Most of us work with computer screens nearly every day of our lives. Whether it is e-mail at work or surfing the net at home, working with computer screens can lead to visual fatigue, eyestrain, burning, irritated eyes, blurred vision, tiredness or irritability, headaches, migraines, and nausea. Medical experts typically consider these symptoms as a temporary discomfort rather than indications of any long-term disease, but they can seriously impact work performance. There are several very simple things you can do to reduce eyestrain.
- Make sure your vision is properly corrected for close-up work.
- Use floor or table lamps instead of overhead lighting.
- Use a monitor that has a good anti-glare coating.
- Use the monitor for no more than four hours of screen-based work per day.
- Make sure that you schedule regular rest breaks during screen-based work.
If you've taken all these precautions and still experience any of the above symptoms there may be one more very important thing you can do: adjust the monitor's refresh rate. First, a little explanation is in order.
How It Works
For the typical CRT-type monitor screen, the inside of the front glass is coated with a phosphor that glows when electrons emitted from another part of the tube fly into it. In modern color monitors, the inside of the front glass has a very fine array, in a specific pattern, of red, green, and blue phosphors. Each phosphor dot will glow briefly when an electron released from one of three electron guns (one for each color) strikes it. The guns spray these electrons from left to right, beginning at the top of the screen and moving to the bottom covering the entire array of phosphor dots. For each time the array has been covered, the monitor has displayed a "frame." The guns are repeatedly spraying these electrons throughout the array so fast that the human eye can barely detect it. Barely.
The ability of the human eye to "fuse" separate frames together into seamless motion is called the "persistence of vision" and it's the phenomenon all motion picture projectors rely on to work. The eye's light receptors and the human brain retain an image's impression for a fraction of a second longer than the light from the image was actually there.
When you watch a movie, what you are actually seeing is individual still frames of film projected at 24 frames per second. Each of these frame impressions is separated by an interval of darkness. While watching a film, you are actually sitting in a pitch-dark theater about half of the time. Persistence of vision makes one image blend into the next, giving the illusion of movement and continuity. The dark intervals are "ignored" by the brain. Or so it was thought.
Saccades
Sometimes, however, the human eye can detect the dark interval. When reading text, the human eye moves to focus on the lettering by making a series of imperceptible jerky movements along the object being read. These jerky movements are called saccades. (See Institute of Behavioural Sciences, University of Derby UK, for an oversimplified [not!] explanation of saccades.) The eyes make these movements so quickly that there is time for only one or perhaps two frames during the transition. Clinical tests have shown that the brain smoothes out these jerky motions by ignoring or blanking-out the image of the text. This process is called "saccadic suppression." When we see text on a computer screen, the pulsation of the image, called the screen's refresh rate, interferes with this saccadic suppression and increases the number of rapid eye movements required for reading. This in turn causes eye strain and can lead to migraines in some individuals, especially those individuals who are already susceptible to headaches.
Fortunately modern computers are becoming faster and the newer machines allow you to modify the number of times the screen refreshes (flickers) each second. The refresh rate, the number of times per second that the entire screen is redrawn from top to bottom, is typically measured in Hertz (Hz). If a monitor refreshes the screen 10 times per second, it's refresh rate would be 10 Hz. It has been shown that a refresh rate below 70 Hz causes many people to suffer from eyestrain. As a result, the industry has settled on a standard 75 Hz refresh rate for nearly all monitors. The rate of saccadic suppression varies from person to person and some people may not notice a flickering that sends others to the emergency room with a migraine.
In Windows 95, changing the refresh rate depends heavily on a separate control panel supplied by the video card drivers. If this extra control wasn't supplied, there is very little you can do to change the refresh rate. To change the refresh rate of your monitor in Windows 98 and above, reducing the potential for eyestrain, follow these steps.
Right-click on an empty area of your desktop and select Properties from the menu that pops up. Click the Settings tab, then the Advanced button. The Adapter tab contains a drop-down list of all available refresh rates supported by the combination of your card and monitor. Be aware that this list will not be present if Windows does not know what kind of monitor is being used.
The Monitor tab contains the brand and model of the monitor Windows thinks is connected to the system. If it says "Generic Monitor" or "Unknown," changing the refresh rate is disabled. Why? Because running a monitor at a refresh rate for which it wasn't designed can actually cause the monitor to destroy itself. (Be aware that changing the refresh rate may also cause the position and size of your desktop to shift within your monitor's display area. You will have to use the positioning controls on the monitor's front panel to re-center the display.)
Note: The eye's retina is essentially divided into two zones: the central vision area (macula) and the peripheral vision area. The central vision can discern fine detail, has long persistence, but isn't very sensitive at low light levels. The peripheral area is just the opposite. The persistence of the peripheral area is very short, so that a monitor display that does not seem to flicker when looked at directly, will flicker horribly when looked at askance, especially in poorly lit rooms or office cubicles.
There is also a sub-level flickering that is produced when two flickering objects, one flickering slightly slower than the other, both make their impressions on the retina. The resultant flicker is about two to three flashes per second and is highly discomfiting. To reduce this sub-level flicker, use a bright incandescent lamp placed off to the side. Fluorescent lamps are the worst back illumination one can use in a computer office. The best is sunlight.
