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Perceived performance of daylighting systems: lighting efficacy and agreeableness
Pergamon
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Solar Energy Vol. 73, No. 2, pp. 83–94, 2002 2002 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X / 02 / $ - see front matter
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PERCEIVED PERFORMANCE OF DAYLIGHTING SYSTEMS: LIGHTING EFFICACY AND AGREEABLENESS M. FONTOYNONT † ´ ´ ˆ Departement Genie Civil et Batiment, Ecole Nationale des Travaux Publics de l’Etat, Rue M. Audin, F-69518 Vaulx-en-Velin Cedex, France Received 17 August 2001; revised version accepted 28 March 2002
Abstract—Can daylighting systems be assessed through objective procedures? On one hand, they can be considered as lighting techniques and deserve to be characterised through the same type of parameters: illuminances, uniformity, luminances, colour temperature, colour rendering indices, etc. On the other hand, two major aspects differentiate them from artificial lighting installations. Firstly, the daylight source is variable, requiring a long term approach and an assessment of the duration of the phenomena per day, month or year. Secondly, the brightness of the window cannot be totally disconnected from the content of the view and its agreeableness. This suggests that psychological well-being may be in some cases as important as visual comfort issues. For the window component industry, it appears that a serious concern about the two aspects of windows — i.e. lighting efficacy and agreeableness — should be carefully approached for each design. 2002 Elsevier Science Ltd. All rights reserved.
on the control of comfort conditions for occupants. Most recommendations for artificial lighting (A.F.E., 1993; CIE, 1988; CIBSE, 1994; CEN, 1998) were originally issued in relation to achieving a satisfactory level of visual performance in relation to the activity. This means that historically the first concern was visual acuity (vision of details for manufacturing, reading and writing) and also security reasons. Therefore the efficacy of a lighting installation could be considered through its ability to fulfil the visual requirements in relation to a given activity. This approach was mainly physiological. In the 1980s, most progress in lighting fixtures was related to the search for comfort: the concern became the reduction of glare and this was particularly crucial for work places with computer screens. The interest in research in the field of visual comfort increased. International collaboration led to an agreement on the rating of glare through a Unified Glare Ratio (CIE, 1994b) although there are still other proposals to rate visual comfort such as VCP (IESNA, 1993) and the J index (Meyer et al., 1993). The UGR attempts to provide a rating for the luminous scene in the visual field of an observer with respect to glare: how glary is each source by comparison to the luminance of the background, with respect to its luminance, its size and its location in the field of vision. The development of low luminance luminaries which started at a large scale in the 1980s demonstrates that lighting
1. INTRODUCTION
Daylight is the primary source of light, and most of the developments in artificial lighting occurred in the 20th century. Fluorescent lighting brought new possibilities of reaching high illuminances in buildings (a few hundred lux) in the 1950s. The interest for daylighting came back in the late 1970s after the energy crisis. It was re-discovered as an efficient way to reduce energy consumption in buildings used mainly during the day: offices, educational buildings, and factories. Thanks to progress in glazing technologies such as double glazing and later low-e coatings, the energy balance of daylighting techniques was improved: glazing areas were no longer solely associated with heat loss concerns. Logically, it took up its rightful place beside solar technologies within the general effort to save energy. In areas of buildings where daylight is available (i.e. where illuminances are roughly 2 to 5% of the simultaneous outdoor illuminance for overcast conditions), daylight brings enough light to meet lighting requirements of 50 to 70% of the occupancy period in the temperate zone of the earth, and even more around the equator (CIE, 1970). Extensive work has been carried out in the field of performance assessment of artificial lighting installations, both on bringing light into space and †
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installations were still aiming at providing the best ‘support’ for the task, and not necessarily in generating the most pleasant environment. On the occasion of some field monitoring activities from our group, we found more and more cases of occupants rejecting low luminance environments in judging them too ‘dull’. The halogen torchieres with their high electrical consumption (300 to 500 W) demonstrated that there was a need to bring large amounts of light without glare. It also demonstrated the demand for dimming, a new feature which had to be supplied in some lighting installations. In the 1990s, the supply of lamps for indoor lighting became wider: low voltage halogen, high beam lamps, compact fluorescent lamps, later T5 fluorescent lamps: the trend for miniaturisation was launched. Hence new possibilities to enhance contrast. Instead of distributing light evenly over the space, the trend became to focus light on important surfaces: the work plane, decoration on walls, access to rooms, etc. Although this trend is still marginal in offices and educational buildings, it can easily be noted in circulation areas, hallways, sanitary rooms, reception desks and some workplaces. This means that concerns for psychological well-being is now being added to the basic physiological concerns, supposed to be matched with standard solutions. The consequences of this trend should not be underestimated. It is in fact a real revolution in the approach to lighting: lighting becomes a visual construction of the built environment. It is highly related to finishing and design options. The occupant gives a final and global rating in which the ‘amount’ of light available is only one of the parameters which defines visual well-being. For scientists involved in research and development activities, it is becoming indispensable to define descriptors of lighting quality, beyond only the field of performance. In this spirit, the CIE Division on Interior Lighting launched a new Technical Committee aimed at proposing Lighting Quality Descriptors (Chairwoman J. Veitch, 1999).
(A) The natural light source varies in intensity and colour, continuously, suggesting an approach with average, maximum, minimum values as well as probabilities to exceed given thresholds. (B) It is almost impossible to forget the role played by the attractiveness of the view toward the outside, which tends to broaden the margin of luminous acceptance of occupants in comparison with artificial lighting conditions.
2.1. The high variability of daylight Daylight is a highly variable light source: illuminances on building facades can reach 100,000 lx when the sun is located in front of them. Under overcast conditions, illuminances in the middle of the day can be as low as a few thousand lux. Over a given clear day (in Lyon, France, June 15) we have measured the variations of vertical global illuminances on vertical planes facing East, West and South. They show variations between 4000 and 70,000 lx between 09:00 and 18:00 legal time, a ratio of 1 to 17 (Fig. 1). To assess the performance of daylighting systems, a good understanding of the daylight source is required, more particularly, the luminance and
2. DAYLIGHT, A LIGHTING SOURCE, VARIABLE IN INTENSITY AND COLOUR
The optical performance of daylighting and artificial lighting systems can be described with the same parameters: illuminance distribution, luminance distribution and contrasts, glare indices, colour rendering and colour temperature. The major differences can be condensed into two themes.
Fig. 1. Frequency to exceed 20 klux of diffuse horizontal illuminance between 08:00 and 18:00 h. Source www.satellight.com.
Perceived performance of daylighting systems: lighting efficacy and agreeableness
colour distribution on the sky vault as well as the occurrence of these values. This means that the daylight source needs to be characterised in space (luminance distribution on the sky vault, location on the earth) and time (hour of day, month, etc.). This suggests specific assessment protocols (Fontoynont et al., 1991) involving optical transfer functions from each patch of the sky vault to each patch inside a building. Models for sky luminance distribution have been proposed and some of them standardised. The International Lighting Commission (CIE — Commission Internationale de l’Eclairage) has proposed standards for overcast sky and clear skies (CIE, 1994a) and is working on intermediate skies (Matsuura, 1999). All-weather sky luminance models have been proposed (Perez et al., 1990; Perraudeau, 1990). These models have been partly developed for prediction of energetic and luminous performances of daylighting systems (Kittler, 1986). Regarding the colour of daylight, the CIE has proposed standard illuminants such as the D65 for overcast sky conditions and others for sunlight (CIE, 1986, 1989). More recently, new models have been proposed to relate the luminance models to models supplying the distribution of colour temperatures and spectral distribution functions on the sky vault (Chain et al., 1998, 1999a,b). The statistical analysis requires data obtained through continuous measurements of parameters and not integrated values. A few ground stations record luminous data worldwide: 50 stations measure ground illuminances and irradiances (Dumortier and Koga, 1999). Sky luminance distributions are measured in less than 20 stations worldwide since they require costly sky scanners delivering large data sets. Spectral distribution of light for various locations in the sky is recorded on a continuous basis in Vaulx-enVelin, a suburb of Lyon, France, by C. Chain (Chain et al., 1999a). Due to the lack of available data in sky luminance distribution, the state-of-the art in this field consists of determining sky luminance distribution on the sky vault from data which are more available such as horizontal irradiances or illuminances when they are available (Perez et al., 1990). Nowadays, such models are being used to determine the luminous patterns of the sky vault as seen from ground level on the base of satellite observations. The new Satel-light server (Fontoynont et al., 1997; Dumortier et al., 1999) provides useful luminous parameters such as diffuse and global illuminances, and information on the sky, every 30 min, all over Western and Central Europe (Fig. 2). These data are indispens-
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Fig. 2. Variations of global illuminances on a building facade in Lyon for a clear day in June for East-facing, South-facing, and West-facing fac¸ades.
able to approach the performance of daylighting systems with respect to time and climate.
2.2. The various colours of daylight This second noteworthy aspect is related to the fact that daylight is not a single spectrum light source, as opposed to a fluorescent lamp for instance. It is a mixture of lights coming from clouds (white, grey), haze (white, warmer hew), blue sky (cool white), light reflected from facades, ground and vegetation. The mixture is often subtle but with training one may observe these differences clearly. Since it is well known that the colour temperature of artificial light modifies the value of preferred illuminance level on work (Kruithoff, 1941), it is clear that all models on visual comfort have to take the colour characteristics of incoming daylight into account. The colour temperature of daylight varies typically from 3000 K to 30,000 K. Before entering a window, it is often mixed with the greenish light reflected by the vegetation and more yellow-brown colours obtained by reflection on the built environment (Figs. 3 and 4). After transmission through a window and reflection on the various indoor surfaces, the contrasts in colour temperature of incident daylight are reduced, but still cover a significant range of values between 3000 and 8000 K (Chain et al., 1999b) (Fig. 5). This suggests testing user preferences with daylight of various colour temperatures. A first set of results suggests that the role of the outdoor colour temperature is higher than that of the air
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Fig. 3. Spectral distribution of various daylight sources reaching a vertical window pane.
temperature or season on user assessment of their luminous environment (Laurentin et al., 1998) 3. THE NON-UNIFORMITY OF DAYLIGHTING IN ROOMS
Is a ‘lux’ supplied by a daylighting system comparable with that supplied by an artificial lighting installation? Most systems controlling the dimming or the extinction of lamps as a function of daylight assume more or less that the goal is to
Fig. 4. Characterisation of sky luminance and spectral distribution function with a spectrophotometer.
maintain the same value of illuminance on a given plane, whatever the source (natural or artificial). At first glance, this approach is sound and logical. But if we carefully observe the two ways light is brought into the room, it is clear that what can be observed in the field of view is quite different (Fig. 6). Daylight comes from the side, and is a mixture of spectra. Artificial light comes from lamps, usually located above the occupant, with fixed spectra. Recommendations by the CIE (CIE, 1988) and national institutions (A.F.E., 1993; CIBSE, 1994) tend to promote uniformity on the work plane as one important parameter to guarantee the space will appear well lit, with no visible local luminous deficit. Although this seems logical in classrooms and multi-occupant rooms, it is certainly not indispensable in offices with a single occupant. Moreover, the pictures in Fig. 6 seem to demonstrate that the non-uniformity which is often associated with daylight penetration may be one of the reasons why daylit space appears more attractive. Here is one of the major difficulties, i.e. assessing the performance of a daylighting system in comparison with an artificial lighting installation: the uniformity of daylight distribution cannot be a real criterion, or the criterion must not be as tight as in artificial lighting. For instance, it is clear that in a side-lit room, illuminances vary by more than 10 from near the window to near the back wall. These conditions differ greatly from those typically obtained in artificial lighting where illuminances on a task are not supposed to drop by more than 20% below the average illuminance. It is clear also that in bringing daylight into a room through the roof or through secondary windows, the space appears rapidly brighter. An experience in France in college ‘La Vanoise’ (Fontoynont, 1999) demonstrates that classrooms benefiting from daylight from two sides appear very bright although the minimum value of the daylight factor in the centre of the room is as low as 1%. It has been noticed that in such rooms, decisions to switch on the lights start at around 100 lux. This means that the balanced distribution of daylight may lead to a reduction of the minimum illuminance acceptance threshold. This is relevant to former studies suggesting that the (spectral) quality of light may compensate for the lower illuminance obtained at the source, a result obtained in test rooms which is consistent with user preferences expressed in field studies (Levermore and Leventis, 1997). This suggests that although the issue of uni-
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Fig. 5. Colour of incident daylight measured in the same room for two sky conditions.
formity should not be skipped in daylighting, it should not be approached as strictly as for artificial lighting. ‘Reasonable’ ratios are necessary, in accordance with the sensitivity of the eye. Velds (2000) suggests a minimum uniformity ratio on a desk of 0.7. What can be said about the room itself? Other uniformity thresholds would be useful to describe the general quality of the daylighting of the room. These ratios will however be much lower than the ones required in artificial lighting today. 4. COMPARISON BETWEEN ARTIFICIAL AND DAY-LIGHTING PERFORMANCE CRITERIA
The quality of daylighting system’s approach, as it is perceived by occupants, needs to capitalise on the information available in the field of
artificial lighting. Below we propose a comparison of existing descriptors of performance in artificial and natural lighting. Table 1 clearly shows the major difficulty in characterising the performance of daylighting systems with respect to lighting performance and glare control. Beyond the fact that the daylight source is highly variable, suggesting a statistical approach to uniformity, glare, etc., a daylighting system is not a fixture. It is a building component relating indoor to outdoor environments. It is judged with criteria which are highly psychological and subjective (Collins, 1975, 1976). • How aesthetically is the daylight component (window, blind, glass material, shutter, etc.) associated with daylight penetration? • How pleasant (and indispensable) is the quality of the view?
Fig. 6. Light patterns of daylight and artificial light in a room.
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Table 1. Comparison of performance indices in artificial lighting and daylighting Artificial lighting
Daylighting
Illuminance level on plane Average illuminance on horizontal plane Uniformity on reference plane
Daylight factor on plane Average daylight factor on horizontal plane Uniformity on reference plane Minimum value of daylight factor at half the depth of the room (for instance) Absolute minimum value of daylight factor on reference plane Level of attenuation of shading system (luminous transmittance, etc.)
Colour temperature of light source (lamp)
Colouring effect of glazing or paint since colour temperature of daylight varies, typically from 3000 K to 25,000 K. Reduction of colour rendering index due to colouring of glazing; since daylight has a CRI of about 100 Frequency to exceed a given illuminance or indoor luminance over the operating period Impact on energy for lighting Impact on energy for HVAC Contrast of luminances around task (near and further away) Maximum acceptable luminance of scene seen through window, with respect to indoor luminances or task luminances (Velds, 2000) Glare rating DGI
Colour rendering indices (CRI) of light sources (between 80 and 100 for most indoor lighting sources)
Contrast of luminances around task (near and further away), wall luminances (Berrutto et al., 1997) Maximum luminance of luminaries for given angle of ¨ observation (charts by Bodmann and Sollner, 1965) Glare rating UGR, J Psychological aspects: Comfort and performance versus amenity (Boyce and Eklund, 1995; Boyce, 1998), search for new quality descriptors Luminary aesthetics
Long term perception Existing situation versus expected situation User friendliness of lighting controls Social status brought by lighting fixtures
• How agreeable are the light patterns indoor (light distribution, rays of light, reflections, colour, etc.)? It appears necessary to bring examples of daylit rooms and try to discuss the opportunity of daylighting quality descriptors, both objective and subjective. 5. PERCEIVED PERFORMANCE OF DAYLIGHTING SYSTEMS
The performance can be judged instantaneously and in the long term. Rating of visual comfort requires fixed luminous conditions. In her PhD dissertation, Enrech-Xena (1999) identifies the perceived quality in relation to the difference between the actual environment and the expected
Dependence of preferences in illuminances and luminances as a function of the quality of view Window aesthetics, quality of view, presence of nature, view to the sky, etc. Temporary effects such as sunlight penetration, sunlight reflections on external ground, reflection on snow, sunsets, etc. Long term perception Existing situation versus expected situation User friendliness of shading controls Social status brought by number of windows and quality of views
environment. In studying occupants in windowless environments, she could have a better understanding of what daylighting means for a work place. Through a test room equipped with an artificial window, allowing the simulation of various luminous climates, she demonstrated that the sensation for daylight could be obtained at low illuminance levels (Enrech-Xena et al., 1997). Through various field surveys, she recorded the desire of occupants for vision toward nature, toward movement outside (animals, pedestrians, etc.), toward climatic information. She developed techniques such as open interviews and visual reactivation, which appeared to be more efficient than written questionnaires to explore the way occupants daily perceive their luminous environ-
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ment, and how they imagine their ‘ideal’ lighting environment. Such studies could be used for clients before refurbishment of buildings. It can identify the most important changes to carry out in the lighting installation as a function of the priorities expressed by users. The information which is delivered can be considered as a complement of monitoring techniques conducted by specialists, using a calibrated CCD concern for luminance measurements (Berrutto and Fontoynont, 1995) the tool-kit (Boyce and Cuttle, 1994), and the lighting monitoring procedures (Velds and Christoffersen, 1997). Study of user reaction (Veitch et al., 1993; Veitch and Newsham, 1998a,b), user assessment (Love and Nemeskeri, 1995), occupant preferences (Loe et al., 1994) and Post Occupancy Evaluations ¨ (Hygge and Lofberg, 1997) is of course the best way to explore quality issues beyond the fulfilment of standard criteria. But listening to people’s reaction, however interesting it may be, is a task which leads to providing information on quality for decision-makers in the field of industry, building construction and building owners. A link to ‘lighting quality descriptors’ is expected at the end of the international programme of a new Technical Committee of CIE (Veitch, 1999). 6. DISCUSSION OF DAYLIGHTING PERFORMANCE ON VARIOUS EXAMPLES
What is the daylighting performance of a stained glass window (Fig. 7)? Most certainly its overall transmittance affects the amount of light penetrating a room. Typically below 40%, and often below 20% this suggests that it is not an efficient daylight transmitter. However, the benefits need not be looked for elsewhere: is the colouring of daylight suitable for the space? Does it increase its amenity? Are the colours appropriate? Do the occupants like the theme? Is it obstructing an interesting view? • Objective parameters: luminous transmittance, spectral change in transmitted daylight, daylight factors, luminance of stained glass windows. • Subjective parameters: aesthetics of stained glass windows, suitability to the function of the room, semi-transparency, . . . Wooden shutters used in southern Europe break the view and glare (Fig. 8). However they bring daylight (mainly sunlight) to the interior, reflected from the ground outside. • Objective parameters: light transmission, angle of lamellae, reflectance of lamellae, aperture ratio in the shutter.
Fig. 7. Stained glass window, poor light transmitter but aesthetical component.
• Subjective parameters: aesthetics under sunny and non-sunny conditions, practical aspects related to their manoeuvrability, appearance, design. In the last few years, various techniques of films or panels have been developed, attempting to redirect a fraction of daylight deep into the interior? This is most attractive when there are high obstructions outside reducing the view toward the lowest section of the sky vault which produces daylight able to penetrate deeply in rooms behind windows. Among them, light deviating prisms tend to block a fraction of the view toward the outside and increase daylight penetration (Fig. 9). • Objective parameters: increase in daylight factors in the room, increase or reduction of
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Fig. 8. Sunblocking shutters.
glare for indoor occupants as a function of their location, colouring effects, etc. • Subjective parameters: presence of non-transparent material in the upper part of the window, blocking of the view to the sky, indoor light patterns, etc. There are no limits to the patterns associated with daylight and sunlight penetration in a build-
ing. In his ‘Institut du Monde Arabe’ in Paris, Architect Jean Nouvel has designed a modern version of classical moucharabieh found in the Arabic culture (Fig. 10). Although the dynamics of transmission (1 to 4 roughly) can be considered as limited in comparison with the large fluctuations of daylight outside, the object has had a strong esthetical presence and its ‘performance’
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Fig. 9. Light deviating prism.
cannot be judged only on its ability to protect glare. It can be found sometimes that solutions which generate glare and produce poor daylighting, may be preferable because they are attractive. This is found however when the visual task is not too demanding, or when the task is felt to be so
tedious that what is expected from the daylighting system is some excitement. Daylight can offer much more than illuminances and view (Fig. 11). It can excite our senses and participate in an overall rating of quality where it is not necessarily expected, such as in underground parking places, subway sta-
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Fig. 10. Modern design of moucharabieh (Institut du Monde Arabe, Paris).
tions, control-rooms, sanitary rooms, circulation areas, etc. It is no more the quality of daylight which is judged, but the fact that there is some daylight, arriving in a manner which enhances the quality of the indoor luminous environment. 7. CONCLUSION
Occupants perceive the quality of daylight in a room through a mixed sensation. On the one hand, daylight is expected to fulfil requirements regarding the level of illuminances at task, and without generating glare. On the other hand, it is expected that the results be ‘visually’ agreeable.
Cuttle (1999) believes that it is difficult to address lighting quality without references to the indoor architecture and furniture. There are certainly many compensation effects among occupants: is a daylit office really preferable to a windowless office with luxury furniture? Is daylight still desired if it means overheating during summer? Is it more the quality of the control of daylight penetration which is preferred to the amount of available daylight? We all have our own view on the topic. Surveys and monitoring campaigns (Christoffersen and Velds, 1999) should help us to find the right rank of daylighting in the overall rating of quality in a
Fig. 11. Daylight distribution pattern under a light well.
Perceived performance of daylighting systems: lighting efficacy and agreeableness
space. Specific protocols have to be defined to explore preferences, sometimes in stimulating interest (Enrech-Xena et al., 1998) or to identify parameters for visual comfort (Velds and Van Der Voorden, 1996). This is the topic covered by various teams worldwide. We should soon obtain a better understanding of what makes day lighting quality as it is perceived by building occupants.
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Solar Energy Vol. 73, No. 2, pp. 83–94, 2002 2002 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0038-092X / 02 / $ - see front matter
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PERCEIVED PERFORMANCE OF DAYLIGHTING SYSTEMS: LIGHTING EFFICACY AND AGREEABLENESS M. FONTOYNONT † ´ ´ ˆ Departement Genie Civil et Batiment, Ecole Nationale des Travaux Publics de l’Etat, Rue M. Audin, F-69518 Vaulx-en-Velin Cedex, France Received 17 August 2001; revised version accepted 28 March 2002
Abstract—Can daylighting systems be assessed through objective procedures? On one hand, they can be considered as lighting techniques and deserve to be characterised through the same type of parameters: illuminances, uniformity, luminances, colour temperature, colour rendering indices, etc. On the other hand, two major aspects differentiate them from artificial lighting installations. Firstly, the daylight source is variable, requiring a long term approach and an assessment of the duration of the phenomena per day, month or year. Secondly, the brightness of the window cannot be totally disconnected from the content of the view and its agreeableness. This suggests that psychological well-being may be in some cases as important as visual comfort issues. For the window component industry, it appears that a serious concern about the two aspects of windows — i.e. lighting efficacy and agreeableness — should be carefully approached for each design. 2002 Elsevier Science Ltd. All rights reserved.
on the control of comfort conditions for occupants. Most recommendations for artificial lighting (A.F.E., 1993; CIE, 1988; CIBSE, 1994; CEN, 1998) were originally issued in relation to achieving a satisfactory level of visual performance in relation to the activity. This means that historically the first concern was visual acuity (vision of details for manufacturing, reading and writing) and also security reasons. Therefore the efficacy of a lighting installation could be considered through its ability to fulfil the visual requirements in relation to a given activity. This approach was mainly physiological. In the 1980s, most progress in lighting fixtures was related to the search for comfort: the concern became the reduction of glare and this was particularly crucial for work places with computer screens. The interest in research in the field of visual comfort increased. International collaboration led to an agreement on the rating of glare through a Unified Glare Ratio (CIE, 1994b) although there are still other proposals to rate visual comfort such as VCP (IESNA, 1993) and the J index (Meyer et al., 1993). The UGR attempts to provide a rating for the luminous scene in the visual field of an observer with respect to glare: how glary is each source by comparison to the luminance of the background, with respect to its luminance, its size and its location in the field of vision. The development of low luminance luminaries which started at a large scale in the 1980s demonstrates that lighting
1. INTRODUCTION
Daylight is the primary source of light, and most of the developments in artificial lighting occurred in the 20th century. Fluorescent lighting brought new possibilities of reaching high illuminances in buildings (a few hundred lux) in the 1950s. The interest for daylighting came back in the late 1970s after the energy crisis. It was re-discovered as an efficient way to reduce energy consumption in buildings used mainly during the day: offices, educational buildings, and factories. Thanks to progress in glazing technologies such as double glazing and later low-e coatings, the energy balance of daylighting techniques was improved: glazing areas were no longer solely associated with heat loss concerns. Logically, it took up its rightful place beside solar technologies within the general effort to save energy. In areas of buildings where daylight is available (i.e. where illuminances are roughly 2 to 5% of the simultaneous outdoor illuminance for overcast conditions), daylight brings enough light to meet lighting requirements of 50 to 70% of the occupancy period in the temperate zone of the earth, and even more around the equator (CIE, 1970). Extensive work has been carried out in the field of performance assessment of artificial lighting installations, both on bringing light into space and †
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installations were still aiming at providing the best ‘support’ for the task, and not necessarily in generating the most pleasant environment. On the occasion of some field monitoring activities from our group, we found more and more cases of occupants rejecting low luminance environments in judging them too ‘dull’. The halogen torchieres with their high electrical consumption (300 to 500 W) demonstrated that there was a need to bring large amounts of light without glare. It also demonstrated the demand for dimming, a new feature which had to be supplied in some lighting installations. In the 1990s, the supply of lamps for indoor lighting became wider: low voltage halogen, high beam lamps, compact fluorescent lamps, later T5 fluorescent lamps: the trend for miniaturisation was launched. Hence new possibilities to enhance contrast. Instead of distributing light evenly over the space, the trend became to focus light on important surfaces: the work plane, decoration on walls, access to rooms, etc. Although this trend is still marginal in offices and educational buildings, it can easily be noted in circulation areas, hallways, sanitary rooms, reception desks and some workplaces. This means that concerns for psychological well-being is now being added to the basic physiological concerns, supposed to be matched with standard solutions. The consequences of this trend should not be underestimated. It is in fact a real revolution in the approach to lighting: lighting becomes a visual construction of the built environment. It is highly related to finishing and design options. The occupant gives a final and global rating in which the ‘amount’ of light available is only one of the parameters which defines visual well-being. For scientists involved in research and development activities, it is becoming indispensable to define descriptors of lighting quality, beyond only the field of performance. In this spirit, the CIE Division on Interior Lighting launched a new Technical Committee aimed at proposing Lighting Quality Descriptors (Chairwoman J. Veitch, 1999).
(A) The natural light source varies in intensity and colour, continuously, suggesting an approach with average, maximum, minimum values as well as probabilities to exceed given thresholds. (B) It is almost impossible to forget the role played by the attractiveness of the view toward the outside, which tends to broaden the margin of luminous acceptance of occupants in comparison with artificial lighting conditions.
2.1. The high variability of daylight Daylight is a highly variable light source: illuminances on building facades can reach 100,000 lx when the sun is located in front of them. Under overcast conditions, illuminances in the middle of the day can be as low as a few thousand lux. Over a given clear day (in Lyon, France, June 15) we have measured the variations of vertical global illuminances on vertical planes facing East, West and South. They show variations between 4000 and 70,000 lx between 09:00 and 18:00 legal time, a ratio of 1 to 17 (Fig. 1). To assess the performance of daylighting systems, a good understanding of the daylight source is required, more particularly, the luminance and
2. DAYLIGHT, A LIGHTING SOURCE, VARIABLE IN INTENSITY AND COLOUR
The optical performance of daylighting and artificial lighting systems can be described with the same parameters: illuminance distribution, luminance distribution and contrasts, glare indices, colour rendering and colour temperature. The major differences can be condensed into two themes.
Fig. 1. Frequency to exceed 20 klux of diffuse horizontal illuminance between 08:00 and 18:00 h. Source www.satellight.com.
Perceived performance of daylighting systems: lighting efficacy and agreeableness
colour distribution on the sky vault as well as the occurrence of these values. This means that the daylight source needs to be characterised in space (luminance distribution on the sky vault, location on the earth) and time (hour of day, month, etc.). This suggests specific assessment protocols (Fontoynont et al., 1991) involving optical transfer functions from each patch of the sky vault to each patch inside a building. Models for sky luminance distribution have been proposed and some of them standardised. The International Lighting Commission (CIE — Commission Internationale de l’Eclairage) has proposed standards for overcast sky and clear skies (CIE, 1994a) and is working on intermediate skies (Matsuura, 1999). All-weather sky luminance models have been proposed (Perez et al., 1990; Perraudeau, 1990). These models have been partly developed for prediction of energetic and luminous performances of daylighting systems (Kittler, 1986). Regarding the colour of daylight, the CIE has proposed standard illuminants such as the D65 for overcast sky conditions and others for sunlight (CIE, 1986, 1989). More recently, new models have been proposed to relate the luminance models to models supplying the distribution of colour temperatures and spectral distribution functions on the sky vault (Chain et al., 1998, 1999a,b). The statistical analysis requires data obtained through continuous measurements of parameters and not integrated values. A few ground stations record luminous data worldwide: 50 stations measure ground illuminances and irradiances (Dumortier and Koga, 1999). Sky luminance distributions are measured in less than 20 stations worldwide since they require costly sky scanners delivering large data sets. Spectral distribution of light for various locations in the sky is recorded on a continuous basis in Vaulx-enVelin, a suburb of Lyon, France, by C. Chain (Chain et al., 1999a). Due to the lack of available data in sky luminance distribution, the state-of-the art in this field consists of determining sky luminance distribution on the sky vault from data which are more available such as horizontal irradiances or illuminances when they are available (Perez et al., 1990). Nowadays, such models are being used to determine the luminous patterns of the sky vault as seen from ground level on the base of satellite observations. The new Satel-light server (Fontoynont et al., 1997; Dumortier et al., 1999) provides useful luminous parameters such as diffuse and global illuminances, and information on the sky, every 30 min, all over Western and Central Europe (Fig. 2). These data are indispens-
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Fig. 2. Variations of global illuminances on a building facade in Lyon for a clear day in June for East-facing, South-facing, and West-facing fac¸ades.
able to approach the performance of daylighting systems with respect to time and climate.
2.2. The various colours of daylight This second noteworthy aspect is related to the fact that daylight is not a single spectrum light source, as opposed to a fluorescent lamp for instance. It is a mixture of lights coming from clouds (white, grey), haze (white, warmer hew), blue sky (cool white), light reflected from facades, ground and vegetation. The mixture is often subtle but with training one may observe these differences clearly. Since it is well known that the colour temperature of artificial light modifies the value of preferred illuminance level on work (Kruithoff, 1941), it is clear that all models on visual comfort have to take the colour characteristics of incoming daylight into account. The colour temperature of daylight varies typically from 3000 K to 30,000 K. Before entering a window, it is often mixed with the greenish light reflected by the vegetation and more yellow-brown colours obtained by reflection on the built environment (Figs. 3 and 4). After transmission through a window and reflection on the various indoor surfaces, the contrasts in colour temperature of incident daylight are reduced, but still cover a significant range of values between 3000 and 8000 K (Chain et al., 1999b) (Fig. 5). This suggests testing user preferences with daylight of various colour temperatures. A first set of results suggests that the role of the outdoor colour temperature is higher than that of the air
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Fig. 3. Spectral distribution of various daylight sources reaching a vertical window pane.
temperature or season on user assessment of their luminous environment (Laurentin et al., 1998) 3. THE NON-UNIFORMITY OF DAYLIGHTING IN ROOMS
Is a ‘lux’ supplied by a daylighting system comparable with that supplied by an artificial lighting installation? Most systems controlling the dimming or the extinction of lamps as a function of daylight assume more or less that the goal is to
Fig. 4. Characterisation of sky luminance and spectral distribution function with a spectrophotometer.
maintain the same value of illuminance on a given plane, whatever the source (natural or artificial). At first glance, this approach is sound and logical. But if we carefully observe the two ways light is brought into the room, it is clear that what can be observed in the field of view is quite different (Fig. 6). Daylight comes from the side, and is a mixture of spectra. Artificial light comes from lamps, usually located above the occupant, with fixed spectra. Recommendations by the CIE (CIE, 1988) and national institutions (A.F.E., 1993; CIBSE, 1994) tend to promote uniformity on the work plane as one important parameter to guarantee the space will appear well lit, with no visible local luminous deficit. Although this seems logical in classrooms and multi-occupant rooms, it is certainly not indispensable in offices with a single occupant. Moreover, the pictures in Fig. 6 seem to demonstrate that the non-uniformity which is often associated with daylight penetration may be one of the reasons why daylit space appears more attractive. Here is one of the major difficulties, i.e. assessing the performance of a daylighting system in comparison with an artificial lighting installation: the uniformity of daylight distribution cannot be a real criterion, or the criterion must not be as tight as in artificial lighting. For instance, it is clear that in a side-lit room, illuminances vary by more than 10 from near the window to near the back wall. These conditions differ greatly from those typically obtained in artificial lighting where illuminances on a task are not supposed to drop by more than 20% below the average illuminance. It is clear also that in bringing daylight into a room through the roof or through secondary windows, the space appears rapidly brighter. An experience in France in college ‘La Vanoise’ (Fontoynont, 1999) demonstrates that classrooms benefiting from daylight from two sides appear very bright although the minimum value of the daylight factor in the centre of the room is as low as 1%. It has been noticed that in such rooms, decisions to switch on the lights start at around 100 lux. This means that the balanced distribution of daylight may lead to a reduction of the minimum illuminance acceptance threshold. This is relevant to former studies suggesting that the (spectral) quality of light may compensate for the lower illuminance obtained at the source, a result obtained in test rooms which is consistent with user preferences expressed in field studies (Levermore and Leventis, 1997). This suggests that although the issue of uni-
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Fig. 5. Colour of incident daylight measured in the same room for two sky conditions.
formity should not be skipped in daylighting, it should not be approached as strictly as for artificial lighting. ‘Reasonable’ ratios are necessary, in accordance with the sensitivity of the eye. Velds (2000) suggests a minimum uniformity ratio on a desk of 0.7. What can be said about the room itself? Other uniformity thresholds would be useful to describe the general quality of the daylighting of the room. These ratios will however be much lower than the ones required in artificial lighting today. 4. COMPARISON BETWEEN ARTIFICIAL AND DAY-LIGHTING PERFORMANCE CRITERIA
The quality of daylighting system’s approach, as it is perceived by occupants, needs to capitalise on the information available in the field of
artificial lighting. Below we propose a comparison of existing descriptors of performance in artificial and natural lighting. Table 1 clearly shows the major difficulty in characterising the performance of daylighting systems with respect to lighting performance and glare control. Beyond the fact that the daylight source is highly variable, suggesting a statistical approach to uniformity, glare, etc., a daylighting system is not a fixture. It is a building component relating indoor to outdoor environments. It is judged with criteria which are highly psychological and subjective (Collins, 1975, 1976). • How aesthetically is the daylight component (window, blind, glass material, shutter, etc.) associated with daylight penetration? • How pleasant (and indispensable) is the quality of the view?
Fig. 6. Light patterns of daylight and artificial light in a room.
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Table 1. Comparison of performance indices in artificial lighting and daylighting Artificial lighting
Daylighting
Illuminance level on plane Average illuminance on horizontal plane Uniformity on reference plane
Daylight factor on plane Average daylight factor on horizontal plane Uniformity on reference plane Minimum value of daylight factor at half the depth of the room (for instance) Absolute minimum value of daylight factor on reference plane Level of attenuation of shading system (luminous transmittance, etc.)
Colour temperature of light source (lamp)
Colouring effect of glazing or paint since colour temperature of daylight varies, typically from 3000 K to 25,000 K. Reduction of colour rendering index due to colouring of glazing; since daylight has a CRI of about 100 Frequency to exceed a given illuminance or indoor luminance over the operating period Impact on energy for lighting Impact on energy for HVAC Contrast of luminances around task (near and further away) Maximum acceptable luminance of scene seen through window, with respect to indoor luminances or task luminances (Velds, 2000) Glare rating DGI
Colour rendering indices (CRI) of light sources (between 80 and 100 for most indoor lighting sources)
Contrast of luminances around task (near and further away), wall luminances (Berrutto et al., 1997) Maximum luminance of luminaries for given angle of ¨ observation (charts by Bodmann and Sollner, 1965) Glare rating UGR, J Psychological aspects: Comfort and performance versus amenity (Boyce and Eklund, 1995; Boyce, 1998), search for new quality descriptors Luminary aesthetics
Long term perception Existing situation versus expected situation User friendliness of lighting controls Social status brought by lighting fixtures
• How agreeable are the light patterns indoor (light distribution, rays of light, reflections, colour, etc.)? It appears necessary to bring examples of daylit rooms and try to discuss the opportunity of daylighting quality descriptors, both objective and subjective. 5. PERCEIVED PERFORMANCE OF DAYLIGHTING SYSTEMS
The performance can be judged instantaneously and in the long term. Rating of visual comfort requires fixed luminous conditions. In her PhD dissertation, Enrech-Xena (1999) identifies the perceived quality in relation to the difference between the actual environment and the expected
Dependence of preferences in illuminances and luminances as a function of the quality of view Window aesthetics, quality of view, presence of nature, view to the sky, etc. Temporary effects such as sunlight penetration, sunlight reflections on external ground, reflection on snow, sunsets, etc. Long term perception Existing situation versus expected situation User friendliness of shading controls Social status brought by number of windows and quality of views
environment. In studying occupants in windowless environments, she could have a better understanding of what daylighting means for a work place. Through a test room equipped with an artificial window, allowing the simulation of various luminous climates, she demonstrated that the sensation for daylight could be obtained at low illuminance levels (Enrech-Xena et al., 1997). Through various field surveys, she recorded the desire of occupants for vision toward nature, toward movement outside (animals, pedestrians, etc.), toward climatic information. She developed techniques such as open interviews and visual reactivation, which appeared to be more efficient than written questionnaires to explore the way occupants daily perceive their luminous environ-
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ment, and how they imagine their ‘ideal’ lighting environment. Such studies could be used for clients before refurbishment of buildings. It can identify the most important changes to carry out in the lighting installation as a function of the priorities expressed by users. The information which is delivered can be considered as a complement of monitoring techniques conducted by specialists, using a calibrated CCD concern for luminance measurements (Berrutto and Fontoynont, 1995) the tool-kit (Boyce and Cuttle, 1994), and the lighting monitoring procedures (Velds and Christoffersen, 1997). Study of user reaction (Veitch et al., 1993; Veitch and Newsham, 1998a,b), user assessment (Love and Nemeskeri, 1995), occupant preferences (Loe et al., 1994) and Post Occupancy Evaluations ¨ (Hygge and Lofberg, 1997) is of course the best way to explore quality issues beyond the fulfilment of standard criteria. But listening to people’s reaction, however interesting it may be, is a task which leads to providing information on quality for decision-makers in the field of industry, building construction and building owners. A link to ‘lighting quality descriptors’ is expected at the end of the international programme of a new Technical Committee of CIE (Veitch, 1999). 6. DISCUSSION OF DAYLIGHTING PERFORMANCE ON VARIOUS EXAMPLES
What is the daylighting performance of a stained glass window (Fig. 7)? Most certainly its overall transmittance affects the amount of light penetrating a room. Typically below 40%, and often below 20% this suggests that it is not an efficient daylight transmitter. However, the benefits need not be looked for elsewhere: is the colouring of daylight suitable for the space? Does it increase its amenity? Are the colours appropriate? Do the occupants like the theme? Is it obstructing an interesting view? • Objective parameters: luminous transmittance, spectral change in transmitted daylight, daylight factors, luminance of stained glass windows. • Subjective parameters: aesthetics of stained glass windows, suitability to the function of the room, semi-transparency, . . . Wooden shutters used in southern Europe break the view and glare (Fig. 8). However they bring daylight (mainly sunlight) to the interior, reflected from the ground outside. • Objective parameters: light transmission, angle of lamellae, reflectance of lamellae, aperture ratio in the shutter.
Fig. 7. Stained glass window, poor light transmitter but aesthetical component.
• Subjective parameters: aesthetics under sunny and non-sunny conditions, practical aspects related to their manoeuvrability, appearance, design. In the last few years, various techniques of films or panels have been developed, attempting to redirect a fraction of daylight deep into the interior? This is most attractive when there are high obstructions outside reducing the view toward the lowest section of the sky vault which produces daylight able to penetrate deeply in rooms behind windows. Among them, light deviating prisms tend to block a fraction of the view toward the outside and increase daylight penetration (Fig. 9). • Objective parameters: increase in daylight factors in the room, increase or reduction of
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Fig. 8. Sunblocking shutters.
glare for indoor occupants as a function of their location, colouring effects, etc. • Subjective parameters: presence of non-transparent material in the upper part of the window, blocking of the view to the sky, indoor light patterns, etc. There are no limits to the patterns associated with daylight and sunlight penetration in a build-
ing. In his ‘Institut du Monde Arabe’ in Paris, Architect Jean Nouvel has designed a modern version of classical moucharabieh found in the Arabic culture (Fig. 10). Although the dynamics of transmission (1 to 4 roughly) can be considered as limited in comparison with the large fluctuations of daylight outside, the object has had a strong esthetical presence and its ‘performance’
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Fig. 9. Light deviating prism.
cannot be judged only on its ability to protect glare. It can be found sometimes that solutions which generate glare and produce poor daylighting, may be preferable because they are attractive. This is found however when the visual task is not too demanding, or when the task is felt to be so
tedious that what is expected from the daylighting system is some excitement. Daylight can offer much more than illuminances and view (Fig. 11). It can excite our senses and participate in an overall rating of quality where it is not necessarily expected, such as in underground parking places, subway sta-
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Fig. 10. Modern design of moucharabieh (Institut du Monde Arabe, Paris).
tions, control-rooms, sanitary rooms, circulation areas, etc. It is no more the quality of daylight which is judged, but the fact that there is some daylight, arriving in a manner which enhances the quality of the indoor luminous environment. 7. CONCLUSION
Occupants perceive the quality of daylight in a room through a mixed sensation. On the one hand, daylight is expected to fulfil requirements regarding the level of illuminances at task, and without generating glare. On the other hand, it is expected that the results be ‘visually’ agreeable.
Cuttle (1999) believes that it is difficult to address lighting quality without references to the indoor architecture and furniture. There are certainly many compensation effects among occupants: is a daylit office really preferable to a windowless office with luxury furniture? Is daylight still desired if it means overheating during summer? Is it more the quality of the control of daylight penetration which is preferred to the amount of available daylight? We all have our own view on the topic. Surveys and monitoring campaigns (Christoffersen and Velds, 1999) should help us to find the right rank of daylighting in the overall rating of quality in a
Fig. 11. Daylight distribution pattern under a light well.
Perceived performance of daylighting systems: lighting efficacy and agreeableness
space. Specific protocols have to be defined to explore preferences, sometimes in stimulating interest (Enrech-Xena et al., 1998) or to identify parameters for visual comfort (Velds and Van Der Voorden, 1996). This is the topic covered by various teams worldwide. We should soon obtain a better understanding of what makes day lighting quality as it is perceived by building occupants.
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