Lighting for Video Display Terminals

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Alaina Porter
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Lighting for Video Display Terminals odern ofjices require lighting systems that can work for two very dtfferent tasks: reading printed text and viewing video display terminals.

1 Lighting for Video Display Terminals odern ofjices require lighting systems that can work for two very dtfferent tasks: reading printed text and viewing video display terminals. To accommodate both tusks, a lighting system must be jlexible and the surrounding environment must be designed to enhance the lighting system. This Technology Update explains the special needs of offices with video display terminals, offers recommendations on how to design lighting systems for today's offices, and provides a handy checklist for evaluating your lighting environment. Office lighting in the past has been relatively straightforward because the predominant occupation has been one of reading dark letters (print) on light background (paper). However, with the prolific installation of video display terminals (VDTs) in the workplace as part of CAD/CAM (computer aided design/manufacture) or data or word processing, the task now becomes one of reading light text on a dark screen. Thus, the level and technique of lighting need to be reassessed in order to reduce or effectively eliminate VDT screen problems. Problems arise not just from the lighting system but also from its interaction with the surrounding environment, including wall surfaces, windows, and furniture. VDT Lighting Requirements Areas containing VDTs require the usual office lighting methods, but modified to provide a higher degree of lighting quality and control flexibility. The ambient lighting should be sufficient for an operator to perform all tasks, including reading VDT text and any printed or written matter without strain. Generally, a mix of natural sunlight and indirect fluorescent lighting is preferable, but fluorescent lighting only is an acceptable alternative. The main areas that need to be addressed are screen glare, VDT location/layout, lighting controls, environment, and design recommendations. Glare Natural and artificial light sources have the potential to produce glare in the VDT room. Standard office lighting will cause direct and indirect glare on the VDT screen. Figure 1 illustrates how direct room lighting causes glare on the VDT screen, which can lead to operator eyestrain and inefficiency. Glare can also originate from light reflection on desk surfaces, walls, internal windows, fabrics, and graphics. The "mirror test" can be used to detect sources of glare within a VDT area. Toll-free FAX Electronic Bulletin Board Bonneville PO WE R ADM I N I ST RATION

Hold a mirror in front of the VDT and scan it over the screen. Any offending reflecting surface will then be clearly identified, and corrective action can be taken.

2 Hold a mirror in front of the VDT and scan it over the screen. Any offending reflecting surface will then be clearly identified, and corrective action can be taken. Fitting luminaires with grid-type glare shields will alleviate direct illumination (non-hatched area in Figure 2). If glare persists, the following changes should be made: Drape windows (use finely meshed plain bright drapes). Try to avoid blinds because they produce daylight striations. Figure 1. VDT screen glare Locate VDT screen at normal angles (at 90 degrees) to glare sources such as windows and luminaires. Fit appropriate screen filter to minimize reflections (see the following paragraph). Use specular parabolic louver reflectors or baffles on direct (overhead) lighting or widespread beam (greater than 120 degrees) on indirect lighting. Place luminaires out of the glare angle (45 degrees from the horizontal). Use screen plaiforms or tiltturntables if more than one person operates the VDT to allow the screen angle to be adjusted for minimum glare. VDT operator fatigue and eyestrain can be reduced by installing an anti-glare, anti-reflection screen filter. The filter polarizes light rays and permits only rays Figure 2. Direct and reflected glare at a VDT il i 2

from a particular angle or direction to be transmitted. Room light can pass through the filter to reach the screen, but the returning reflected rays are absorbed by the polarizer.

3 from a particular angle or direction to be transmitted. Room light can pass through the filter to reach the screen, but the returning reflected rays are absorbed by the polarizer. In addition, the filter traps stray light rays from the phosphor screen that cause glare. Image enhancement also occurs through an increase in text contrast and sharpened resolution. Figure 6 shows the benefits of fitting an optical filter to a VDT screen. VDT Location/Layout A VDT should be placed in a location where the screen will not receive direct natural or artificial light. If it cannot be shielded from direct overhead lighting, reorient the terminal such that the operator s line of viewing is parallel to the light sources, as shown in Figure 3. Also, equip the screen with a visor and an optical filter. If feasible, do not place the VDT against a bright background such as windows or white walls (Figure 4). When the operator s eyes move from document or text reading to surrounding areas in the room, they make muscular adjustments to accommodate brightness differences between the surfaces. Excessive brightness contrasts will ultimately lead to overexertion of the eye muscles and cause eyestrain. Figure 5 shows an acceptable VDT workstation layout. The screen is set at a normal angle, parallel to the overhead lighting. The surrounding walls and floor have a low reflectance value to prevent glare on the screen. Figure 3. Recommended work station Figure 4. Do not place the VDT against a bright background such as windows or white walls 3

4 ~ Lighting Controls The illumination level in a VDT area can be varied easily by installing manual or automatic controls. Individual light switches or dimmers have the advantages of low cost and simplicity, but they must be situated close to the VDT to be effective. Figure 5. Acceptable work station. Automatic controls featuring programmable controllers, timers, dimmers, people sensors, and photocells allow for a more sophis tica ted lighting environment. VDT Environment Environmental features other than lighting that must be addressed are noise and office decor. Ambient sound level should be minimized in order to eliminate distractions that may reduce operator comfort and efficiency. Particular noise sources that disturb concentration include constantly ringing telephones; typewriters; passing traffic; loud music; heating, ventilating, and air conditioning (WAC) systems; and the printer used with the computer. Locate VDTs away from telephones (or reduce bell sound level) and typewriting stations. To shield traffic noise, select a room away from the street or treat the room with carpeting and acoustical tiles on walls/ceiling. Printers can be remote from the VDT, but if they have to be close, a noise shield should be considered. All brightness within the surrounding space must be controlled. Walls, graphics, and other surfaces should be diffuse and have a light reflectance value of less than 50 percent. Decor that is non-intmsive and does not include highly contrasting or drab colors such as dark green, brown, and grey can be advantageous. Mixing neutral tones, including beige, pearl grey, and light orange, contribute strongly to a comfortable working environment while minimizing reflected glare on the VDT screen. VDT operators should also avoid wearing white shirts or blouses because these reflect light onto the screen and obscure vision. Lighting Design Recommendations Reading VDT text is enhanced by reduced ambient or environmental illumination. Light characters on a dark screen require less light for reading than do dark characters on a light background. When the operator s task involves both VDT and document reading, illumination and reflectance levels must be bright enough for paperwork but sufficiently low enough to maintain screen contrast and avoid glare. The following are lighting suggestions for a room containing a VDT. H Approximately 28 to 46 footcandles for direct lighting is adequate, with 19 to 28 footcandles available for indirect (individual) Lighting, particularly for illuminating document holders. Both horizontal and vertical luminances are important at the VDT workplace. A document placed on a desktop is illuminated by horizontal illuminance; whereas, the VDT receives vertical illuminance. An office fitted with overhead lighting usually has a vertical/horizontal illuminance ratio of between 0.3 and 0.5. If, for example, a room illuminance is quoted as 46 footcandles, that would be the horizontal illuminance, the ver- - 4

tical (or screen) illuminance might be 14 to 23 footcandles. Figure 6. Using an optical filter enhances the image through an increase in text contrast and sharpened resolution.

5 tical (or screen) illuminance might be 14 to 23 footcandles. Figure 6. Using an optical filter enhances the image through an increase in text contrast and sharpened resolution. H With respect to lighting quality, the following luminances are recommended: VDT characters 12 to 53 footlamberts (23 to 47 +/- is considered optimum); VDT background 3 to 6; VDT screen to far wall luminance ratio not to exceed 1. Reduce fluorescent flicker by using lead-lag ballasts or connect alternate luminaires on different phases of a threephase power system. BConnect a1 ternate luminaires on different switchable circuits. Thus, if a room is to be converted from a general office area to one containing VDTs, half of the luminaires can be switched off convenien tly. BVDT lighting is a relatively new and, as yet, uncharted territory. The scope for innovative design, perhaps featuring indirect high intensity discharge (HID) sources, is large and limited only by the ability of the specialist. An example of indirect lighting which was acceptable to all staff is the following. In an open-plan office area where staff perform general office tasks and use VDTs. The overhead fluorescents were removed because of screen glare, and mercury vapor luminaires were installed to produce indirect (ceiling-reflected) illumination. Consideration should be given to plug-in type ceiling luminaires. Thus, if the VDT layout should be altered, the illumination level can be changed in an effective and economical manner. Multilevel switching and individual or small-zone switching should be considered, along with automatic lighting controls that use a photocell to monitor and maintain a constant ambient illumination level with varying room daylight. Lighting Retrofit Office spaces not specifically designed for VDT operation can be modified conveniently as follows: Disconnect luminaires placed in the offending glare zone (use the mirror test). Retrofit remaining luminaires with parabolic louver lenses after determining that their number and placement are correct. Rewire luminaires to facilitate zone control by switching or dimming. Eliminate sources of natural glare (skylights, windows) by installing drapes or blinds. Use individual desk lights for illuminating objects such as document holders. H Fit VDT screen with shroud and/or optical filter. 5

6 VDT Lighting Checklist Use this checklist to evaluate your present VDT lighting environment. You may be able to identify areas where improvements can be made with minimum disruption and inconvenience. Is the illuminance between 28 and 46 footcandles? Is the operator s field of vision free of direct reflections from the display screen, keyboard, desk, papers, and other sources? Is the operator s field of vision free from glare sources such as lights and windows? Are the luminaires equipped with prismatic or grid-type glare shields? Is the lighting system controllable and equipped with dimmers or switching? Are the VDT workplaces positioned such that the operators line of vision is parallel to luminaires and windows? Are the windows fitted with internal or external blinds? Are task (individual) light sources available to supplement the general lighting? Are the windows fitted with drapes, with a reflectance in the range of 50 to 70 percent? Is the average reflectance of the ceiling greater than 60 percent? Is the reflectance of the walls between 30 and 80 percent? Is the reflectance of the floor about 30 percent? Has regular cleaning and maintenance of the luminaires been properly considered? 6

7 I The Electric Ideas Clearinghouse is a comprehensive information source for commercial and industrial energy users. It is operated by - the Washington State Enerpj Office and is part of the Electric ldeas technology transfer program sponsored by participating utilities and - the Bonneville Power Administration. I Technology Update CH - 27 I Electric Ideas Clearinghouse 925 Plum St. S.E. P.O. Box Olympia, WA I Toll-free Hotline: Fax: Electronic Bulletin Board: I To obtain this factsheet in an a1 ternate format, contact the Clearinghouse. TDD (206) DOEIBP June C Adapted from B.C. Hydro. Lighting for Video Display Terminals. Guides to Energy Management. November 1983 Reprinted April With permission from B.C. Hydro, British Columbia. 7

8 * GPO

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