Ординатура / Офтальмология / Английские материалы / Diagnosing and Treating Computer-Related Visual Problems_Sheedy, Shaw-McMinn_2003

134 Diagnosing and Treating Computer-Related Vision Problems

Glare—Other Visual Problems

Another problem that occurs with large disparities in the brightness of objects in the field of view is that, when looking from brighter objects to darker objects or vice versa, there is always a brief period of time after the eye movement during which the eye has to adapt to the new brightness level. This is called transient adaptation. Entering or leaving a dark movie theater or restaurant in the daytime is a common example of dark and light adaptation. These are situations in which the eye needs to completely adapt to a different range of brightness. This same effect occurs on a smaller scale when the eye needs to fixate back and forth from bright to dark objects in a given environment. When the eye is not properly adapted to the brightness level at which it is looking, the vision is not good. This is a particular problem when constantly looking back and forth from a dark background computer display to a bright white reference document (see Box 8-2).

Patients who have macular degeneration are susceptible to symptoms caused by difficulties with light adaptation. The lower number of healthy cone photoreceptors requires a longer period to recover from being dazzled by glare. Consider this factor as a source of problems for older patients.

Illumination Levels

A goal of illuminating a computer work environment should be not to cause glare discomfort. Therefore, the 1 to 3 and 3 to 1 recommended maximum luminance ratios can be used to establish recommended illumination ranges based on ratios to the screen luminance. Because the computer display is the primary visual target, it is important to balance the luminance of the display with the lighting in the work environment. The luminance (brightness) of most objects that are visually adjacent to the computer are dependent on the amount of the room illumination and on the reflectance of each of those objects. Ideally, the luminance of the computer display is matched (within a factor of 3) with the general luminance of nearby objects in the environment. If this is not the case, then consider adjusting the display brightness or the room illumination to accomplish parity.

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Is Glare Discomfort Present?

Subjective Method

One way to test for glare discomfort is to have the worker look at his or her computer screen and be aware of any bright lights in the worker’s peripheral vision. The worker can use his or her hand to shield the eyes like a baseball-cap visor and should note whether he or she senses an immediate improvement

in comfort. If the glare is from the window, then shield the eyes from the window. The worker should try this two to three times. If he or she notices an immediate sense of improved comfort by eliminating the lights from peripheral vision, then the worker is experiencing glare discomfort. Most people, when performing

this test in the presence of bright overhead fluorescent lights, notice the discomfort glare. If they can notice an immediate improvement in comfort, the cumulative effects of an entire workday are quite obvious.

Measurement Method

A luminance meter can be used to measure the luminance of key objects in the field of view to determine whether they may cause glare discomfort. Measurements should be made with the instrument viewing from approximately the same location as the user’s eyes. It is best to use a fairly large aperture size (approximately 1 degree) so that the spatially averaged luminance value of a given stimulus can be determined. The luminance levels of detail (e.g., the luminance of the text letters) are not important; rather, the mean luminance of large areas within the visual field is important. Typical measurements would be to measure the luminance level of the screen (large aperture to include text and screen background), the reference document, the wall or desk area behind the computer, general office luminance, desk lamp, window, and overhead lights. For each measurement, it is also important to note the angle between the normal line of sight to the computer and the line toward the glare source. This angle can be mea-

136 Diagnosing and Treating Computer-Related Vision Problems

sured or estimated by viewing the computer user from the side and viewing the geometry through a clear protractor.

The luminance ratios of the peripheral objects to the computer display can then be determined and compared to the maximum recommended luminance ratios established by the IES. The luminance ratio should not exceed 1 to 3 or 3 to 1 between the task and the immediate visual surroundings (within 25 degrees), nor should the ratio exceed 1 to 10 or 10 to 1 between the task and more remote visual surroundings. The dividing angle of 25 degrees can be estimated as being 50% percent (actually 46.6%) of the distance from the eyes. For example, for a fixation point at a viewing distance of 24 in., an object that is 12 in. (11.2 in.) laterally from the fixation point would be 25 degrees away.

Clinical Method

Although there is not a clinical method for measuring discomfort glare (tests of disability glare do not test for glare discomfort), it is quite easy to demonstrate to patients as follows. In most eye care offices, there is an overhead fluorescent fixture that is quite bright. If this is not available, a bright open window or a desk lamp can also serve for the demonstration. Instruct the patient to look straight ahead at a fixation point with the bright light in his or her peripheral vision. Encourage the patient to continue viewing the fixation point while shielding and unshielding his or her eyes several times. If the patient notes glare discomfort, then advise the patient to perform the same test while looking at his or her computer. Positive results help the patient to identify sources of glare discomfort and, hence, what he or she should attempt to treat or eliminate in his or her work environment.

Solutions to Glare Discomfort

If bright lights are deemed to contribute to discomfort, then they should be removed or mitigated in some manner. There are several ways in which the situation can be improved. Many of the solutions below are suggestions that can be implemented fairly inexpensively by the computer user and without major office redesign.

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1.Turn off the offending fluorescent lights. Sometimes, it is a single offending fluorescent fixture in the ceiling that is immediately in front of the person. The entire fixture can often be turned “off ” by loosening one of the fluorescent tubes. Very often, a single offending fixture can be turned off without creating lighting deficiencies. Many offices have too much light anyway. When turning off lights, however, consideration must be given to coworkers, who may rely on the light coming from that particular fixture.

2.Many offices have two wall switches controlling the fluorescent light fixtures. Each switch controls half of the bulbs in all of the fixtures. Often, both switches are turned on for the day. If the room is brighter than the computer displays, consider turning off half of the lights. Very frequently, however, people are more comfortable with only one switch turned on, effectively cutting the room illumination in half. The first reaction of many people on turning off half of the lights is negative. However, request that they try it for a few minutes. After they have worked in the reduced illumination for a while, most people forget that the light level is lower and appreciate the improvement.

3.Some fluorescent light fixtures can be retrofitted with parabolic louvers that direct the light straight down into the room. The reason that fluorescent lights cause discomfort glare is that most of the light fixtures direct the light so that it leaves the fixture in all directions. This results in light leaving the fixture toward the eyes of workers who are looking horizontally in the room. A parabolic louver (the louver is the egg crate–like cover that directs light into the room) directs the light from the fluorescent tubes straight down; hence, the light is not directed into anyone’s eyes.

4.Reorient the workstation so that bright lights are not in the field of view. Sometimes the work desk can be rotated 90 or 180 degrees so that the fluorescent lights or bright windows are not in the field of view. Many workers make the mistake of placing their computer display immediately in front of a window. In this situation, the brightness ratio between the screen and the window can be excessive. In a large office with rows of overhead fluorescent lights, it is usually better if workers look along the rows of lights while viewing the computer display instead of across the rows.

138Diagnosing and Treating Computer-Related Vision Problems

5.Wear a visor. This is actually a very efficient

way to eliminate the brightness of overhead fixtures. A person can wear a visor for a day or two as a test to determine the extent to which the visor alleviates discomfort at the end of the day. This can help the worker and his or her manager or doctor gauge the

extent to which overhead brightness is contributing to the worker’s discomfort and the extent to which solutions should be implemented.

6.Avoid bright reflective surfaces. In some work environments, the desktops are white. This is not good because it results in the desktop surface becoming a discomfort glare source. Desktops and other furnishings should have a matte, medium-reflective surface. Ceilings should be painted white, and walls should be medium light.

7.Use blinds or drapes on windows. This is often the most difficult change to implement because people like window views. However, if the window view is considerably brighter than objects in the room (as it normally is), then the window is serving as a discomfort glare source. Usually, the best solution is to use blinds. The blinds should be adjusted so that the worker cannot see the light coming from the window, but the light from the window should be directed upward onto the ceiling or sideways so that it is not entering the worker’s eyes. (The light should also be directed so that it is not directly hitting the computer screen, which causes screen reflections.)

8.Evaluate whether auxiliary lighting (e.g., a desk lamp) on reference documents is causing glare discomfort. Auxiliary lighting is not often necessary and may contribute to glare discomfort. General room illumination is often adequate to see the reference documents clearly. Placing the auxiliary lighting on reference documents often causes them to become considerably brighter than the computer display, especially if it is a dark background display. The bright reference document not only creates a discomfort glare source, but it also results in transient adaptation problems when looking back and forth from the dark screen to the bright reference documents. The first impression on turning off auxiliary lighting is often negative; therefore, longer-term comfort should serve as the guide to changes.

9.Change the brightness of the screen. The brightness of the screen should be adjusted to match the brightness of the visual objects

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that immediately surround it. This is usually accomplished quite easily with a display with black characters on a white background. It is not possible to accomplish this if the display has light characters on a dark background—the worker is looking into a “black hole” surrounded by a much brighter room. This results in a fundamentally poor visual environment and is one of the primary reasons why light background displays are preferred over dark background displays for most purposes.

10. Hang or erect partitions. Very often, the offending light sources can be eliminated from the field of view by erecting or moving partitions.

In summary, lighting is often the most significant factor in the work environment resulting in vision discomfort at the computer. Patients must be educated as to the effect of improper lighting and instructed on how to improve it. Appendix 8-1 shows a sample handout that may be given to patients to self-evaluate the lighting in their work area. Chapter 9 addresses a second major work environment factor, reflections.

References

ANSI/IESNA RP-1-1993. American National Standard Practice for Office Lighting.

New York: American National Standards Institute, 1993.

Guth SK. Prentice Memorial Lecture: The science of seeing—a search for criteria. Am J Optom Physiol Opt 1981;58:870–885.

140 Diagnosing and Treating Computer-Related Vision Problems

Appendix 8-1

Lighting at a Computer Workstation

Although the amount of light is important, it is more important to have good light distribution. Good light distribution is accomplished when all of the objects in the field of view have approximately equal brightness. Bright lights or windows are common offending sources and cause discomfort. You can

determine if the overhead lights or windows are contributing to discomfort by shielding them from the field of view with a hand or a file folder, which simulates a visor. If a small but immediate sense of relief occurs, the bright source is contributing to discomfort.

Suggestions for improving the lighting include the following:

1.Use blinds or drapes on windows to eliminate the bright light. It is often necessary to change the blind adjustment during the day because of sun movement. The blinds should be adjusted to allow light into the room without being able to see the bright light directly.

2.It is often inadvisable to put additional lighting on reference documents; this makes them too bright compared to the screen.

3.If auxiliary desk lighting is used, it should be low wattage and be directed so that it does not directly enter your eyes or directly illuminate the computer screen.

4.Indirect lighting systems often provide the best visual environment.

5.Wear a visor to shield your eyes from bright overhead lights.

6.Reorient the workstation so that bright lights are not in the field of view.

Appendix 8-1 141

7.Avoid white reflective surfaces. Desktops and other furnishings should have a matte, medium-reflective surface.

8.Ceilings should be painted white, and walls should be medium light.

9.Turn off some fluorescent light fixtures that are in your field of view and are bothersome. Be considerate of the effects on other employees. More information is available at http://www.DoctorErgo.com.