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VDT Reflecitons by Mark Rea

 

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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)

Rensselaer researcher Dr. Mark S. Rea explains how to identify, understand and prevent lighting problems in computerized work environments.

There are many problems associated with viewing visual display terminals (VDTs), but the main one is that of reflected images in the VDT screen. This is related largely to lighting, but it also relates to other aspects of the interior design; it is a pervasive problem that must be addressed by the architect. There are many different ways to solve the problem of reflections.   The purpose of this article is to explain its nature, and with this understanding to solve it in effective and creative ways.

VDT 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 eyes1 .   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.

Two general strategies may be followed that address these issues.   Bright, high-contrast, reflected images should be reduced below the threshold of perception, and distinct, sharply focused, reflected images should be reduced below the acuity limit of perception.   Both of these strategies are based on fundamental properties of the human visual system, and both have specific design implications and recommended solutions.

Contrast fundamentals

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/m2 ) 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


 •  850 cd/m2 at 65°

 •  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 Solution: 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:

  • Locate the VDT in a position that eliminates reflections from windows and luminaires.   This is difficult to achieve with curved screens, in that the reflections cannot be completely eliminated.   Operators will see their own reflections if the VDT placed in front of the window; brightly illuminated operators also create highly visible reflective (placing VDTs in front of windows also create uncomfortable and visually debilitating glare for the operator).   "Permanent solutions" are difficult to achieve because VDT workstations are frequently relocated.   As a general rule, the de solution should not depend exclusively on assumed VDT location.
  • 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.
  • 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.

Conclusions

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.
  • If indirect lighting is used, an adequate distance between the luminaire and the ceiling must be provided (these vary with design of the unit), and the luminaire should have a lightly colored finish on its underside.

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

Recommended reading

Guide on Interior Lighting, CIE 29.2, TLA Consultants (publications office of the U.S. N Committee of the Commission International de L'Eclairage), Salem, Massachusetts (508) 745-6870, 1986, 114pp.


IES Recommended Practice for Lighting Offices Containing Computer Visual Display Terminals, Illuminating Engineering Society of North America, New York (212) 705-7916, 1990, 25 pp.


"Lighting the Display or Displaying the Lighting," P.R. Boyce, in Work with Display '86, B. Knave and P.G. Wideback, editors, Elsevier Science Publishers, New York (212) 589-5800, 1987, pp. 340-349.


Solving the Puzzle of VDT Viewing Problem National Lighting Bureau, Washington, D.C.   457-8437, 1987, 20 pp.


The NLB Guide to Office Lighting and Productivity, National Lighting Bureau, Washington, D.C. 457-8437, 1988, 39 pp.

Side margin notes


            1 Diffusers in direct-lighting luminaires control the distribution of light and reflections on computer screens (1a-c).   Although aluminum-finished plastic louvers (c) virtually eliminate glare in this example, the energy consumption associated with them is twice that of specularly finished deep-cell aluminum louvers (b).   While reducing reflected glare, the strong downlight of sharp cut-off luminaires tends to cast shadows across the face of bookcases and other furnishings that many find disturbing.   More important, the performance of any type of shielding and diffusing medium with respect to VDTs depends on 1 the contrast between the luminaire and the ceiling and 2 the contrast between the reflected images of the ceiling and luminaires on the VDT screen and the VDT display.

            1 Accommodation is the ability of the eye to alter its focal distance with changes to the crystalline lens.   Vergence is the change in positions of the optical axes of the two eyes brought about by the extraocular muscles.   Convergence, for example, describes the process of bringing the lines of sight for the two eyes to a near point in front of the nose.

Calculating contrast

            2 Luminance contrast is a dimensionless value ranging from 0 to 199 percent and may be computed from the simple formula,

             C = 100{(L max + V) – (L min + V)}

             {(L max + V) + (L min + V)}.

            By rearranging,

            C = 100(L max – L min)/(L max + L min + 2V),


            Where C is contrast, L max is the maximum luminance of an image (in cd/m2), L min is the minimum luminance of an image (in cd/m2 ), and V is the luminance of the background on which the image is seen, or the luminance of the veiling reflection (reflected glare) superimposed on the image.

           In illustration 2a, the solid lines represent the luminance profile of white (w) and gray (g) bars reflected in a black screen.   For this example, the black screen luminance (V) equals 0 cd/m 2 , the maximum luminance of the reflected white bars (L max ) is 100 cd/m 2 , and the luminance of the reflected gray bars (L min ) is 50 cd/m 2 , thus producing a contrast of 33 percent.   If the same reflected image is seen on a lighter screen (V = 100 cd/m 2 ), the contrast between the same gray and white bars drops to 14.3 percent (2b).


           3    To control reflections of luminaires in VDT screens, IES recommends that direct luminaires in the reflected visual field behind the VDT operator have cut-off angles no greater that 65 0 .   The preferred maximum average luminance at 65 0 is 350 cd/m 2 , and under no circumstances should the luminance exceed 850 cd/m 2 .   While useful as rules-of-thumb, these guidelines cannot prevent visible reflections under all circumstances.

  • These two values of adaptation levels come from an assumed reflectance value of 0.8 and assumed illuminance levels of 200 and 1000 lux.   A unit of 1 cd/m 2 is sometimes called a nit; it equals 0.2919 foot lamberts, a term that has fallen out of favor. Glass reflects about 8 percent (0.08) of the light falling on it.   In the case of a reflected sight angle of 85 0 , the allowable luminance of the luminaire is 175 cd/m 2 .   If the ceiling luminance is 350 cd/m 2 , then the luminance of their reflected images is 0.08 x 350 and 0.08 x 175, so L max = 28 and L min = 14 cd/m2.

    For a VDT with a bright background luminance V of 100 cd/m 2 , the contrast C on the screen is:

            C = 100(28 – 14)/{28 + 14 + (2 x 100)}

            C = 5.78 percent.

            At a reflected sight angle of 65 0 , the allowable luminance of the luminaire is 850 cd/m 2 (greater than that of the ceiling).   Its reflected image L max = 0.08 x 850 = 68.   The ceiling remains the same at 0.08 x 350 = 28, but it now becomes L min .   For the same VDT display as above, V remains at 100 cd/m 2 , and the contrast on the screen is:

            C = 100(68 – 28)/{68 + 28 + (2 x 100)}

            C = 13.51 percent

  • In this illustration, the dashed lines represent the magnitude and spatial distribution of light at two types of edges.   A rapid transition of light is shown on the left (a) and a more gradual transition is shown on the right (b).

    The two edges show a common luminance difference  j, and thus have the same physical contrast.

    The perceived contrast  Y between the two edges is not the same, however, because of the dimensional difference ("sharpness" of the edge) between the contrasting bodies.   The faster the rate of change in luminance at the edge, the greater its perceived, or subjective contrast.   Both blurring an image and physically "ramping" the luminance variation at the edge (as in the illustration on the right) will make an object appear to have less contrast.
  • Normal visual acuity is considered to be 20/20, which means, in effect, that a person with normal acuity can resolve a high contrast image (black target on a white background) one minute of arc wide.   A pencil at a distance of 20m is about one minute of arc wide.   A person is considered legally blind if his or her acuity is less than 20/200.   Although people with 20/200 acuity still see large objects, they cannot see small ones, because the retinal image is blurred and, as a result, contrast is lost.   A person with 20.200 acuity can just resolve a high contrast object subtending ten minutes of arc – for example, a pencil at 2m.

    If we take the definition of legally blind as a criterion for defining the quality – or lack of quality – of the reflected image, a person with 20/20 vision will be "blind" to the reflected images produced by VDT screens meeting this criteria.   Architects can use a simple test to determine if the VDT screen will render the VDT operator "legally blind" for the reflected image:   On white cardboard, draw two black lines, the distance between them equal to the width of a pencil (approximately 5.8 mm).   To perform the test, hold the card 1m from the VDT screen and view the reflected image from the same distance (twice the optical distance).   If the two black lines cannot be distinguished from each other, then the screen renders the operator legally blind for the reflected image.   By this criterion, the screen would be acceptable.
  • This example cross-section of a fluorescent direct lighting luminaire shows how an "egg-crate" louver blocks light leaving the luminaire at angles greater than the cut-off angle.
  • The blades of a parabolic louver provide a physical cut-off in the same way as an "egg-crate" louver; however, a parabolic louver with a specular finish reflects all light from its curved blades at an angle equal to or less than the louver cut-off angle.
  • Luminaires with a sharp luminance cut-off are beneficial for work areas with VDTs, but may also produce shadows along the top of an adjacent wall.
  • The luminance of the VDT display in part governs the contrast and perceptibility of reflected images in the screen (see 2).   In this instance, the dark half of the screen clearly shows the reflected white blouse of the VDT operator, while the reflection (although present) is masked by the bright display.
  • Direct-indirect luminaires bounce light off the ceiling like indirect luminaires.   They avoid a dark underside by emitting light downward.   This reduces the difference in brightness between the luminaire and the rest of the ceiling in the reflected view on the VDT screen.   The distance between the ceiling and the luminaire must be matched to the design of its optics.

            The author is the Director of the Lighting Research Center at the School of Architecture, Rensselaer Polytechnic Institute, Troy, New York.   From 1978 to 1988, he worked at the National Research Council Canada, where he was manager of the Indoor Environment program.   The Lighting Research Center was established in 1988 through a grant from the New York State Energy Research and Development Authority (NYSERDA).   The mission of the Center is to change architecture by significantly improving lighting energy efficiency and lighting quality. The Center has an ongoing relationship through its Partners Program with Niagara Mohawk Power Corporation, Northeast Utilities, PPG Industries, and the Genlyte Company, as well as NYSERDA.

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