Solving the Problem of VDT Reflections
Solving the Problem of VDT Reflections, by Mark Rea
(Reprinted from Progressive Architecture, October 1991, pages 35-40, with permission of the author and the publisher)
displays commonly provide two competing images for the worker’s attention
(1). Not only does the VDT display provide a view of the electronically
generated alphanumeric text, line drawings, or pictures, but it can also
offer, through reflection, a view of the luminous environment surrounding
the worker and the VDT. These two images are at different optical distances:
The electronically generated image is close while the reflected image
is usually much further away. These two images compete for the attention
of the VDT operator and in doing so require repetitive focusing – or,
more accurately – accommodation and vergence adjustments by the eyes
1 . In essence, all successful design solutions for VDT environments
eliminate or reduce the quality of the reflected image while maintaining
the quality of the electronically generated image.
Why eliminate or reduce bright, high-contrast reflections? To be visible, an object must have a luminous (or color) contrast with its background, that is, the object must have a different brightness than the background has (see sidebar, 2). A minimum contrast must be exceeded to product perception; contrast values below this threshold are functionally invisible. Contrast threshold depends on a number of factors, including the size of the object, how long the object was seen – if for short durations – and the overall adaptation level of the visual system. Contrast threshold values between 2 and 5 percent can be taken as a representative range for all but the smallest objects seen in typical windowless, interior offices, which usually have luminance (“brightness”) levels of about 50 to 250 candelas per square meter (cd/m 2 ). These contrast threshold values can be taken as performance criteria for defining the visibility, or rather invisibility, of reflected images in VDT screens. 2
Most VDT screens are self-luminous; that is, they product light by electrically stimulating phosphors in the VDT display. Dark background displays (“positive contract displays”) may have an average screen luminance of 5 to 10 cd/m 2 in the dark: bright background displays (“negative contract displays”) may have a comparable luminance of 100 cd/m 2 or more.
Glass or an untreated VDT screen will reflect about 8 percent of the light incident on it. An object with a contrast of, say, 33 percent and a maximum luminance of 100 cd/m 2 will produce a much more visible reflection in a dark background display (of 10 cd/m 2 , for example) than in a bright background display (of 100 cd/m 2 ). In both cases, 8 percent of the light from the object is reflected from the screen. The (maximum) luminance of the reflection (8cd/m 2 ) is close to the luminance of the dark background display and will be clearly seen. For the bright background display, however, this is a small amount of light relative to its self-generated light. Indeed, for the bright background display, the contrast of the reflected image described above would be 1.9 percent – below the more conservative contrast threshold criterion of 2 percent. Such a reflected image can be considered invisible.
Bright background displays work rather well in reducing many distracting reflections from the VDT environment, but, because the upper limit of luminance from this type of display is approximately 100 cd/m 2 , they will not be effective for very bright, high contrast objects. Untreated windows and many forms of electric lighting will still product reflections well above the more liberal contrast threshold criterion of 5 percent.
Direct Lighting luminaires
The Illuminating Engineering Society of North America (IES), in its Recommended Practice for Lighting Offices Containing Computer Visual Display Terminals (IES RP-24-1989), states that the average luminance produced by direct lighting luminaires (3) should never exceed the following values (angles are expressed in degrees from vertical, that is walls are 0 and horizontal ceilings are 90
350 cd/m2 at 75°
175 cd/m2 at 85 °
These recommendations cannot be properly evaluated, however, without also considering the contrast of these fixtures against the ceiling. If, for example, the ceiling luminance was 350 cd/m 2 then the fixture would be invisible in any screen at 75 because it has no contrast with the ceiling. At 85 ° , it would have a contrast against the ceiling of 33 percent. However, the contrast of the reflection on a bright background display of 100 cd/m 2 would be only 5.8 percent – close to the more liberal contrast threshold criterion of 5 percent and probably acceptable to the VDT operator. At 65 ° , however, the contrast of the reflected image would be 13.5 percent, certainly visible and perhaps unacceptable to the VDT operator. 3
Clearly, darker ceilings or VDTs with dark background displays would result in higher contrast images and would make the IES recommendations unacceptable. To be meaningful, therefore, designers must specify both the absolute luminance of the luminaire and the luminance of the ceiling, thus providing information on contrast.
Indirect Lighting Luminaires
Indirect luminaires emit all light toward the ceiling. Interestingly, the IES Recommended Practice recommends contrast values for ceiling luminance produced by indirect luminaires. They suggest that the ratio of the ceiling luminance directly above the luminaire (normally the brightest area) to the ceiling illuminance between luminaires should never exceed 10:1 and, preferably, this luminance ration should be limited to 4:1. They go further to state, as they do for direct luminaires, that the maximum luminance on the ceiling should not exceed 850 cd/m 2 for any 0.6 m x 0.6 m (24” x 24”) area. Using the contrast equation presented in the sidebar, the 850 cd/m 2 maximum luminance, the preferred luminance ratio of 4:1, and assuming an 8 percent reflection from the screen, the contrast of the luminous ceiling reflected in a bright-background VDT display of 100 cd/m 2 will be 18 percent – well in excess of the contrast threshold criteria of 2 and 5 percent. Obviously, these recommendations will allow still more distinct reflections in a dark-background VDT display.
It should also be noted that the bottom of the indirect luminaire is relatively dark and the contrast of its image against the bright ceiling will remain high, perhaps 80 percent. Although the IES recommendations for indirect luminaires properly include specifications of both contrast and absolute luminance, they are, by the contrast threshold criteria presented here, inadequate for ensuring the elimination of reflected images in the VDT screen.
Direct-Indirect Lighting Luminaires
Direct-indirect luminaires provide light downward and upward. The IES makes the same recommendations for direct-indirect luminaires as for indirect luminaires, but adds that the luminance of the downward component should not exceed 850 cd/m2. We have already seen that this value and the 4:1 ratio for ceiling luminance can exceed the contrast threshold criterion. However, one advantage of the direct-indirect luminaire is that the contrast between the ceiling and the underside of the luminaire can be significantly reduced. When properly adjusted with respect to the distance from the ceiling, the luminance contrast between the images reflected in the VDT screen of the ceiling and luminaires can be very low, while the luminaires still maintain a high ambient level of illumination.
Prescriptive guidelines like those provided by the IES must be carefully considered. It is usually necessary to make suitable luminance measurements and some simple calculations to properly assess the suitability of a given installation. A variety of light measurement tools known as luminance photometers can be used to measure the general luminance of the electronically generated display and the reflectance of the screen. These devices work much like the light meter in a 35 mm camera, the difference being that the calibrated luminance photometer provides a numerical measurement of brightness suitable for calculating contrast and visual response.
It is also desirable to calculate screen brightness in the design phase. With luminance data supplied by the luminaire manufacturer and the methods outlined above, it is fairly easy to determine whether a particular luminaire will be suitable for any of a variety of VDT applications. The impact of window glazings and coverings can be approached in the same way, but the variability of sky conditions and ground reflectances complicates the analysis. With a luminance photometer and a mock-up, however, it is possible to determine typical luminance values for alternative window glazings and coverings, and wall finishes.
Edges in the Visual Field
Why eliminate or reduce the distinctness of edges of illuminated areas or objects in the visual field? Up to this point, we have considered only the absolute luminance and contrast of the reflection without regard to the spatial distribution of this light. As will be shown below, our perception of contrast and brightness depends not simply on physical luminance, but also on how that luminance is distributed across the retina in the eye.
Our visual system is designed to respond efficiently to edges of images focused on the retina. Edges are so important to the visual system that an architect can effectively represent the appearance of a building facade from a simple line drawing.
Conversely, a defocused image – which has reduced the contrast and sharpness of the lines and edges – is very difficult to see; defocused images may cause an uneasy feeling that deters viewers from looking at them. Edges are so important to the visual system that it will, in fact, enhance their perceived or subjective contrast (4).
It is usually impossible to reduce the perceived contrast of the reflected image in a VDT screen through lighting alone. Indirect and direct-indirect lighting will help to some extent because the light pattern on the ceiling changes luminance gradually, as opposed to the abrupt change from bright to dark characteristic of direct lighting. Attention must be given primarily to the optical fidelity of the VDT screen when approaching this problem. Matte-surface screens that diffuse reflected light should be specified by owners and facility managers when purchasing hardware; although beyond the control of designers, they can, nevertheless, educated clients about these issues as they relate to lighting. Importantly, where the contrast of the reflected image is higher than the threshold contrast criteria offered above (which will often be the case), a matte-surface screen will reduce the subjective contrast of the reflected image, often making the reflection of such low quality that it will be unnoticed by the VDT operator. Although somewhat crude and unconventional, the quality of the image reflected in the VDT screen can be assessed with a pair of lines drawn on a white index card (5).
Design Solutions: Reducing Brightness
Knowing the essence of the underlying reasons behind the two strategies offered here – reducing brightness and subjective contrast (“sharpness of focus”) – it is now possible to examine some practical solutions to the problem of multi-images in the VDT Display. Lighting techniques and treatments to the VDT screen are complimentary approaches. The first step is to reduce absolute brightness of the reflected image; this will also tend to reduce the contrast of the image reflected in the screen. Reflections of ceiling luminaires and windows are of primary concern sign measures include:
2 Turn the electric lights off or obscure light windows that produce bright reflected images in screen. This is not a design solutions, but probably the most common remedy chosen operators where no consideration has been given to the VDT work environment. It should be noted that the employer often incurs the expense of task lighting for work surfaces in these other dark environments.
3 Shield the screen from reflections using optical controls. perhaps the most common shielding approach is to provide sharp cut-off louvers direct lighting ceiling luminaires. Both “egg-crate” (6) and parabolic louvers (7) are available for nearly every manufacturer of direct lighting luminaires. The luminance values of these louvers range from near-zero upward, depending on the type and finish of the louver material as well as the angle of view.
Sharp-cut-off luminaires are not a panacea, however. Strongly directional down lighting will create very dark areas along the ceiling-wall juncture (8). They will also create high contrast images in VDT screens by producing strong shadows under shelves and bright horizontal surfaces. VDT screen treatments and supplemental lighting (wall washing and task lighting) may be required to overcome the objections of workers in the VDT environment.
Attachments to the VDT screen can also eliminate a direct view of the luminaires. Much like the black louvers sometimes placed on the rear window of “fast-back” sports cars, a screen mesh (9) will shade the screen from high angle illumination from ceiling luminaires, while providing a view of the display directly through the mesh. These meshes will not eliminate reflected light from windows because, like the VDT operator, the walls have a “direct view” of the screen though the mesh. Given the electric charge of the terminal, screen meshes also attract dust, which obviously will reduce the quality of the screen image and may, with poor handling, be compressed down so that direct viewing of the VDT display is obscured. Still, with proper maintenance, meshes can be effective in eliminating bright reflections from ceiling luminaires.
Circular polarizer attachments to VDT screens are a second approach to blocking reflections. In essence, the circular polarizer works like a key hole in a lock. Light passing through the circular polarizer is oriented in one direction, as a key has to be oriented to pass through a key hole. Reflection from the VDT screen causes the light to be reoriented and, in doing so, prevents the light from passing back through the circular polarizer “key hole” again. This technology has been used effectively for many years as treatments to radar screen installations.
Design Solution: Reducing Subjective Contrast
Reducing the prominence or frequency of sharp edges in the image will reduce subjective contrast. Attention must be given primarily to the VDT display, although lighting techniques can also be partially effective.
Screens with matte finishes will diffuse the reflected light and physically reduce the brightness of the reflected image. This approach tends to equalize the light reflected in any given direction. More important, perhaps, it reduces subjective contrast by eliminating well defined edges in reflected images. The matte finish does reduce the quality of electronically generated images to a small extent, but the visual advantages outweigh the disadvantages.
Using a VDT with a bright background display will reduce the contrast of the reflected image (9). A similar effect can be achieved by “washing” the surface of the VDT screen with light, but this is not recommended. This “solution” requires a task lamp and wastes electricity. Further, because there are limits to the brightness of the screen, too much illumination on the screen can reduce the contrast of the electronically generated image.
Indirect or direct-indirect lighting can be effective. Diffuse light created by indirect luminaires softens sharp shadows in the environment and thus reduces the subjective contrast of reflected images. The light reflected from the ceiling is typically of lower luminance than that produced by direct luminaires, and the distribution of the luminous pattern on the ceiling is less sharply defined. Nevertheless, the dark underside of a totally indirect luminaire can produce a high contrast, distinct, reflected image in the screen. Luminaires that combine direct and indirect optical control can often balance the brightness of the ceiling and the underside of the fixture, thereby reducing the contrast between bright and dark. Direct-indirect fixtures are ideal for producing a relatively high level of ambient brightness without producing a high contrast image of the fixture in the VDT screen. Obviously, the distance between the luminaire and the ceiling must be carefully considered, as should the material and finish of the optical control for the luminaire lens (10). Designers should know how to read and interpret photometric reports for luminaires or seek assurances from the luminaire manufacturer that the criteria for brightness and contrast discussed here are met.
The architect should also consider splaying the inside edges of windows and skylights to reduce the sharp transition between the bright scene outdoors and the relatively dark interior wall.
Although there are several problems with the visibility of VDT displays, the main problem to solve is the occurrence of reflections on the screen. A basic understanding of the human visual system can guide architects and interior designers in the selection of VDT design alternatives. This knowledge empowers them to move away from prescriptive guidelines for VDT lighting and to provide more creative and effective design solutions. The following guidelines should minimize reflections in the VDT display, and at the same time provide adequate illumination for other tasks throughout the room.
VDT environments should normally be uniformly illuminated. Uniform lighting tends to reduce contrast and to lower absolute levels of luminance. Direct, indirect, and direct-indirect luminaires can all be used to achieve these results. The guiding principle in every case is to avoid very bright, high contrast, sharp-edge reflections in the screen.
If sharp cut-off luminaires are used to reduce reflected images in the screen, the walls should also be illuminated.
VDT screens with a matte finish and bright-back-ground displays will reduce the contrast of reflected images. Meshes and circular polarizer screen treatments can be used to block reflections from poorly designed electric lighting or skylights. Attention should also be given to VDT position, particularly with regard to windows. Reflected images from windows are a major problem to be solved in the VDT environment, and careful attention be given to window treatments, again following the principle that very bright, sharp-edge reflection should be avoided. Splaying the interior of the window can reduce the subjective contrast of the image.
Finally, these general recommendations only sensible in the abstract: Effective solutions depend on actual conditions. An understanding of the fundamentals and the simple evaluation tools offered here enable the architect to go beyond a prescriptive approach to VDT visibility, and to begin to diagnose existing problems and to create better design solutions. Mark S. Rea