Lighting and visual ergonomics for the display screen environment

Symbiosis of Human and Artifact Y. Anzai, K. Ogawaand H. Mori (Editors) 1995 Elsevier Science B.V.

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L i g h t i n g a n d V i s u a l E r g o n o m i c s for t h e D i s p l a y S c r e e n Environment M J Perry and P J Littlefair Building Research Establishment (BRE), Garston, Watford, England, WD2 7JR 1. I N T R O D U C ~ O N The rise in the use of computers and their associated display screen equipment (DSE) has been rapid in the developed industrial nations, particularly in office environments. The widespread introduction of DSE into the office environment has been accompanied by problems with the ergonomics and the environment of the DSE work place. It is important to address these problems to create a work place environment for DSE t h a t is comfortable for users and non-users. An important issue raised by the introduction of DSE workstations was how best to light the environment [1]. Electric lighting needs to be arranged to minimize screen reflections and glare. The use of daylight for the DSE environment can address both the ergonomic requirements of the users, and the need for energy efficient use of electric lighting, but needs careful attention at the design stage. The BRE is carrying out a programme of work investigating visual ergonomics of electric lighting and daylighting for DSE. This is described in this paper, with guidance for daylighting of DSE [2,3]. 2. E I ~ E ~ C

LIGHTING I S S U F ~

2.1 LuminMres a n d the visual envirmmmnt One of the major differences between paper based and screen based tasks is the presence of a glass screen on the DSE which acts as a partially reflecting filter between the user and the task on the phosphor screen. Reflections of light fittings on the screen are a potential problem for users, and designers now specify lower brightness luminaires [1] to overcome the problems of reflections. The shift in use from wrap around type luminaires, with light output distributed sideways, upwards and downwards, to low brightness luminaires with very directional downward light output has produced a general change in the luminous environment of office spaces. When using low brightness

624 luminaires, vertical surfaces receive much less illumination, and modelling effects on people and objects in the space can be harsher, particularly where the ceiling and walls have low surface reflectance values. People can find this subjectively unacceptable. Low brightness luminaires may also adversely affect energy consumption. In principle, for a given level and uniformity of illuminance across the horizontal working plane a larger installed load may be needed to achieve the required iUuminance and uniformity if low brightness luminaires are used. So, while DSE installations using low brightness luminaires m a y conform to the relevant recommendations, and the lighting is suitable for screen based tasks, the lighting may be unsuitable for other users carrying out non-screen tasks in the space eg discussions with colleagues. (Screen users are very likely to be non-users at some time during the working day.) The distribution of the light m a y also result in adverse subjective responses about the appearance of the space. These potential negative effects on people in the space may be, in principle, aggravated by the requirement for a higher installed load when using low brightness luminaires. These problems can be ameliorated by including supplementary lighting to increase room surface brightnesses. Individual lighting control, possibly using dimming, can help with the transitions between screen and non-screen tasks. 2.2 DSE screens One of the important findings of a study investigating user responses to DSE was that, for the study sample, few users complained of reflections in their screens. Most of those who did complain of reflections were mainly concerned with the images of windows or of window coverings. Complaints of reflected lighting equipment generally relate to extremes, such as images of high luminance spots above uplighters next to walls. Those using negative contrast screens, where the background is lighter t h a n the characters, suffer far fewer screen reflection problems t h a n those with dark background screens. The common impression t h a t 'white' screens are rapidly overtaking 'dark' screens in commercial offices is exaggerated. Certainly there are increasing numbers of newer 'white' screens in use but there are also m a n y older colour screens of low resolution running old software. There are also m a n y screens t h a t are running software t h a t default to white on blue, or similar compromises. Certainly for the sample use in the study, the colour screens out numbered the mono dark background screens. It is likely to be a n u m b e r of years before the light background screens outnumber dark background screens. In the study there were no adverse subjective responses to jitter or flicker on the screens. This was true for both light background screens and dark background screens. Possibly if more large screen users, or more workers from

625 darker working environments, had been included in the study then some comparative feel for the effects of different screen set ups might have been possible. 2.3 Design issues In the sample of buildings included in this study at least one example of each of the main types of lighting equipment available to light DSE areas was found. There were no distinct user preferences t h a t emerged from the interview data attributable solely to the lighting. For instance although the combined up/downlights and concealed wall washing systems were favourably commented on, this may have been related to their being installed in offices which had a high staff morale and company loyalty and where the office space itself was of an interesting nature. Because of confounding factors it was not possible to determine whether lighting designed for DSE areas using the current guidance would result in more energy usage. On theoretical grounds it would appear so, but this could not be substantiated. An important finding of the study was the absence of integrated design procedures for the interior luminous environment, particularly for DSE. Thus the lighting design for a space is, in the great majority of cases, developed in isolation from consideration of surface colours and reflectance values, and what furniture and other objects may be placed within the space. Changes in the interior environment from those assumed at the design stage will have a profound effect on the lighting levels, energy consumption, and appearance of the space. When planning the building layout, it is important to consider all spaces where DSE is likely to be used, not just offices. For example, factory and workshop spaces often contain DSE, and the reception areas of buildings usually have DSE as well as often having large areas of glazing. If client expectations about the lighting design for a space are to be fulfilled by the final installation it is essential t h a t a well written specification is developed in consultation with the client. Any changes to the environment of the space should be fed back into the design process to establish the effect of t h a t change on the lighting design. The client should be made aware of the changes to establish t h a t they are acceptable. 3. DAYLIGHTING FOR DISPLAY-SCREEN EQUIPMENT 3.1 Background People generally like having daylight in their workplaces, and its use can make buildings more energy efficient. In interiors with DSE, occupants are still very positive about windows and daylight. For example, a German study [4] found t h a t DSE users preferred to be near windows. Daylight can brighten up a room

626 by supplying extra light to vertical surfaces, and windows supply contact with the outside world. For DSE users, an external view can help relax the eyes. However the provision of daylight can lead to problems: Sunlight in the eyes of the DSE user 2. Sunlight reflecting off the screen 3. Reflections of windows in the screen 4. Reflections of interior sunlit patches in the screen 5. Glare from interior sunlit patches 6. Glare from bright patches of sky 7. High illuminances on light coloured surfaces at the workstation making the screen look dark in comparison °

3.2 Building a n d window design In a new building it is possible to control the form of the building and the disposition of glazing to promote a high quality daylit environment. Ideally, the long sides of the building should face north and south. East and west facing glazing should be avoided, because low altitude sunlight can enter and cause glare and screen reflection problems. Screen reflection and glare difficulties tend to be worse in wide, open plan spaces with continuous 'ribbon' glazing. Cellular offices seldom cause problems. Where possible, wide, open plan spaces should be divided into a succession of smaller spaces. Long runs of glazing should also be separated, into a series of smaller windows. Windows on adjacent walls give good daylighting distribution but make it difficult to avoid screen reflections and glare. For this reason, the corners of buildings need special attention in design. Windows solely on opposite walls make it easier to align DSE workstations to avoid glare problems. Glare from windows can be reduced by decreasing the contrast between glazing and the window wall. All the following can help: • a light coloured window wall • light coloured glazing bars • splayed, deep, light coloured reveals • supplementary electric lighting on the window wall Windows should be positioned to give a good view out. In particular, the sill needs to be low enough, especially on the upper floors of tall buildings, to give a view of the horizon and foreground. In a deep building, an atrium can offer some of the characteristics of an external view while bringing in some daylight and sunlight. However, it is important to consider the possibility of sunlight coming through the atrium glazing and reaching adjoining spaces with DSE. In daylit spaces, screen orientation is very important. DSE screens and their users should face parallel to the window wall, not towards or away from it, and they should be at least 2m from the windows. This may require careful planning of workstation positions, and circulation routes. Where windows are

627 holes in the wall r a t h e r t h a n continuous ribbon glazing, workstations could be placed so t h a t the users or their screens face blank areas of outside wall [5]. Appropriate lighting controls are vital to make the most of available daylight. For spaces with DSE the most suitable solution is often localised m a n u a l control, for example using infra- red controllers or luminaire pull cords. Occupants value individual user controlled dimming or step switching, and it will save energy in a room with intensive DSE use. Controls m u s t be easy to u n d e r s t a n d and operate. 3.3 S h a d i n g devices Adjustable shading t h a t the occupants can control is usually the best option. Conventional venetian blinds can be adjusted to retain a view while restricting incoming sunlight and sky glare. For east or west facing windows vertical slat louvre blinds can cut out sunlight while keeping a view out and some incoming diffuse daylight. However, problems can occur if the blind material is translucent. A blind illuminated by sunlight can be very bright, causing reflection in DSE screens and distracting glare. This problem can occur with other non-opaque shading devices when sunlight enters. Translucent roller blinds, net curtains and diffusing glazing can all act as secondary sources of glare and screen reflections. While tinted solar control glazing and window films can reduce sky glare and incoming solar heat, they are not effective at controlling glare from the sun. If sunlight is likely to enter a space with tinted glazing, extra shading will be necessary, usually adjustable blinds. Heavily tinted glazing can make a room, and the view out, look dull. Careful design is required if rooflit spaces contain DSE. Sunlight can come through the rooflights and reach working areas, and rooflights can be reflected in screens. Dome type rooflights with a deep 'collar' are suitable. Northlights (either vertical or sloping) will restrict sunlight entry, but can cause screen reflections if DSE screens face the glazing. Other rooflight types will need shading if they let sunlight reach a DSE workstation or if they are visible at less t h a n 15° - 35 ° to the horizontal (the angle depends on the category of DSE use [1]) from a workstation. 3.4 Innovative daylight systems Innovative daylight systems [6], like the light shelf, mirrored louvres and prismatic glazing, can control and redistribute direct sunlight and improve the uniformity of light within a room. For DSE spaces they are of interest for several reasons: (a) They reduce the high illuminances experienced close to the window. These high illuminances can increase the contrast between the screen and source documents, causing discomfort for DSE users. (b) They can reduce window luminances, decreasing the severity of screen reflections and discomfort glare.

628 (c)

They maintain a reasonable daylight level deep within a space, although BRE research [7] suggests they rarely increase core illuminances compared with an unshaded window. Whatever system is chosen, it needs to work properly for all sun positions. In some cases supplementary blinds will be needed. CONCLUSIONS AND F U R O R

WORK

The use of low brightness luminaires in DSE spaces can lead to an unacceptable luminous environment with dark walls and harsh modelling. Supplementary lighting on room surfaces can help overcome this. Daylighting can bring important benefits to DSE users, but measures must be taken to avoid screen reflections and glare. These include appropriate shading devices and workstation orientation. Innovative daylighting systems can help but need careful design to ensure they can control sunlight at all times of year. The next stage of the project is currently underway and is investigating good practice DSE lighting installations. The study includes a review of the prospects for developing integrated lighting design procedures, and to investigate the effect of DSE lighting on lighting energy consumption. ACKNOWl ,EDGEMENT This paper has drawn on work by Tanya Heasman of System Concepts Limited, Lorraine Gardner, formerly with the Institute for Consumer Ergonomics, Loughborough, and Paul Ruffles for Building Health Consultants. T h e i r contributions are gratefully acknowledged. REFERENCF_~ 1. Chartered Institution of Building Services Engineers. 'Lighting Guide LG3. Areas for visual display terminals' London, CIBSE, 1989. 2. M J Perry and L Gardner, 'Daylighting requirements for display-screen equipment' Building Research Establishment Information Paper IP 14/93. Garston, BRE, 1993. 3. P J Littlefair, 'Daylighting design for display-screen equipment' Building Research Establishment Information Paper, to be published. 4. Cakir A E. 'An investigation on state-of-the-art and future prospects of lighting technology in German office environments'. Berlin, Ergonomics Institute for Social and Occupational Sciences Research Co Ltd. 1991. 5. W Bordass, T Heasman, A Leaman and M Perry, 'Daylight use in openplan offices: the opportunities and the fantasies' Proc CIBSE National Lighting Conference, Cambridge 251-259, 1994. 6. P J Littlefair, 'Innovative daylighting systems' Building Research Establishment Information Paper IP 22/89. Garston, BRE, 1989. 7. M E Aizlewood, 'Innovative daylighting systems: an experimental evaluation' Ltg Res & Technol 25 (4) 141-152, 1993. © British Crown copyright - Building Research Establishment