Reprinted from The RSI Network - Issue 34 - January'99 Dr. Jeffrey R. Anshel, BS, OD Corporate Vision Consulting Encinitas, California 1-800-383-1202
Improving Visual Comfort At Your PC
Visual discomfort occurs at a computer workstation when the visual demands of your task exceed your visual abilities. The problem can be resolved by treating your visual condition, if any, or by making your visual task less demanding.
A thorough eye examination is necessary to determine whether you have any visual problems. The examination must include analysis of the functioning of your eyes at near working distances. It’s also important to provide the examining doctor information about your viewing distance (from your eyes to the screen) and the location of the screen in your field of view.
The following suggestions are for the improvement of visual comfort at a computer workstation.
Although the quantity of illumination is important, it’s probably more important to have good light distribution. This is accomplished when all the objects in your field of view have approximately equal brightness. Bright lights or windows are common offending sources that can cause discomfort. To determine whether overhead lights or windows are a problem, shield them from your field of view with your hand or a file folder, simulating a visor; if you experience a small but immediate sense of relief, the bright source is contributing to your discomfort.
Here are some suggestions and tips for improving the lighting:
- Use blinds or drapes on windows to eliminate bright light. Blinds should be adjusted during the day to allow light into the room without enabling you to see bright light directly.
- Wear a visor to shield your eyes from bright overhead lights. (Indirect lighting systems often provide the best visual environment.)
- Reorient the workstation so that bright lights aren’t in your field of view. Turn off fluorescent light fixtures in your field of view if they’re bothersome (though of course be considerate of the effect on others who are working nearby).
- Most offices have too much light: 75-150 foot candles. The recommendation for computer displays with a dark background screen is 18-46 foot candles; higher light levels are OK with a white background screen. Too much room illumination makes the room too bright compared to the display screen, resulting in visual discomfort and too many screen reflections.
- If you use auxiliary desk lighting, it should usually be low wattage and should be directed so that it doesn’t directly enter your eyes or directly illuminate the display screen. It’s usually inadvisable to put additional lighting on reference documents, because this makes them too bright compared to the screen.
- Avoid white reflective surfaces. Desktops and other furnishings should have a matte, medium reflective surface. The ceiling should be white and the walls should be medium light, also with a matte finish.
Reflections on the screen decrease the visibility of text on the screen by decreasing contrast. To determine whether this is a problem, you can temporarily use light baffles (such as file folders) to shield the screen from offending light sources. If this results in a noticeable increase in the contrast and clarity of the text, the reflections are a problem and should be addressed.
- An antireflection screen can be placed over the display. Glass screens perform better than mesh screens. Look for screens that have been approved by the American Optometric Association. (Alternatively, a hood can be purchased and placed over the display to shield it from offending sources; however, hoods often don’t perform as well as antireflection screens.)
- Eliminate or cover the sources of the reflections—typically windows and other bright lights behind you.
- Use dark characters on a light background; they are less affected by reflections than are light characters on a dark background.
Screen Flicker Problems
Some people experience a flickering sensation when viewing the screen. If this is bothersome, try turning down the brightness, or use a dark background instead of a light one. If all else fails, using a display with a higher refresh rate might solve the problem.
- Good screen resolution is important, especially for extended work. Screens with more pixels generally provide better resolution. Monochrome monitors usually have better resolution than color monitors; if the job doesn’t require color, it’s often best to use a monochrome monitor.
- Adjust the screen brightness and contrast so that character definition and resolution are maximized. The screen brightness should match the general background brightness of the room (this is much easier to do with light background screens).
- Black characters on a white background is probably the best combination. Other combinations can be comfortable as long as the contrast between the characters and the background is high. It’s best to avoid dark backgrounds.
- The size of the text should be three times the size of the smallest text you can read. You can test this by viewing the screen from three times your usual working distance; you should still be able to read the text.
- Although 60 Hz is the most common refresh rate, higher refresh rates are preferred.
- For color monitors, small dot pitches (less than 0.28 mm) are desirable.
Computer Furniture and Arrangement
- Your table should allow for adjustability in the keyboard height, have adequate space for both the monitor and reference material, and provide adequate knee space. It's usually best for the keyboard to be 3-5 inches below the standard desktop height of 29 inches; you should not be reaching up to the keyboard.
- Your chair should be easily adjustable in height, provide adjustable lower-back support, and have a flexible, woven seat covering. Five legs provide greater stability than four. Full armrests are not recommended, because they often preclude moving the chair under the table. However, support for the elbow or forearm can relieve strain on the shoulder, arms, and wrists.
- The height of your monitor should be adjustable. The center of the screen should be 10-20 degrees below your straight-ahead gaze (4-9 inches below your eyes, for normal working distances). In most cases the top of the screen should be just below your eyes. A screen that is higher than these recommendations adversely affects your posture.
- Reference documents should be located close to the screen, with adjustable copy holders. The documents and the screen should be the same distance from your eyes, and the brightness of the documents should match that of the screen.
About the Author
Dr. Jeffrey Anshel graduated from the Illinois College of Optometry and served as a lieutenant in the U.S. Navy in San Diego, where he established the Navy's first vision therapy center. In 1990, he published Healthy Eyes, Better Vision, a layman’s reference book containing practical advice regarding vision care. Taylor & Francis has just published his second book, Visual Ergonomics in the Workplace. Dr. Anshel is currently the principal of Corporate Vision Consulting, where he addresses issues related to visual demands on computer users. He also maintains a private practice in Carlsbad, California.