PLEASE READ THIS: The information contained herein is not intended as a substitute for medical advice and care from qualified, licensed health care providers. The reader should regularly consult a physician in matters relating to his or her health and particularly with respect to any symptoms that may require diagnosis or medical attention. The information is presented here to educate and inform and to guide you to an understanding of cause, prevention as well as treatment.
By E. Lawrence Bickford, O.D.
The use of computers and the associated video monitors presents a situation which may cause a certain degree of "eye discomfort" for the user. Sooner or later, almost all users will experience "computer related eye strain" (CRES).
This report will explain the issues behind this phenomena, both the causes, symptoms, treatments and suggestions to remedy this condition.
Many thanks to those readers who have written with their comments and technical suggestions and have contibuted to this report!
Your feedback is always appreciated!
1. the distance from your eyes to monitor
2. the angular position of the monitor relative to the line of site
3. the brightness of the image on the screen
4. the contrast of the image on the screen
5. the size of the image
6 the color of the image and the color of the background on the screen
7. the quality of the image which relates to:
* resolution: the number of pixels per unit area and dot pitch
* interlaced vs. non-interlaced image transfer
* screen refresh rates
* screen focus
8. the ambient (surrounding) room illumination
9. air-conditioning and dry eyes
Some of these items (notably the first four) are user adjustable. The remaining are related to the manufacturing of the hardware and environmental conditions.
1. difficulty maintaining clear focus on screen image, unstable
2. distance vision blurred after looking at monitor
3. burning and/or itching feeling
4. dry, sandy feeling
5. aching and "tension" in the brow and forehead area
7. neck ache
Eyeglasses and computer use
These issues are discussed based on the assumption that, if you normally use vision correction (eyeglasses or contact lenses), it is current and correct. The use of an incorrect vision correction can certainly contribute to symptoms of CRES.
The human vision system is designed to be at rest when viewing a distant image at optical infinity, generally a distance further than about 6 meters (20 feet) from the eyes. When you view a closer image, the crystalline lens inside your eye adjusts to focus on the near target. Each of your eyes turn inward so that they are aligned on target, and the iris pupil constricts . A total of seven muscles and six major cranial nerves are involved in this process, accounting for approximately 25% of the total energy output of the brain.
The neurological issues involved in processing an image into electrical signals that go to the brain is yet another issue. The photoreceptors in the retina in your eye use brightness, contrast, size and line/edge/border information to create an image. Computer screens have varying degrees of brightness and contrast and no "real" lines with sharp borders! Pixels are small squares lined up edge to edge with spaces between them. And the dot pitch finally defines the effective screen resolution. A line drawn on a screen is actually a series of dots. There is no coherent sharp edge. A line drawn on a piece of paper is a solid structure with a sharp edge. On the computer monitor, your eye and brain are forced to "imagine" that the series of pixels is actually a solid object, creating a very unusual and unexpected, difficult to process set of neurological messages.
As you can see, near vision tasks require a high degree of coordination and energy output. Anything that disrupts this system may result in discomfort.
Now let's talk about ergonometrics---your body position in relation to the monitor.
Here's what can you do to maximize your body's efficiency at using your eyes in this hostile environment:
Control your workspace
1. Adjust monitor distance
For most people, there is a distance somewhere between 56 cm (22") and 91 cm (36") that is most comfortable for the vision system. Try positioning the monitor and a given distance and experience that difference for at least 20 minutes. Move the monitor further out in 2.5 cm (1") increments and repeat until you find the place that feels best for your eyes.
2. Adjust screen image size
As the monitor is moved further from the eyes, a larger image size is often required. There is, unfortunately, a point of diminishing returns. As you increase the image size, you decrease the total amount of information observed on the screen, which may make your work more difficult.
3. Adjust monitor horizontal position
A monitor at 60 cm should be located as follows: Measure a line from the top of the screen to a point on the center of your forehead between your eyes. The screen should be slightly below the horizontal, creating an angle of about 5-10 degrees, although some people are comfortable with viewing a screen as much as 20 degrees below the horizontal. this depends upon the size of your face and the size of monitor. This angle decreases as the distance from the eyes increases, and visa versa.
4. Adjust brightness and contrast
Generally, the higher the contrast relative to the brightness is best. Setting at high contrast and medium brightness is best for most people.
5. Screen color
Black on white is usually best, as this creates the highest contrast. People with certain visual deficits, however, will find amber or green background screens with white images more comfortable.
6. Ambient lighting
This is a common and often overlooked problem. Some light in your working environment is critical for viewer comfort, Fluorescent lights create additional screen glare and conflicting flicker. Standard fluorescent flicker at 60 hertz. Combine that with a monitor flickering at 60 hertz and you can have trouble.
Two ways around this problem is to use electronic ballast fluorescent (much higher flicker frequency), incandescent lighting and/ or monitors with higher refresh/flicker rates. Natural lighting from outdoor sunshine is, obviously, the best option.
7. As "cool white" fluorescent tubes are color skewed toward the blue end of the spectrum, eyeglass lenses or screen filters with a 10% pink/rose tint can offset and neutralize this effect.
8. Screen image quality:
Finally, there is the issue of image stability on the monitor. This relates to flicker/refresh rates, intrelace, pixel count and dot pitch. The "easiest on the eyes" monitors have the most stable images. LCD screens do not flicker and are quite stable, but unless you have a high resolution screen with high pixel count, the line/edge resolution, lacking in any event, becomes more of an issue than the flicker. Conversely, a CRT monitor with a high refresh rate, high dot pitch, at low pixel count is likewise a poor combination.
Most of the better screens will offer high speed multiscan, interlace/non-interlace options, 75+ hertz refresh rates, high pixel counts and low dot-pitch (.28mm or less). And they may cost a thousand dollars and more! If you're setting up a new business or home system and you are going to be spending a lot of time in front of the monitor, invest heavily in the very best. Your eyes deserve it.
Comments from some readers of this report suggest that you should look for, in order of importance: lowest dot pitch (and never more than .30mm), horizontal refresh rates of at least 70hz, user *easily* adjustable and, when possible, programmable contrast and brightness controls, accessable screen focus, and pixel counts appropriate for the task (word processing vs, graphics, for example).
And remember to check screen focus! This is often an internal adjustment and may require the services of a technician. Consider that, as the monitor's power supply ages, the screen focus can change. An annual "tune up" can be a worthwhile investment, especially after the first few years of use.
So, lets review what you can do in your home or in the work place, when you just don't have the option of using the very best quality monitor:
1. adjust monitor position
2. adjust brightness and contrast
3. adjust room lighting
4. Rest your eyes periodically
Take breaks at least every ten minutes. A break is defined as looking away from the screen to re-focus on a distance (6m (20f)+ distant) object for a few seconds. Every twenty minutes, get up, stretch your back and neck and look around. Move your eyes and move your body, change your position.
5. Watch out for dry eyes!
When you look at the computer monitor, there is a natural tendency toward a reduced blink rate. The less you blink, the more likely you are to experience dry eye symptoms of burning, sand-in-the-eye, heavy lids, etc. You can use tear replacement drops, but being aware of the need to blink is the real fix. The normal blink rate is averages 12 times per minute. Computer users usually blink 5 times per minute. The longer the eye remains open between blinks, the more likely the cornea is to dehydrate, burn or ache. Then, finally, you blink. But the damage, although minor and easily repaired, is already done. Your eyes sting, burn, sting and otherwise feel miserable. You tear, feel better, then start the process all over again. Eventually, the disruption to the corneal tissue causes a blurred image to go along with the other symptoms. You stop work, fall asleep and your body fixes the insult.
Much has been discussed about eyeglasses and contact lenses for CRES. Along with dealing with all the other issues, this is another step to investigate. Your eye doctor can test your eye prescription and visual function specifically at the special working distances required by your computer use. Before your eye appointment, carefully measure all of the position issue discussed above and bring this information to your doctor.
There is no easy "rule of thumb" with regard to vision correction, but here's a summery of the related issues:
* If you do not normally require a vision correction, computer eyeglasses with low power plus lenses and sometimes a light tint is often helpful. The color of the tint depends upon the screen backround color, ambient room lighting, and your prescription.
* If you use a vision correction for myopia (nearsightedness), a reduced prescription is often recommended, designed for the near working distance. Just taking off your eyeglasses or contacts for near work is generally not a good idea, with some exceptions. (The demand to focus is reduced, but the need to align the eyes is not and a conflict could result.)
* If you use a vision correction of hyperopia (farsightedness) or if you are farsighted and do not generally use eyeglasses, the extra demands of computer use often require a specific vision correction to deal with the specific demands on the vision system.
* If you have astigmatism, it is often very important to have this optical error fully corrected.
Now that you have all this information, you are prepared to analyze your specific issues and speak intelligently with your eye doctor. Please remember that computer use presents a unique set of visual demand upon your vision system. We all respond to these demands differently, with different symptoms and degrees of discomfort. Likewise, there are different remedies for these conditions.
There has been much discussion about ultraviolet (UV) and electromagnetic (EM) radiation from CRT computer monitors. The current thinking is that UV radiation is hardly an issue, as the amount of exposure is insignificant especially when compared to a few seconds out in the sunlight or even that which emitted from fluorescent lights (also very, very low levels).
EM is somewhat different story. Many of us have experienced EM effects from our monitors. Leave a floppy/diskett too close to the computer and, puff---no more data. Although there is no hard data on the physiological effects from EM exposure, many of us prefer to take the cautious approach. Remember that the EM radiation declines as the distance from the monitor increases, and by a factor of the square of that distance. At 50cm (20 inches) there is virtually no measurable EM radiation (except, perhaps, with the very largest monitors!). So keep your eyes, brain, and body as far from the screen as possible. Don't have cables carrying power sitting on your lap and take other sensible precautions.
Do consult your eye care professional for remedies and regular eye examinations.