.)
occurs when a light coloured object is displayed against a strong coloured background. For example, if the background is a strong green colour then the visual process will adjust to this as the norm and distort the object colours to the complementary colour and have a pale red or pink appearance.
15.3 Illumination and colour metrology Illumination design is a relatively new element of environmental design being only since the early 20th century, when the first practical electric light sources were developed. Over the last 100 years it has developed from providing the basic needs of
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vision to lit environments for every conceivable facility for both interior and exteriors together with a serious consideration of energy consumption. Over this period it has developed four primary units of measurement to define the basic design parameters. These are illuminance, luminance, a colour rendering index (CRI) and a light colour appearance index currently described by the CCT.
15.3.1 Illuminance This defines the amount of light energy incident on a surface directly from either an electric lamp or daylight. It will also include light being reflected from an intermediary surface. The unit of measurement is the lux (or lumens/m2). Its primary use is in defining the amount of light on a task. This allows the lighting designer to provide an appropriate illuminance for particular tasks whether simple or complex see Table 15.1 and The Society of Light and Lighting (2009).
15.3.2 Luminance This is sometimes referred to as ‘apparent brightness’ because it refers to the ‘light’ appearance of a light source, or an illuminated object or surface, or an element within a particular scene. The unit is the light intensity in candelas/metre2 in a particular direction and over a particular field size. This measure allows the designer to ascertain the luminance pattern in a proposed design. For example, is the illumination appearance appropriate in terms of the particular activity? Is the amount appropriate; or is the pattern too uniform or too extremely different in terms of the particular situation or to be comfortable or uncomfortable in terms of glare? Luminance is particularly important because it is one of the elements along with colour that defines our visual experience in terms of appearance.
15.3.3 Reflectance This is the amount of the light reflected from a surface in a particular direction and is defined in terms of a percentage of the intensity of the illumination incident on the surface, to the intensity of the light reflected. The colour of the reflected light will be determined by the colour of the light and the spectral reflectance of the surface. Table 15.1 The relationship between task illuminance and a range of applications. Task requirement Limited to large tasks with poor contrast General movement of people Simple visual tasks for limited time Moderate task difficulty Very small tasks critical sight Courtesy of the Society of Light & Lighting.
Illuminance in lux
Examples
50 100 200
Corridors and store rooms Foyers and simple factory tasks
500 1000
General offices Assembly of small components
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The relationship between illuminance and luminance requires the interaction of the reflectance of the material. And, unless the surface has a perfectly matt surface, it will also depend on the direction of the measurement. If the surface is without a gloss finish and is matt then the reflected luminance will be the same in all directions.
15.3.4 Light measurement standards Prior to the invention of electric lamps, some countries had developed independent light measurement standards for measuring the light output of oil and gas lamps. The standards were usually based on the wax candle hence the early measurement of light intensity was the candela – lamps were classified in candle power rather than power consumed in watts. Hence there was a need to create an internationally acceptable standard based on the human spectral response. It is important to remember that at this time there were no photo-electric measurement instruments so photometric procedures were devised using visual comparison optical devices. There were a number of instruments developed which because of their narrow field of view depended on foveal vision. Work was carried out in a number of laboratories and in 1924, the Commission International de l’Eclairage (CIE), accepted the Photopic Vλ spectral distribution as the standard distribution on which all light measurements would be made and has been ever since.
15.3.5 Colour appearance As different light sources were developed it became clear that although they were nominally ‘white’ in colour they were often different shades of white depending on the way that the light was produced. However, if the difference is not too large the visual system can often adapt to overcome the difference. In other words the brightest area in the scene will be seen as ‘white’ and other colours will be seen relative to that colour but slightly incorrectly. But if different lamps are compared in a side by side demonstration the difference in colour appearance will be clear. The CIE developed a system using the concept of a black-body. This is a hypothetical source that emits a continuous spectrum of radiation determined solely by its temperature in degrees Kelvin. The filament of an incandescent lamp is a good approximate example. The colour of the light produced changes with the temperature. As the temperature increases the light appearance becomes cooler and vice versa. Because lamp light output can be produced in different ways and are not usually true black-bodies the temperature scale used is the CCT. Table 15.2 gives examples. Table 15.2 The relationship between light colour appearance and Correlated Colour Temperature (CCT). Colour appearance
Correlated colour temperature (CCT)
Warm Intermediate Cool
Up to 3300 K Between 3300 and 5300 K Above 5300 K
Courtesy of the Society of Light & Lighting.
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15.3.6 Colour rendering index This measure was seen as important because as discharge lamps were introduced it was clear that their colour rendering effects were different to the incandescent lamp that had preceded them. In addition the ability to measure spectral distributions was also not readily available. This led the members of the CIE to propose a system of measurement based on a series of colour samples from around the hue circle. Comparisons are made between the appearance of each of the samples under the light source in question and a standard incandescent light source with a similar colour temperature. The differences of all the samples were then amalgamated into a single number. However, because the value is an amalgamation of eight independent readings a particular CRI value will not necessarily give the same visual effect for two different light sources. This has provided a useful practical measure in spite of its limitations. Table 15.3 gives examples. However, with the relatively recent introduction of light emitting diodes (LED) as practical illumination light sources it has indicated that CRI is not appropriate for all lamp types. Currently work is being carried out by the CIE to develop a new system of measurement to introduce a supplementary measure possibly based on a gamut area, or spectral capacity indicating degrees of quality.
15.3.7 Future light and colour measurements So what of illuminant colour performance measurements in the future? It is clear that with the development of different light sources a more exact system is needed to define the colour rendering accuracy and the light colour appearance. It would seem that neither is likely to be quantified sufficiently accurately by a single number. Also it is likely to require a consideration of the full range of the human visual spectral range. A possible solution is that suggested by Crawford (1963) i.e. to divide the visual spectral range into a number of spectral bands and compare the value of each to the values of a standard source similar to daylight. For some time now, there has been an interest in the luminance distribution of the human field of view as a way of defining illumination appearance. The hypothesis being, that people react to different light patterns, either positively or negatively, relative to the particular purpose of the facility or activity (Loe and Rowlands, 1996). For example, would people in a restaurant prefer a different distribution of light to that of a hospital ward and if so is there some benefit other than pure appearance preference? Table 15.3 The relationship between colour rendering quality and colour rendering index (CRI). Colour rendering quality
Colour rendering index
Near exact colour rendering High colour rendering Good colour rendering Poor colour rendering
Ra = 100 Ra = 90 Ra = 80 Ra ≤ 79
Courtesy of the Society of Light & Lighting.
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The same could apply to colour and colour combinations in an environment, again which might have some additional human sensation that might be beneficial or not, which might have a psychological effect either positive or negative? Research in this area has used subjective assessments of different light patterns and comparing these to the luminance distributions (Loe et al., 1994). Initially the luminance distributions were measured with a luminance measuring scanning devise. Since that work some researchers have been using digital cameras to measure the distribution and converting the pixel reading into values of luminance. Now if they are to be truly values of luminance in candelas/m2 the camera would need to have a spectral response distribution of the CIE Vλ standard. But by including a known luminance in the output as a calibrator, apparently the results are acceptable. This is puzzling. Clearly a digital camera must be able to measure accurately all colours in the visible spectrum to produce accurate colour images. And since human vision enables the experience of all colours across the range it could provide a basis for a new measurement system for light and colour measurements. Finding information about a particular digital camera spectral response is not readily available but it seems likely it would be similar to daylight? This is purely a hypothesis, but one that deserves further investigation.
15.4 Light sources 15.4.1 Daylight At this time perhaps it would be helpful to consider human evolution. Mankind has been living on planet earth for many thousands and thousands of years and for most of that time the only illuminant was the sun combined with light from the sky and sometimes diffused by cloud cover. This has a spectral power distribution covering the human spectral response (Fig. 15.5). This is an illuminant that is universally trusted as being correct – note how a discerning customer will take a garment to study its colour by the shop window and particularly if matching it to an accessory. This raises the question as to perhaps daylight, or an ‘idealised’ daylight spectral distribution, should be the basis of all light measurement including colour? Recently Mark Rea has proposed what he has called a universal luminous efficiency function Uλ based upon the spectral sensitivity of all five known photoreceptors in the human retina (Rea, 2015) (see Fig. 15.6). Its similarity to daylight makes a further reason for serious consideration, though its response in the ‘red’ region of human response is low which seems odd since red is an important colour in vision? But research and international agreement will be required to determine this. Nonetheless it is a potential step forward.
15.4.2 Flame sources These ranged from fire light to candles and oil and gas lamps. These extended the working day after daylight had faded and improved productivity. Fig. 15.7 shows the spectral distribution of a candle together with that of an incandescent lamp showing
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Relative spectral power distribution
1.0
Clear sky Cloudy sky
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 400
450
500
550 600 650 Wavelength (nm)
700
750
800
Fig. 15.5 Daylight spectral power distributions. (Courtesy of the National Physical Laboratory, UK.)
1.0
V (λ)
0.9
U (λ)
0.8 2° L-cones
0.7 Efficiency
2° M-cones 0.6
2° S-cones
0.5
10° S-cones
0.4
Rods ipRGCs
0.3 0.2 0.1 0
400
450
500 550 Wavelength (nm)
600
650
700
Fig. 15.6 Spectral response curves for the known human receptors within the eye including the Vλ distribution used for illumination photometry encased by the Uλ distribution proposed by Rea as the spectral response as an alternative basis for photometry. (Courtesy of Rea, M.S., 2015. The lumen seen in a new light: making distinctions between light, lighting and neuroscience. Light. Res. Technol. 47, 259–280.)
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Relative spectral power distribution
0.9
Incandescent lamp
0.8
Candle flame
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 400
450
500
550 600 Wavelength (nm)
650
700
750
Fig. 15.7 Spectral power distribution of candle light and an incandescent lamp. (Courtesy of the National Physical Laboratory, UK.)
the similarities. Flame sources have also been around for thousands of years, which suggest that human evolution could also have been influenced by that too.
15.4.3 Incandescent lamps This was the first practical form of electricity generated light. It was preceded by the arc lamp but at the time its uses were limited. The tungsten filament lamp was invented in the late 19th century. It comprised a coil of fine tungsten wire mounted in a glass bulb, initially with a vacuum, but later filled with an inert gas and connected across an electrical supply. At the time of the tungsten lamp introduction there was no lighting industry and very few electricity supplies. At the outset it meant that an electricity generation plant was also required before lighting could take its place in improving and extending human performance. The dimensions of the lamp and its components and the applied voltage produced a light of colour temperature of about 2700 K and SPD as in Fig. 15.7. Over the first half of the 20th century, there were many refinements introduced resulting in a lamp with a life of around 1000 hours and an efficacy of around 12 lumens/W depending to some extent on the size of the lamp. Efficacy relates the lamp light output in lumens to the amount of power it consumes to allow comparisons between different lamp types to be made. In the 1950s, using the same basic construction, integral reflectors were incorporated to create spotlights of varying beam shapes. At a similar time halogens were introduced into the bulb to increase lamp life. One of the problems of the basic lamp is that as it emits light it also discharges tungsten particles onto the inside of the glass bulb. This has the effect of reducing the life. The introduction of halogens and a quartz bulb enabled the lamp to be run at a higher colour temperature which increased
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the CCT to around 3100 K and a life of around 2000 hours with an efficacy of around 20 lumens/W making it a more efficient light source. Another benefit of the tungsten lamp was that the filament was small in size, which meant that it responded well to the use of optics to focus the light into beams and into particular distributions. Colour filters were also introduced for particular applications. Incandescent lamps can be dimmed relatively easily but the halogen lamp should not be used continuously in the dimmed condition otherwise the extra light output and life will not result.
15.4.4 Fluorescent low pressure discharge lamps The fluorescent discharge lamp was introduced in Britain in 1940, but was available earlier in America. The lamp comprised a glass tube 5 ft (1.5 m) in length and 1½ʺ (38 mm) in diameter containing mercury and a gas filling together with a fluorescent coating on the inside of the tube. At either end a small tungsten filament was connected to a lamp holder that enabled it to be connected to an electrical supply via control gear to enable an arc discharge to be ignited and maintained. The emission from the arc was mostly in the ultraviolet region of the spectrum with little emission in the visual region but with its interaction with the fluorescent coating it produced a large amount of light efficiently. The early lamps had an 80 W power rating and a light output of around 2800 lumens. The lamp and associated control gear had an efficacy of around 35 lumens/W and a life of around 2000 hours. The lamp was used initially in factories and later in commercial buildings and beyond. However, the initial colour performance including colour rendering and colour appearance was poor particularly for skin tones but it did enable higher illuminance values which aided productivity. And although modern day lamps have more efficient solid state control gear and a higher performance in terms of life, light output and efficacy, they operate in a similar manner. They are also available in a range of sizes and shapes including those that have the arc tube folded into a compact shape to fit into small luminaires labelled compact fluorescent lamps (CFL) largely aimed at the domestic market and as a replacement for tungsten lamps. Although the spectral distribution varied for different colour types and manufacturer, the distribution tended to be rather spiky and not as smooth as the tungsten lamp. Also there was generally a lack of light emission in the red end of the spectrum which tended to limit the quality of the appearance of skin tones particularly, which was important in hospitals and other health care facilities where skin colour can be an aid to diagnosing particular illnesses. By the mid-1960s, research had improved colour rendering quality to the approval of hospital and health care staff. But the improved colour quality came at a cost with lower lamp efficacy and the suggestion was made that with the improved colour rendering, was it possible that users could accept a lower task illuminance, thus moderating the lower efficacy. At least one trial was carried out using the traditional Landolt ring tasks, both coloured and monochromatic using a range of different lamp types,
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but the results showed that colour rendering had little or no effect on performance. A later investigation, where the visual tasks were studying works of graphic art in a general sense rather than a detailed sense, suggested that it did (Loe et al., 1982). Was this a case of the observers using larger field of view and hence different areas of the retina? However, the problem diminished with the lamp scientists discovering what was termed the tri-phosphor or multi-phosphor coating which improved both colour rendering and efficacy – see Fig. 15.8. In the 1980s, a new example of fluorescent lamp emerged – the CFL. They were prompted by the need to wean users, particularly in the domestic area, off relatively energy inefficient incandescent lamps. They comprised of a narrow fluorescent tube folded into a compact shape, of different designs and with integrated electronic control gear, And, although they used less electricity than a supposedly similar light output to the equivalent tungsten lamp they were met by considerable resistance. Complaints included ‘room appeared dim on switch on’ and an unpleasant colour. Eventually people tended to use them in spaces of a utilitarian nature where the colour performance and other features of annoyance were acceptable. They are still available, although they are being phased out by some manufacturers. However, the experience showed that a lamp had to have a performance that was acceptable to the users in many aspects – good energy efficiency is not sufficient.
15.4.5 High pressure discharge lamps Although the technology of these lamps is different and they tended to be used in different situations i.e. exterior lighting and large volume spaces like factories, the colour requirement tended to be less demanding. That is until relatively recent times when these lamps, particularly metal halide lamps started to be used in retail premises and other spaces where colour was important. This was because of their small size which enabled good beam control in spot lamp luminaires and good efficacy. The lamp scientists have developed lamps with at least ‘good’ colour rendering. Tri-Phosphor 835 CCT: 3500 K CRI: 80 - 89 100%
60% 40% 20% 0% 400
Tri-Phosphor 841 CCT: 4100 K CRI: 80 - 89 100%
80%
Relative power
80%
Relative power
Relative power
Tri-Phosphor 830 CCT: 3000 K CRI: 80 - 89 100%
60% 40% 20% 0%
500
600
Wavelength (nm)
700
400
80% 60% 40% 20% 0%
500
600
Wavelength (nm)
700
400
500
600
700
Wavelength (nm)
Fig. 15.8 Typical spectral power distributions of three tri-phosphor fluorescent lamps c.1980s. (From Illuminating Engineering Society of North America, 2011. The Lighting Handbook, 10th ed., with permission from IES – the Illuminating Engineering Society.)
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15.4.6 Light emitting diodes This is the most recent lamp development and is the result of semiconductor technology. It depends on a chip of a semiconductor device comprising a wide number of possible materials each of which creates light of a particular spectral emission. The chip is connected to a direct current (DC) supply which is derived often from an integral circuit. There is also the possibility of combining the device with a phosphor coating. The technology is continuing to develop, life is currently measured in many 1000s of hours and energy consumption is low hence a high efficacy is typical. They were initially popular where energy efficiency was important. LEDs come in a number of different forms but an important feature is that the light output can be regulated, or dimmed relatively easily, making them attractive where this provides a benefit. Another feature that has recently been introduced is by having two chips, one of a cool colour and the other a warm colour combined within a single unit. This enables the user to adjust the colour from warm to cool. Fig. 15.9 shows the spectral distribution of two separate chips one warm and one cool. Note also that the curves are smooth rather than the spiky distribution of a discharge lamp. Experiments have been carried out into the possibility of producing planar LEDs or OLEDs but this is still work in progress.
15.4.7 Graphene lamps This is a very new technology/material which can be used to produce thin ribbon incandescent filaments that are strong and potentially can be heated to 2800 K enabling light emission with a very long life and presumably a spectral distribution similar to a 1
Relative spectral power distribution
0.9 Warm white LED
0.8
Cool white LED
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 400
450
500
550 600 Wavelength (nm)
650
700
750
Fig. 15.9 Spectral power distribution of warm white and cool white LED light sources. (Courtesy of the National Physical Laboratory, UK.)
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tungsten filament lamp. At this time there are no lamps on the market but if they can produce light similar to the incandescent lamp but with an enhanced performance of increased efficacy together with good colour performance and can be easily dimmed then they could be welcome source – but time will tell.
15.5 Surface colours and reflectance When light is incident on a surface, some may be absorbed, some transmitted and some reflected. Reflectance is the term used for the percentage of the amount of light reflected to the amount of incident light. For example, a ‘light’ pastel colour may have a reflectance of 80% or more, but a dark colour could be 10% or less. The reflected light may be contained within a very small range of angles, termed specular reflection e.g. a mirror surface, or over a very wide range of angles termed diffuse reflection, like the reflections from a matt surface e.g. blotting paper. Some materials combine both specular reflection and diffuse reflection as in a coloured material that has a glazed finish as in a piece of china where the material is coloured but with a glazed transparent skin which gives the product visual sparkle. These properties apply whether the material is neutral in colour as in a white surface, or highly coloured, although the amount of light reflected will depend on the reflectance of the material and of the amount of incident light. In the case of specular surfaces it may be more appropriate to compare the intensity of the incident light to that reflected. If the surface is multi coloured then it will be necessary to determine the area-weighted reflectance. For example, the overall reflectance of a brick wall where it will be necessary to take into account the reflectance value of the brick and its area, together with the reflectance value of the mortar together with area of mortar. Area weighted reflectance calculation: Area of brick = Ab Area of mortar = Am Reflectance of brick = Rb Reflectance of mortar = Rm Area weighted reflectance =
( Ab ´ Rb ) + ( Am ´ Rm ) ( Ab + Am )
An approximation of the degree of reflectance of a matt material can be measured using an illuminance metre by placing the photocell on the surface of the material and measuring the incident illuminance in lumens/m2 and comparing the reading to the amount of light reflected in lumens/m2 by aiming the photocell downward at an angle
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Fig. 15.10 Reflectance assessment chart showing an approximate match of a sample indicating an approximate reflectance. (Courtesy of the Society of Light and Lighting, 2001. Lighting Guide 11: Surface Reflectance and Colour, its Specification and Measurement for Designers.)
of approximately 45° to the surface. If a precise value is required then it will usually require the services of a photometric laboratory which has the equipment to carry out the measurement. The appearance of the colour will depend on the visual conditions. That is, the amount and spectral properties of the illumination. It can also depend on the surrounding colours which can cause a distortion in the colour appearance. Hence the designer needs to be aware of these things in making a choice of materials. Also lighting designers will need to know the reflectance values of the main building surfaces of a room and their intended or expected reflectance values to enable them to design the lighting with due consideration of the amount light reflected back on to the main task surfaces e.g. the desk tops in a school classroom. An aid to assessing reflectance values of matt surfaces can be assessed by visual comparison using a reflectance chart as produced by the Society of Light and Lighting containing known reflectance values for a range of colours and intensity (The Society of Light and Lighting, 2001) (see Fig. 15.10).
15.6 Material damage and colour degradation caused by light Light incident on a material can cause permanent colour degradation in some materials. The degree of the problem is caused by the responsiveness of the material itself and the spectral power distribution of the light source. The problem is caused mainly
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by short wave radiation below 400 nm and particularly ultraviolet light. It will also depend on the length and intensity of the light exposure. This is a particular problem for the curators of museums, art galleries and retail outlets where exhibits will be on public display and where the display needs to be seen to the best effect. There is an added problem where the items are exposed to daylight as in a shop window. The problem is creating a balance between the opposing requirements which can be in conflict with one another and in particular where there are paying visitors or shoppers interested in making a purchase who have a reasonable expectation of good viewing conditions (The Society of Light and Lighting, 2015).
15.7 Considering each of the elements Exhibit responsiveness to damage: this is mainly a problem with organic materials and a curator or retail display manager will need to be judge of this but Table 15.4 provides initial guidance: Light source spectral power distribution: as was mentioned above the main problem is ultraviolet radiation, hence this should always be excluded for medium and high responsive materials. This will include daylight where, if possible, the glass should include a UV inhibitor either within the glass itself or with a UV filter applied, though the latter has usually a limited life. Another solution is to exclude daylight outside gallery or shop opening hours by installing blinds or some other obstructing device. For electric light sources it will depend on the UV component in the spectral power distribution. The problem will usually apply to discharge light sources where, if necessary, a filter to exclude UV can be used. Light exposure: this is a combination of the intensity of the light incident on the exhibit in lux and the length of the exposure in hours, presented as a product of the two. The problem is selecting an illuminance appropriate for the viewer to see exhibits sufficiently well in terms of the overall quality of the work including a reasonable appreciation of the colour and detail in the particular work.
Table 15.4 The recommended maximum annual light exposure, in lux hours, for a range of different materials. Conservation category Irresponsive Low responsivity Medium responsivity High responsivity
Material with illuminance examples – e.g.
Annual exposure in lux hours
Stone, ceramics, glass & metals Oil & acrylic paintings Illuminance 200 lx Water colours, textiles & paper. Illuminance 50 lx Animal & plant materials Illuminance 50 lx
Usually no limit
Courtesy of the Society of Light & Lighting.
600,000 150,000 15,000
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Fig. 15.11 Art gallery illumination, using spotlights to illuminate the paintings individually, together with a low wall reflectance to limit the luminance range resulting in good visibility of the paintings.
Viewing conditions: a study carried out at the Bartlett School of Architecture, UCL into the viewing conditions of works of art with a particular interest in the display of paintings. It involved the construction of a small gallery displaying five painting i ncluding oil and water colours covering a number of different subject matter and styles. The gallery was illuminated in different ways including a range of different painting illuminance, and with different lamps to ascertain a solution that served both viewers and curators. A number of observers were asked to make visual assessments of the different presentations using subjective assessment scales. The results indicated that an illuminance on the painting surface of a minimum 200 lx was required to provide acceptable viewing conditions in terms of the detailed discrimination and of the overall quality of the work. The results also suggested that in terms of the overall lighting of the gallery there was a preference for a degree of accent illumination on the paintings i.e. soft edged spot lighting (Loe et al., 1982). This has the effect of focusing the viewer’s attention on the paintings. It also lowers the brightness visual adaptation level and enhancing the brightness of the paintings. The effect will be further enhanced by lowering the surround reflectance – see Fig. 15.11. It will be obvious that the control of illumination for protecting the works involves a number of possible solutions, but the curator needs to take a balanced view of this with the experience of the viewer in mind. There is nothing more disappointing than trying to view an Impressionist style painting of a sunlit composition with an illuminance of 50 lx. It appears drab and not what the artist had intended. To some extent this can be overcome by limiting the exposure time. In addition to colour degradation in some materials, in particular fabrics used as window curtains and drapes, physical damage can be caused by light. To avoid this it is recommended that they be lined. This problem can also apply to displays of costumes and tapestries also of organic materials e.g. animal skins, etc.
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15.8 Illumination, colour and psychological response So far, I have only considered what might loosely be described as the physical relationship of light and colour but there is also the possibility of psychological effects. Some psychologists are beginning to talk about the conscious and unconscious mind to describe the difference between consciously gathering information as in doing a task, as compared to a sensation that is imposed unconsciously. The question that needs to be asked is do psychological effects have a physiological effect that we have not yet discovered? For some time now, researchers and lighting designers in the field of environmental design have begun to consider psychological effects that may induce sensations of, for example, wellbeing, alertness or lethargy. This requires lighting that goes beyond task lighting to a consideration of lighting for appearance. The research so far is fairly limited but there is some evidence, and growing, to suggest that the luminance pattern within a horizontal viewing band of approximately 40° wide has some influence on these psychological sensations (Loe et al., 2000). Some designers, particularly from an architectural back ground, or stage and film lighting designers, with the aim of creating illumination that complements either the architectural form or style or the facility or activity of the environment. Or in the case of stage and film lighting designs to support or enhance the narrative of the script. This is probably a natural part of their purpose but if this is the case perhaps by taking a similar approach in what might be described as ‘everyday’ situations whether it may be a school classroom or a reception area in a commercial organisation or a church for a wedding. If these enhance the built environment and create, perhaps subjectively, a human benefit, or a greater experience relative to the purpose of the environment, they would need to be considered. After all when illumination is provided just for task purposes, it will inevitably affect the appearance of the whole environment. The next question is: if this is the case does the colour scheme have a similar effect? One can imagine that if a muted colour scheme is provided it would have a calming effect on the occupants and vice-versa? Or perhaps by having a vibrant colour scheme it creates a stimulant? One way in which this may be tested is by studying artist’s paintings and labelling them in terms of the subject and comparing these results to the light patterns and the colour designs of the composition. If the results suggest that the results are positive then the next step would be to measure human physiological parameters, perhaps using subjective assessment or brain activity by exploring brain scanning methods, to see if there is a result positive or negative? Some people will respond positively to a well-lit interior or to a colour design or both – but is this just a question of preference dependent on personality or education?
15.9 Conclusions In the preceding discussion of the interaction of the human response to light, illumination and colour, together with systems of measurement and specification I have attempted to explain the science of where we are. This has been fashioned largely
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by where we have come from and the limitations of technology and some research. This has led me to hypothesise what we need to investigate further for the benefit of improved environmental design. A good starting point is to recognise that a visual environment design needs to address two main parameters the activities of the users which are often a primary concern. This might be desk work in an office or children playing in a nursery but there will also be supplementary tasks all of which need to be considered like navigating the space. But any illumination and colour design will create an appearance, either by accident or by intention. This idea is not new; early in the 20th century the American architect Frank Lloyd Wright used the term ‘Form and Function’ as a design guide but I think activity and appearance are more explicit and need to be the bedrock of design. But to achieve this will require a better understanding of the human visual system together with its interaction with the brain. It will also require a system of measurement and specification that will help designers to a more appropriate and better solution. We are on the edge of major developments in lamp technology together with ways of light source control and not to have development in design would be a lost opportunity. But more research will be required from ophthalmologists, neurologists and designers et al to achieve change.
Acknowledgements To write this chapter I have had the benefit of consultations with many experts during my long career in lighting but in particular I would like to thank the input of Peter Boyce, Michael Pointer, Mark Rea, and Peter Tregenza.
References Boyce, P.R., Cuttle, C., 1990. Effect of CCT on the perception of interiors and colour discrimination performance. Light. Res. Technol. 22, 9–36. Chen, H.-S., Chou, C.-J., Lou, H.-W., Lou, M.R., 2016. Museum lighting environment: designing a perception zone map and emotional response models. Light. Res. Technol. 16 (5), 589–607. Crawford, B.H., 1963. Colour rendering tolerances and the colour-rendering properties of light sources. Trans. Illum. Eng. Soc. 28, 50–64. Hopkinson, R.G., Collins, J.B., 1970. The Ergonomics of Lighting. Macdonald Technical and Scientific, London (also further reading (Boyce, 2014)). Kruithof, A.A., 1941. Tubular luminance lamps for general illumination. Philips Tech. Rev. 6, 65–96. Loe, D.L., 2016. vision and illumination: the interaction revisited. Light. Res. Technol. 48, 176–189. Loe, D.L., Rowlands, E., 1996. The art and science of lighting: a strategy for lighting design. Light. Res. Technol. 28, 153–164.
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Loe, D.L., Rowlands, E., Watson, N.F., 1982. Preferred lighting conditions for the display of oil and watercolour paintings. Light. Res. Technol. 14, 173–192. Loe, D.L., Mansfield, K.P., Rowlands, E., 1994. Appearance of lit environment and its relevance in lighting design: experimental study. Light. Res. Technol. 26, 119–133. Loe, D.L., Mansfield, K.P., Rowlands, E., 2000. A step in quantifying the appearance of a lit scene. Light. Res. Technol. 32, 213–222. Rea, M.S., 2015. The lumen seen in a new light: making distinctions between light, lighting and neuroscience. Light. Res. Technol. 47, 259–280. The Society of Light and Lighting, 2001. Lighting Guide 11: Surface Reflectance and Colour, its Specification and Measurement for Designers. National Physical Laboratory and Society of Light and Lighting. The Society of Light and Lighting, 2015. Lighting Guide 8: Lighting for Museums and Art Galleries. Society of Light and Lighting.
Further reading Boyce, P.R., 2014. Human Factors in Lighting, third ed. CRC Press, Boca Raton, FL. Hunt, R.W.G., Pointer, M.R., 2011. The Measurement of Colour, fourth ed. John Wiley & Sons Inc, New York. Illuminating Engineering Society of North America, 2011. The Lighting Handbook, 10th ed. IESNA, New York. Murdoch, J.B., 2003. Illuminating Engineering, second ed. Visions Communication, Larose, LA. The Society of Light and Lighting, 2009. The SLL Lighting Handbook. CIBSE, London. The Society of Light and Lighting, 2012. The SLL Code for Lighting. CIBSE, London. Tregenza, P., Loe, D.L., 2014. The Design of Lighting, second ed. Routledge, London.
The Society of Light and Lighting produce a number of lighting guides, covering a range of topics.
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D.L. Loe Fellow of the Society of Light and Lighting (FSLL), Chartered Institution of Building Services Engineers (CIBSE), London, United Kingdom
15.1 Introduction Light is a fundamental element of human life. It allows us to accomplish visual tasks from the simple to the complex, and can have a bearing on health and wellbeing through its intensity distribution and colour. It can also create psychological human responses that can enhance man’s relationship with the built environment. Much of the latter is still not properly understood and is an area that needs further investigation. The following explores the limitations and hypothesises of electric lighting and colour specification based on human response by drawing from the historical development of the subject as well as from past research with the aim of furthering improvements in design. Recent developments in technology could enable better systems of measurement of light and colour and hence specification. Light sources are also changing, making new lighting solutions possible for the benefit of the occupants together with improved energy efficiency. Along with light and illumination, colour is something we tend to take for granted until we are faced with an anomaly – something appears not quite right or that an error occurs because of how we describe and measure colour which ends up with a mistaken choice of colour or colour combinations or even the choice of a product in terms of its attractiveness or the freshness of natural produce. The colour fidelity, as perceived of an object, a material or a surface, will depend primarily on the interaction of the inherent colour/s of the object i.e. its spectral reflectance or transmission. It will also depend on the colour of the light that illuminates it, that is its spectral emission in terms of wavelength. Both of these elements are usually constant but human colour response is more complicated and has degrees of adjustment largely due to visual adaptation to both colour and brightness but stray outside the limits then errors of judgement can occur. Bearing this in mind, it is perhaps strange, with the benefit of hindsight, that so much of light metrology is only based on the spectral distribution of the centre of our visual field – the fovea which has the greatest density of visual receptors which enables critical sight. The following discusses these issues and how some of the apparent anomalies might be overcome. It also discusses how we deal with light and colour in appearance design.
15.2 Light and vision Light can be described as the transmitter of information from the world around us to the brain. It allows us to carry out tasks ranging from the complex, such as threading Colour Design. http://dx.doi.org/10.1016/B978-0-08-101270-3.00015-1 Copyright © 2017 Elsevier Ltd. All rights reserved.
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a needle, to the more general, for example, allowing us to move easily through a crowded retail outlet. It allows us to discriminate between different features of the three-dimensional scene by luminance and colour differences. It is also an element that can induce a sense of wellbeing as with a sunlit room which has areas of light and shade which can be described as ‘light’ and ‘cheerful’ or one as ‘gloomy’ and depressing when a scene is illuminated relatively uniformly with low levels of illumination – as one might describe the illumination from an overcast sky on a dull day. Light can also be described as electromagnetic radiation ranging approximately from 380 to 780 nm the spectral range of the human eye. Ultraviolet radiation ranges from around 100 to 400 nm, and although it is invisible to the eye, it can be a source of concern because it can damage certain materials. It can also interact with fluorescent materials which can provide special visual effects either decorative or functional, or as in retro-reflective warning signs on a motorway. The human eye has an optical system which focuses the light information on to the inside surface of the retina at the back of the eye, which has a network of light receptors. The eye has a conical field of view of approximately 100° wide, with the most sensitive area a cone of approximately 2° wide which is described as the fovea. This is the area of the retina with the highest density of light sensors thus allowing critical sight. But the spectral response of the fovea is limited to a relatively narrow colour distribution centred on 550 nm and what is described by the CIE as the Vλ photopic spectral distribution and is the standard spectral response by which light and illumination is measured. This is surprising since human vision usually spans the full colour range and not as that described by the Vλ distribution (see Fig. 15.1). There are, however, some people with colour defective vision to which this does not apply.
Relative sensitivity of the human eye: the Vλ curve 400 nm
Gamma radiation 10
Xrays
100 1000 picometres, pm 1
500
Ultraviolet
600
Visible
Microwave Radar
1 10
Infra-red
700
100 1000 nanometres, nm
10 100 1000 micrometres, µm, 1
RADIO TV
MW 1
10
10
100 1000 metres, m
1000 100 millimetres, mm
Fig. 15.1 Electromagnetic spectrum described by wavelength in metres together with the human visual spectrum and CIE Photopic Vλ spectral distribution. (Courtesy of Peter Tregenza, with permission from Taylor & Francis Group.)
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This observation, together with some research evidence, has led me to suggest that there are three main areas of the retina (Loe, 2016). The fovea with critical vision and limited colour response which I have labelled as the Micro field which embraces a conical field of view of approximately 10° wide and a secondary, annular field of view around the Micro field to a total of 40° wide which I have labelled as the Macro field (Fig. 15.2). The dimension of the Macro field was indicated in research into lighting for appearance (Loe et al., 1994). It can also be demonstrated by placing a pencil (approximately 6 mm diameter) with a dark colour outer skin vertically and touching the eyebrow. Then by fixating on the pencil and thus masking the fovea, it is possible to see an out of focus but coloured image. This suggests that there are two images, or sources of information, from which the brain selects information and combines them, or not, depending on the requirement. The third field is the peripheral field of view beyond the Macro field which, as far as we know, only responds to sudden movement and has a very limited colour response. A further effect reported in 1941is the Kruithof effect (Kruithof, 1941), in that the colour of the light and the task illuminance are linked. This being that when the task illuminance is low, say around 50 lux and below, people prefer a warm appearance light. Logically this would seem likely particularly if the reverse is considered i.e. that cool appearance illumination is likely to be unacceptable at low levels of illuminance which tends to indicate a sense of gloom and foreboding. However, later works carried out by Boyce and Cuttle (1990), where colour discrimination visual tasks were used to assess the
Number of rods or cones per square millimeter x 104
18 Blind spot
16 14
Rods
Rods
12 10 8 6 4 2
Cones
Cones
0 70° 60° 50° 40° 30° 20° 10° 0° Temporal
10° 20° 30° 40° 50° 60° 70° 80° Nasal
Perimetric angle in degrees
Peripheral
Mac.
Mac.
Peripheral
Mic.
Fig. 15.2 The distribution of rod and cone photoreceptors across the retina with suggested zones for the micro (foveal), macro (para-foveal) and peripheral fields of view. The divisions between the zones are approximate. (Adapted from Pirenne, M.H., 1967. Vision and the Eye, second ed. Chapman and Hall, London.)
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effect which presumably would have required foveal vision, the effect was not confirmed. More recently in work by Chen et al. (2016), where work was carried out to study the relationship between the light colour appearance and illuminance i.e. the correlated colour temperature (CCT) and task illuminance suggested that the Kruithof effect may exist. To some extent this supports the suggestion that there are two prime areas of the retina, as described by the Micro and Macro fields that operate differently in terms of light colour appearance depending on the primary task in hand. This will also suggest that the eye/brain combination recognises different colours across the spectrum using both fields of view. And to base illumination only on foveal vision, except for task lighting, where the primary concern is a difference in luminance i.e. as in black type on white paper seems unsound. A further effect, although only loosely understood and receiving little attention in lighting design, is that of adaptation, either in terms of light but also of colour. Human vision includes an automatic process which adjusts to the amount of light entering the eye, presumably relative to the average luminance of the visual field or something similar. It operates similar to an automatic exposure camera which adjusts the size of the aperture and length of the exposure relative to the average luminance of the field. This means that any areas of luminance close to the average luminance will be properly displayed but areas with a luminance much higher than the average will be over exposed and those much lower will be under exposed. Ralph Hopkinson was one of the earlier researchers of this (Hopkinson and Collins, 1970). Fig. 15.3 shows his hypothesis which led to his extensive work on discomfort glare. An example of this is shown in an art gallery when a white sculpture is displayed against a light background and a black sculpture also against a light background. In the first example the form of the sculpture is clear but in the second example only the silhouette is visible (Fig. 15.4A and B). A similar effect 104
Object luminance (cd/m2)
Glare 102
100 Discrimination 10–2 Black shadow 10–4 10–4
102 10–2 100 2 Adaptation luminance (cd/m )
104
Fig. 15.3 A schematic illustration of the range of object luminances within which discrimination is possible for different adaptation luminances. The boundaries are approximate. (Adapted from Boyce, P.R., 2014. Human Factors in Lighting, third ed. CRC Press, Boca Raton, FL, figure 2.11, with permission from Taylor & Francis Group.)
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(A)
(B) Fig. 15.4 (A) Light coloured sculpture against a light coloured background shows the form of the sculpture – visibility is enhanced. (B) Dark coloured sculpture against a light coloured background only shows the profile of the sculpture – visibility is limited. (Guggenheim Museum New York.)
occurs when a light coloured object is displayed against a strong coloured background. For example, if the background is a strong green colour then the visual process will adjust to this as the norm and distort the object colours to the complementary colour and have a pale red or pink appearance.
15.3 Illumination and colour metrology Illumination design is a relatively new element of environmental design being only since the early 20th century, when the first practical electric light sources were developed. Over the last 100 years it has developed from providing the basic needs of
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vision to lit environments for every conceivable facility for both interior and exteriors together with a serious consideration of energy consumption. Over this period it has developed four primary units of measurement to define the basic design parameters. These are illuminance, luminance, a colour rendering index (CRI) and a light colour appearance index currently described by the CCT.
15.3.1 Illuminance This defines the amount of light energy incident on a surface directly from either an electric lamp or daylight. It will also include light being reflected from an intermediary surface. The unit of measurement is the lux (or lumens/m2). Its primary use is in defining the amount of light on a task. This allows the lighting designer to provide an appropriate illuminance for particular tasks whether simple or complex see Table 15.1 and The Society of Light and Lighting (2009).
15.3.2 Luminance This is sometimes referred to as ‘apparent brightness’ because it refers to the ‘light’ appearance of a light source, or an illuminated object or surface, or an element within a particular scene. The unit is the light intensity in candelas/metre2 in a particular direction and over a particular field size. This measure allows the designer to ascertain the luminance pattern in a proposed design. For example, is the illumination appearance appropriate in terms of the particular activity? Is the amount appropriate; or is the pattern too uniform or too extremely different in terms of the particular situation or to be comfortable or uncomfortable in terms of glare? Luminance is particularly important because it is one of the elements along with colour that defines our visual experience in terms of appearance.
15.3.3 Reflectance This is the amount of the light reflected from a surface in a particular direction and is defined in terms of a percentage of the intensity of the illumination incident on the surface, to the intensity of the light reflected. The colour of the reflected light will be determined by the colour of the light and the spectral reflectance of the surface. Table 15.1 The relationship between task illuminance and a range of applications. Task requirement Limited to large tasks with poor contrast General movement of people Simple visual tasks for limited time Moderate task difficulty Very small tasks critical sight Courtesy of the Society of Light & Lighting.
Illuminance in lux
Examples
50 100 200
Corridors and store rooms Foyers and simple factory tasks
500 1000
General offices Assembly of small components
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The relationship between illuminance and luminance requires the interaction of the reflectance of the material. And, unless the surface has a perfectly matt surface, it will also depend on the direction of the measurement. If the surface is without a gloss finish and is matt then the reflected luminance will be the same in all directions.
15.3.4 Light measurement standards Prior to the invention of electric lamps, some countries had developed independent light measurement standards for measuring the light output of oil and gas lamps. The standards were usually based on the wax candle hence the early measurement of light intensity was the candela – lamps were classified in candle power rather than power consumed in watts. Hence there was a need to create an internationally acceptable standard based on the human spectral response. It is important to remember that at this time there were no photo-electric measurement instruments so photometric procedures were devised using visual comparison optical devices. There were a number of instruments developed which because of their narrow field of view depended on foveal vision. Work was carried out in a number of laboratories and in 1924, the Commission International de l’Eclairage (CIE), accepted the Photopic Vλ spectral distribution as the standard distribution on which all light measurements would be made and has been ever since.
15.3.5 Colour appearance As different light sources were developed it became clear that although they were nominally ‘white’ in colour they were often different shades of white depending on the way that the light was produced. However, if the difference is not too large the visual system can often adapt to overcome the difference. In other words the brightest area in the scene will be seen as ‘white’ and other colours will be seen relative to that colour but slightly incorrectly. But if different lamps are compared in a side by side demonstration the difference in colour appearance will be clear. The CIE developed a system using the concept of a black-body. This is a hypothetical source that emits a continuous spectrum of radiation determined solely by its temperature in degrees Kelvin. The filament of an incandescent lamp is a good approximate example. The colour of the light produced changes with the temperature. As the temperature increases the light appearance becomes cooler and vice versa. Because lamp light output can be produced in different ways and are not usually true black-bodies the temperature scale used is the CCT. Table 15.2 gives examples. Table 15.2 The relationship between light colour appearance and Correlated Colour Temperature (CCT). Colour appearance
Correlated colour temperature (CCT)
Warm Intermediate Cool
Up to 3300 K Between 3300 and 5300 K Above 5300 K
Courtesy of the Society of Light & Lighting.
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15.3.6 Colour rendering index This measure was seen as important because as discharge lamps were introduced it was clear that their colour rendering effects were different to the incandescent lamp that had preceded them. In addition the ability to measure spectral distributions was also not readily available. This led the members of the CIE to propose a system of measurement based on a series of colour samples from around the hue circle. Comparisons are made between the appearance of each of the samples under the light source in question and a standard incandescent light source with a similar colour temperature. The differences of all the samples were then amalgamated into a single number. However, because the value is an amalgamation of eight independent readings a particular CRI value will not necessarily give the same visual effect for two different light sources. This has provided a useful practical measure in spite of its limitations. Table 15.3 gives examples. However, with the relatively recent introduction of light emitting diodes (LED) as practical illumination light sources it has indicated that CRI is not appropriate for all lamp types. Currently work is being carried out by the CIE to develop a new system of measurement to introduce a supplementary measure possibly based on a gamut area, or spectral capacity indicating degrees of quality.
15.3.7 Future light and colour measurements So what of illuminant colour performance measurements in the future? It is clear that with the development of different light sources a more exact system is needed to define the colour rendering accuracy and the light colour appearance. It would seem that neither is likely to be quantified sufficiently accurately by a single number. Also it is likely to require a consideration of the full range of the human visual spectral range. A possible solution is that suggested by Crawford (1963) i.e. to divide the visual spectral range into a number of spectral bands and compare the value of each to the values of a standard source similar to daylight. For some time now, there has been an interest in the luminance distribution of the human field of view as a way of defining illumination appearance. The hypothesis being, that people react to different light patterns, either positively or negatively, relative to the particular purpose of the facility or activity (Loe and Rowlands, 1996). For example, would people in a restaurant prefer a different distribution of light to that of a hospital ward and if so is there some benefit other than pure appearance preference? Table 15.3 The relationship between colour rendering quality and colour rendering index (CRI). Colour rendering quality
Colour rendering index
Near exact colour rendering High colour rendering Good colour rendering Poor colour rendering
Ra = 100 Ra = 90 Ra = 80 Ra ≤ 79
Courtesy of the Society of Light & Lighting.
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The same could apply to colour and colour combinations in an environment, again which might have some additional human sensation that might be beneficial or not, which might have a psychological effect either positive or negative? Research in this area has used subjective assessments of different light patterns and comparing these to the luminance distributions (Loe et al., 1994). Initially the luminance distributions were measured with a luminance measuring scanning devise. Since that work some researchers have been using digital cameras to measure the distribution and converting the pixel reading into values of luminance. Now if they are to be truly values of luminance in candelas/m2 the camera would need to have a spectral response distribution of the CIE Vλ standard. But by including a known luminance in the output as a calibrator, apparently the results are acceptable. This is puzzling. Clearly a digital camera must be able to measure accurately all colours in the visible spectrum to produce accurate colour images. And since human vision enables the experience of all colours across the range it could provide a basis for a new measurement system for light and colour measurements. Finding information about a particular digital camera spectral response is not readily available but it seems likely it would be similar to daylight? This is purely a hypothesis, but one that deserves further investigation.
15.4 Light sources 15.4.1 Daylight At this time perhaps it would be helpful to consider human evolution. Mankind has been living on planet earth for many thousands and thousands of years and for most of that time the only illuminant was the sun combined with light from the sky and sometimes diffused by cloud cover. This has a spectral power distribution covering the human spectral response (Fig. 15.5). This is an illuminant that is universally trusted as being correct – note how a discerning customer will take a garment to study its colour by the shop window and particularly if matching it to an accessory. This raises the question as to perhaps daylight, or an ‘idealised’ daylight spectral distribution, should be the basis of all light measurement including colour? Recently Mark Rea has proposed what he has called a universal luminous efficiency function Uλ based upon the spectral sensitivity of all five known photoreceptors in the human retina (Rea, 2015) (see Fig. 15.6). Its similarity to daylight makes a further reason for serious consideration, though its response in the ‘red’ region of human response is low which seems odd since red is an important colour in vision? But research and international agreement will be required to determine this. Nonetheless it is a potential step forward.
15.4.2 Flame sources These ranged from fire light to candles and oil and gas lamps. These extended the working day after daylight had faded and improved productivity. Fig. 15.7 shows the spectral distribution of a candle together with that of an incandescent lamp showing
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Relative spectral power distribution
1.0
Clear sky Cloudy sky
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 400
450
500
550 600 650 Wavelength (nm)
700
750
800
Fig. 15.5 Daylight spectral power distributions. (Courtesy of the National Physical Laboratory, UK.)
1.0
V (λ)
0.9
U (λ)
0.8 2° L-cones
0.7 Efficiency
2° M-cones 0.6
2° S-cones
0.5
10° S-cones
0.4
Rods ipRGCs
0.3 0.2 0.1 0
400
450
500 550 Wavelength (nm)
600
650
700
Fig. 15.6 Spectral response curves for the known human receptors within the eye including the Vλ distribution used for illumination photometry encased by the Uλ distribution proposed by Rea as the spectral response as an alternative basis for photometry. (Courtesy of Rea, M.S., 2015. The lumen seen in a new light: making distinctions between light, lighting and neuroscience. Light. Res. Technol. 47, 259–280.)
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Relative spectral power distribution
0.9
Incandescent lamp
0.8
Candle flame
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 400
450
500
550 600 Wavelength (nm)
650
700
750
Fig. 15.7 Spectral power distribution of candle light and an incandescent lamp. (Courtesy of the National Physical Laboratory, UK.)
the similarities. Flame sources have also been around for thousands of years, which suggest that human evolution could also have been influenced by that too.
15.4.3 Incandescent lamps This was the first practical form of electricity generated light. It was preceded by the arc lamp but at the time its uses were limited. The tungsten filament lamp was invented in the late 19th century. It comprised a coil of fine tungsten wire mounted in a glass bulb, initially with a vacuum, but later filled with an inert gas and connected across an electrical supply. At the time of the tungsten lamp introduction there was no lighting industry and very few electricity supplies. At the outset it meant that an electricity generation plant was also required before lighting could take its place in improving and extending human performance. The dimensions of the lamp and its components and the applied voltage produced a light of colour temperature of about 2700 K and SPD as in Fig. 15.7. Over the first half of the 20th century, there were many refinements introduced resulting in a lamp with a life of around 1000 hours and an efficacy of around 12 lumens/W depending to some extent on the size of the lamp. Efficacy relates the lamp light output in lumens to the amount of power it consumes to allow comparisons between different lamp types to be made. In the 1950s, using the same basic construction, integral reflectors were incorporated to create spotlights of varying beam shapes. At a similar time halogens were introduced into the bulb to increase lamp life. One of the problems of the basic lamp is that as it emits light it also discharges tungsten particles onto the inside of the glass bulb. This has the effect of reducing the life. The introduction of halogens and a quartz bulb enabled the lamp to be run at a higher colour temperature which increased
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the CCT to around 3100 K and a life of around 2000 hours with an efficacy of around 20 lumens/W making it a more efficient light source. Another benefit of the tungsten lamp was that the filament was small in size, which meant that it responded well to the use of optics to focus the light into beams and into particular distributions. Colour filters were also introduced for particular applications. Incandescent lamps can be dimmed relatively easily but the halogen lamp should not be used continuously in the dimmed condition otherwise the extra light output and life will not result.
15.4.4 Fluorescent low pressure discharge lamps The fluorescent discharge lamp was introduced in Britain in 1940, but was available earlier in America. The lamp comprised a glass tube 5 ft (1.5 m) in length and 1½ʺ (38 mm) in diameter containing mercury and a gas filling together with a fluorescent coating on the inside of the tube. At either end a small tungsten filament was connected to a lamp holder that enabled it to be connected to an electrical supply via control gear to enable an arc discharge to be ignited and maintained. The emission from the arc was mostly in the ultraviolet region of the spectrum with little emission in the visual region but with its interaction with the fluorescent coating it produced a large amount of light efficiently. The early lamps had an 80 W power rating and a light output of around 2800 lumens. The lamp and associated control gear had an efficacy of around 35 lumens/W and a life of around 2000 hours. The lamp was used initially in factories and later in commercial buildings and beyond. However, the initial colour performance including colour rendering and colour appearance was poor particularly for skin tones but it did enable higher illuminance values which aided productivity. And although modern day lamps have more efficient solid state control gear and a higher performance in terms of life, light output and efficacy, they operate in a similar manner. They are also available in a range of sizes and shapes including those that have the arc tube folded into a compact shape to fit into small luminaires labelled compact fluorescent lamps (CFL) largely aimed at the domestic market and as a replacement for tungsten lamps. Although the spectral distribution varied for different colour types and manufacturer, the distribution tended to be rather spiky and not as smooth as the tungsten lamp. Also there was generally a lack of light emission in the red end of the spectrum which tended to limit the quality of the appearance of skin tones particularly, which was important in hospitals and other health care facilities where skin colour can be an aid to diagnosing particular illnesses. By the mid-1960s, research had improved colour rendering quality to the approval of hospital and health care staff. But the improved colour quality came at a cost with lower lamp efficacy and the suggestion was made that with the improved colour rendering, was it possible that users could accept a lower task illuminance, thus moderating the lower efficacy. At least one trial was carried out using the traditional Landolt ring tasks, both coloured and monochromatic using a range of different lamp types,
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but the results showed that colour rendering had little or no effect on performance. A later investigation, where the visual tasks were studying works of graphic art in a general sense rather than a detailed sense, suggested that it did (Loe et al., 1982). Was this a case of the observers using larger field of view and hence different areas of the retina? However, the problem diminished with the lamp scientists discovering what was termed the tri-phosphor or multi-phosphor coating which improved both colour rendering and efficacy – see Fig. 15.8. In the 1980s, a new example of fluorescent lamp emerged – the CFL. They were prompted by the need to wean users, particularly in the domestic area, off relatively energy inefficient incandescent lamps. They comprised of a narrow fluorescent tube folded into a compact shape, of different designs and with integrated electronic control gear, And, although they used less electricity than a supposedly similar light output to the equivalent tungsten lamp they were met by considerable resistance. Complaints included ‘room appeared dim on switch on’ and an unpleasant colour. Eventually people tended to use them in spaces of a utilitarian nature where the colour performance and other features of annoyance were acceptable. They are still available, although they are being phased out by some manufacturers. However, the experience showed that a lamp had to have a performance that was acceptable to the users in many aspects – good energy efficiency is not sufficient.
15.4.5 High pressure discharge lamps Although the technology of these lamps is different and they tended to be used in different situations i.e. exterior lighting and large volume spaces like factories, the colour requirement tended to be less demanding. That is until relatively recent times when these lamps, particularly metal halide lamps started to be used in retail premises and other spaces where colour was important. This was because of their small size which enabled good beam control in spot lamp luminaires and good efficacy. The lamp scientists have developed lamps with at least ‘good’ colour rendering. Tri-Phosphor 835 CCT: 3500 K CRI: 80 - 89 100%
60% 40% 20% 0% 400
Tri-Phosphor 841 CCT: 4100 K CRI: 80 - 89 100%
80%
Relative power
80%
Relative power
Relative power
Tri-Phosphor 830 CCT: 3000 K CRI: 80 - 89 100%
60% 40% 20% 0%
500
600
Wavelength (nm)
700
400
80% 60% 40% 20% 0%
500
600
Wavelength (nm)
700
400
500
600
700
Wavelength (nm)
Fig. 15.8 Typical spectral power distributions of three tri-phosphor fluorescent lamps c.1980s. (From Illuminating Engineering Society of North America, 2011. The Lighting Handbook, 10th ed., with permission from IES – the Illuminating Engineering Society.)
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15.4.6 Light emitting diodes This is the most recent lamp development and is the result of semiconductor technology. It depends on a chip of a semiconductor device comprising a wide number of possible materials each of which creates light of a particular spectral emission. The chip is connected to a direct current (DC) supply which is derived often from an integral circuit. There is also the possibility of combining the device with a phosphor coating. The technology is continuing to develop, life is currently measured in many 1000s of hours and energy consumption is low hence a high efficacy is typical. They were initially popular where energy efficiency was important. LEDs come in a number of different forms but an important feature is that the light output can be regulated, or dimmed relatively easily, making them attractive where this provides a benefit. Another feature that has recently been introduced is by having two chips, one of a cool colour and the other a warm colour combined within a single unit. This enables the user to adjust the colour from warm to cool. Fig. 15.9 shows the spectral distribution of two separate chips one warm and one cool. Note also that the curves are smooth rather than the spiky distribution of a discharge lamp. Experiments have been carried out into the possibility of producing planar LEDs or OLEDs but this is still work in progress.
15.4.7 Graphene lamps This is a very new technology/material which can be used to produce thin ribbon incandescent filaments that are strong and potentially can be heated to 2800 K enabling light emission with a very long life and presumably a spectral distribution similar to a 1
Relative spectral power distribution
0.9 Warm white LED
0.8
Cool white LED
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 400
450
500
550 600 Wavelength (nm)
650
700
750
Fig. 15.9 Spectral power distribution of warm white and cool white LED light sources. (Courtesy of the National Physical Laboratory, UK.)
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tungsten filament lamp. At this time there are no lamps on the market but if they can produce light similar to the incandescent lamp but with an enhanced performance of increased efficacy together with good colour performance and can be easily dimmed then they could be welcome source – but time will tell.
15.5 Surface colours and reflectance When light is incident on a surface, some may be absorbed, some transmitted and some reflected. Reflectance is the term used for the percentage of the amount of light reflected to the amount of incident light. For example, a ‘light’ pastel colour may have a reflectance of 80% or more, but a dark colour could be 10% or less. The reflected light may be contained within a very small range of angles, termed specular reflection e.g. a mirror surface, or over a very wide range of angles termed diffuse reflection, like the reflections from a matt surface e.g. blotting paper. Some materials combine both specular reflection and diffuse reflection as in a coloured material that has a glazed finish as in a piece of china where the material is coloured but with a glazed transparent skin which gives the product visual sparkle. These properties apply whether the material is neutral in colour as in a white surface, or highly coloured, although the amount of light reflected will depend on the reflectance of the material and of the amount of incident light. In the case of specular surfaces it may be more appropriate to compare the intensity of the incident light to that reflected. If the surface is multi coloured then it will be necessary to determine the area-weighted reflectance. For example, the overall reflectance of a brick wall where it will be necessary to take into account the reflectance value of the brick and its area, together with the reflectance value of the mortar together with area of mortar. Area weighted reflectance calculation: Area of brick = Ab Area of mortar = Am Reflectance of brick = Rb Reflectance of mortar = Rm Area weighted reflectance =
( Ab ´ Rb ) + ( Am ´ Rm ) ( Ab + Am )
An approximation of the degree of reflectance of a matt material can be measured using an illuminance metre by placing the photocell on the surface of the material and measuring the incident illuminance in lumens/m2 and comparing the reading to the amount of light reflected in lumens/m2 by aiming the photocell downward at an angle
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Fig. 15.10 Reflectance assessment chart showing an approximate match of a sample indicating an approximate reflectance. (Courtesy of the Society of Light and Lighting, 2001. Lighting Guide 11: Surface Reflectance and Colour, its Specification and Measurement for Designers.)
of approximately 45° to the surface. If a precise value is required then it will usually require the services of a photometric laboratory which has the equipment to carry out the measurement. The appearance of the colour will depend on the visual conditions. That is, the amount and spectral properties of the illumination. It can also depend on the surrounding colours which can cause a distortion in the colour appearance. Hence the designer needs to be aware of these things in making a choice of materials. Also lighting designers will need to know the reflectance values of the main building surfaces of a room and their intended or expected reflectance values to enable them to design the lighting with due consideration of the amount light reflected back on to the main task surfaces e.g. the desk tops in a school classroom. An aid to assessing reflectance values of matt surfaces can be assessed by visual comparison using a reflectance chart as produced by the Society of Light and Lighting containing known reflectance values for a range of colours and intensity (The Society of Light and Lighting, 2001) (see Fig. 15.10).
15.6 Material damage and colour degradation caused by light Light incident on a material can cause permanent colour degradation in some materials. The degree of the problem is caused by the responsiveness of the material itself and the spectral power distribution of the light source. The problem is caused mainly
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by short wave radiation below 400 nm and particularly ultraviolet light. It will also depend on the length and intensity of the light exposure. This is a particular problem for the curators of museums, art galleries and retail outlets where exhibits will be on public display and where the display needs to be seen to the best effect. There is an added problem where the items are exposed to daylight as in a shop window. The problem is creating a balance between the opposing requirements which can be in conflict with one another and in particular where there are paying visitors or shoppers interested in making a purchase who have a reasonable expectation of good viewing conditions (The Society of Light and Lighting, 2015).
15.7 Considering each of the elements Exhibit responsiveness to damage: this is mainly a problem with organic materials and a curator or retail display manager will need to be judge of this but Table 15.4 provides initial guidance: Light source spectral power distribution: as was mentioned above the main problem is ultraviolet radiation, hence this should always be excluded for medium and high responsive materials. This will include daylight where, if possible, the glass should include a UV inhibitor either within the glass itself or with a UV filter applied, though the latter has usually a limited life. Another solution is to exclude daylight outside gallery or shop opening hours by installing blinds or some other obstructing device. For electric light sources it will depend on the UV component in the spectral power distribution. The problem will usually apply to discharge light sources where, if necessary, a filter to exclude UV can be used. Light exposure: this is a combination of the intensity of the light incident on the exhibit in lux and the length of the exposure in hours, presented as a product of the two. The problem is selecting an illuminance appropriate for the viewer to see exhibits sufficiently well in terms of the overall quality of the work including a reasonable appreciation of the colour and detail in the particular work.
Table 15.4 The recommended maximum annual light exposure, in lux hours, for a range of different materials. Conservation category Irresponsive Low responsivity Medium responsivity High responsivity
Material with illuminance examples – e.g.
Annual exposure in lux hours
Stone, ceramics, glass & metals Oil & acrylic paintings Illuminance 200 lx Water colours, textiles & paper. Illuminance 50 lx Animal & plant materials Illuminance 50 lx
Usually no limit
Courtesy of the Society of Light & Lighting.
600,000 150,000 15,000
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Fig. 15.11 Art gallery illumination, using spotlights to illuminate the paintings individually, together with a low wall reflectance to limit the luminance range resulting in good visibility of the paintings.
Viewing conditions: a study carried out at the Bartlett School of Architecture, UCL into the viewing conditions of works of art with a particular interest in the display of paintings. It involved the construction of a small gallery displaying five painting i ncluding oil and water colours covering a number of different subject matter and styles. The gallery was illuminated in different ways including a range of different painting illuminance, and with different lamps to ascertain a solution that served both viewers and curators. A number of observers were asked to make visual assessments of the different presentations using subjective assessment scales. The results indicated that an illuminance on the painting surface of a minimum 200 lx was required to provide acceptable viewing conditions in terms of the detailed discrimination and of the overall quality of the work. The results also suggested that in terms of the overall lighting of the gallery there was a preference for a degree of accent illumination on the paintings i.e. soft edged spot lighting (Loe et al., 1982). This has the effect of focusing the viewer’s attention on the paintings. It also lowers the brightness visual adaptation level and enhancing the brightness of the paintings. The effect will be further enhanced by lowering the surround reflectance – see Fig. 15.11. It will be obvious that the control of illumination for protecting the works involves a number of possible solutions, but the curator needs to take a balanced view of this with the experience of the viewer in mind. There is nothing more disappointing than trying to view an Impressionist style painting of a sunlit composition with an illuminance of 50 lx. It appears drab and not what the artist had intended. To some extent this can be overcome by limiting the exposure time. In addition to colour degradation in some materials, in particular fabrics used as window curtains and drapes, physical damage can be caused by light. To avoid this it is recommended that they be lined. This problem can also apply to displays of costumes and tapestries also of organic materials e.g. animal skins, etc.
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15.8 Illumination, colour and psychological response So far, I have only considered what might loosely be described as the physical relationship of light and colour but there is also the possibility of psychological effects. Some psychologists are beginning to talk about the conscious and unconscious mind to describe the difference between consciously gathering information as in doing a task, as compared to a sensation that is imposed unconsciously. The question that needs to be asked is do psychological effects have a physiological effect that we have not yet discovered? For some time now, researchers and lighting designers in the field of environmental design have begun to consider psychological effects that may induce sensations of, for example, wellbeing, alertness or lethargy. This requires lighting that goes beyond task lighting to a consideration of lighting for appearance. The research so far is fairly limited but there is some evidence, and growing, to suggest that the luminance pattern within a horizontal viewing band of approximately 40° wide has some influence on these psychological sensations (Loe et al., 2000). Some designers, particularly from an architectural back ground, or stage and film lighting designers, with the aim of creating illumination that complements either the architectural form or style or the facility or activity of the environment. Or in the case of stage and film lighting designs to support or enhance the narrative of the script. This is probably a natural part of their purpose but if this is the case perhaps by taking a similar approach in what might be described as ‘everyday’ situations whether it may be a school classroom or a reception area in a commercial organisation or a church for a wedding. If these enhance the built environment and create, perhaps subjectively, a human benefit, or a greater experience relative to the purpose of the environment, they would need to be considered. After all when illumination is provided just for task purposes, it will inevitably affect the appearance of the whole environment. The next question is: if this is the case does the colour scheme have a similar effect? One can imagine that if a muted colour scheme is provided it would have a calming effect on the occupants and vice-versa? Or perhaps by having a vibrant colour scheme it creates a stimulant? One way in which this may be tested is by studying artist’s paintings and labelling them in terms of the subject and comparing these results to the light patterns and the colour designs of the composition. If the results suggest that the results are positive then the next step would be to measure human physiological parameters, perhaps using subjective assessment or brain activity by exploring brain scanning methods, to see if there is a result positive or negative? Some people will respond positively to a well-lit interior or to a colour design or both – but is this just a question of preference dependent on personality or education?
15.9 Conclusions In the preceding discussion of the interaction of the human response to light, illumination and colour, together with systems of measurement and specification I have attempted to explain the science of where we are. This has been fashioned largely
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by where we have come from and the limitations of technology and some research. This has led me to hypothesise what we need to investigate further for the benefit of improved environmental design. A good starting point is to recognise that a visual environment design needs to address two main parameters the activities of the users which are often a primary concern. This might be desk work in an office or children playing in a nursery but there will also be supplementary tasks all of which need to be considered like navigating the space. But any illumination and colour design will create an appearance, either by accident or by intention. This idea is not new; early in the 20th century the American architect Frank Lloyd Wright used the term ‘Form and Function’ as a design guide but I think activity and appearance are more explicit and need to be the bedrock of design. But to achieve this will require a better understanding of the human visual system together with its interaction with the brain. It will also require a system of measurement and specification that will help designers to a more appropriate and better solution. We are on the edge of major developments in lamp technology together with ways of light source control and not to have development in design would be a lost opportunity. But more research will be required from ophthalmologists, neurologists and designers et al to achieve change.
Acknowledgements To write this chapter I have had the benefit of consultations with many experts during my long career in lighting but in particular I would like to thank the input of Peter Boyce, Michael Pointer, Mark Rea, and Peter Tregenza.
References Boyce, P.R., Cuttle, C., 1990. Effect of CCT on the perception of interiors and colour discrimination performance. Light. Res. Technol. 22, 9–36. Chen, H.-S., Chou, C.-J., Lou, H.-W., Lou, M.R., 2016. Museum lighting environment: designing a perception zone map and emotional response models. Light. Res. Technol. 16 (5), 589–607. Crawford, B.H., 1963. Colour rendering tolerances and the colour-rendering properties of light sources. Trans. Illum. Eng. Soc. 28, 50–64. Hopkinson, R.G., Collins, J.B., 1970. The Ergonomics of Lighting. Macdonald Technical and Scientific, London (also further reading (Boyce, 2014)). Kruithof, A.A., 1941. Tubular luminance lamps for general illumination. Philips Tech. Rev. 6, 65–96. Loe, D.L., 2016. vision and illumination: the interaction revisited. Light. Res. Technol. 48, 176–189. Loe, D.L., Rowlands, E., 1996. The art and science of lighting: a strategy for lighting design. Light. Res. Technol. 28, 153–164.
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Loe, D.L., Rowlands, E., Watson, N.F., 1982. Preferred lighting conditions for the display of oil and watercolour paintings. Light. Res. Technol. 14, 173–192. Loe, D.L., Mansfield, K.P., Rowlands, E., 1994. Appearance of lit environment and its relevance in lighting design: experimental study. Light. Res. Technol. 26, 119–133. Loe, D.L., Mansfield, K.P., Rowlands, E., 2000. A step in quantifying the appearance of a lit scene. Light. Res. Technol. 32, 213–222. Rea, M.S., 2015. The lumen seen in a new light: making distinctions between light, lighting and neuroscience. Light. Res. Technol. 47, 259–280. The Society of Light and Lighting, 2001. Lighting Guide 11: Surface Reflectance and Colour, its Specification and Measurement for Designers. National Physical Laboratory and Society of Light and Lighting. The Society of Light and Lighting, 2015. Lighting Guide 8: Lighting for Museums and Art Galleries. Society of Light and Lighting.
Further reading Boyce, P.R., 2014. Human Factors in Lighting, third ed. CRC Press, Boca Raton, FL. Hunt, R.W.G., Pointer, M.R., 2011. The Measurement of Colour, fourth ed. John Wiley & Sons Inc, New York. Illuminating Engineering Society of North America, 2011. The Lighting Handbook, 10th ed. IESNA, New York. Murdoch, J.B., 2003. Illuminating Engineering, second ed. Visions Communication, Larose, LA. The Society of Light and Lighting, 2009. The SLL Lighting Handbook. CIBSE, London. The Society of Light and Lighting, 2012. The SLL Code for Lighting. CIBSE, London. Tregenza, P., Loe, D.L., 2014. The Design of Lighting, second ed. Routledge, London.
The Society of Light and Lighting produce a number of lighting guides, covering a range of topics.