Video Display Terminals (VTD) and Eye Strain

This article is intended for educational purposes only.

Work at visual display terminals (VDT) is a very common cause of visual fatigue. Most studies would agree that over 60% of VDT users experience eye symptoms while more than 20% experience musculoskeletal problems. Given the millions of people using VDT's this amounts to a very large number of people. Some experts suggest that well over 10 million people a year in the US alone seek out eye care specifically for VDT related problems.

Unfortunately few eye care professionals address VDT ailments. The purpose of this section is to "fill the void", giving the most basic concepts of why VDT's cause problems. Hopefully by understanding this, readers can adjust their work environment to increase their visual comfort at their computer.

The symptoms

Eye problems include eye fatigue, pain, headaches and focusing problems. Musculoskeletal problems usually are related to neck and back pain, with hand fatigue from keyboard use.

Eye symptoms are commonly the result of mild visual problems that become acute with heavy work at near, such as at the computer. Many people can experience relief by simply having an eye exam and wearing the proper correction (glasses or contacts).

Sometimes standard lenses cannot solve the problem and special lenses designed specifically for computers or visual fatigue are needed.

The types of preexisting eye problems

Accommodative ("focusing"): Accommodation is the ability of the eye to change focus from far to near. In order to do this, circular muscles (ciliary muscles) inside the eye constrict to allow the intraocular lens to change shape. When the lens becomes more convex it becomes more powerful, allowing the eye to focus at near. Since seeing at near takes work by the muscle in the eye, it is possible that the muscles fatigues. Just as standing all day can cause your legs fatigue and cramping, focusing at the computer may do the same to the eye muscles.

Cramping or accommodative spasm caused by prolonged near work can cause temporary blur at distance. If you look up from your computer screen and see distance objects (temporarily) more blurry than usual, your eyes are probably being overworked. This is not to be confused with myopia or true nearsightedness, which does not go away in time.

Some studies suggest that accommodative fatigue is not muscle fatigue at all but a neurological response. In effect the brain adapts to the shape and size of the words and paragraphs on the screen. This reduces the stimulus to properly focus, causing blur. Furthermore, the ciliary muscle is made of smooth muscle, which may not fatigue quickly. Regardless of the cause, one thing is for certain, near work causes eyestrain!

Accommodation diminishes with age, beginning about the age of 40 the eye can no longer comfortably focus at near. By age 50, the intraocular lens can hardly change shape at all, causing extreme blur. Presbyopia can cause eyestrain and headaches unless corrected with reading glasses.

Another factor involved in helping the eye to keep in focus is screen resolution. Lower screen resolutions provide poorer targets for the eye/brain tracking system, and can result in fatigue.

Background color of the screen can also help. Bright light constricts the pupil and increases depth of focus, leaving less work for the accommodative mechanism. Therefore white a background with black letters is probably the best combination. However, if the light is too bright the brain may see "flicker" on the screen and cause headaches. Sometimes turning the screen luminance down can decrease eyestrain.

Subtle focusing problems may be hard to diagnose in an eye exam. Unfortunately in depth accommodative and binocular vision is not performed by all doctors. Even with correct diagnoses, many focusing problems can only be corrected by visual training (eye exercises) and not glasses. To make matter worse most insurances will not pay for a costly course of visual training.

Accommodative fatigue can be serious as people can perform poorly at work because of severe eye fatigue. Visual training does work, but finding a professional that is good at it may be the hardest challenge. Ironically, it is becoming a lost art when needed the most.

Refractive errors

These are easy to correct by even the most cursory eye examination.

Myopia (nearsightness)

Small amounts of myopia may actually reduce eyestrain on the computer as the rays of light from a near object (the screen) may focus on the retina without any help from the ciliary muscle. However, this is usually not the case as unequal myopia between eyes may cause an imbalance (the brain cannot focus each eye independently) and the blur in one eye may cause strain. Also, if the person is too nearsighted, they may need to move their head closer to the screen to focus. This can cause neck or back strain due to posture. Many myopes also have astigmatism, which causes blur both at distance and near. This loss of resolution may cause problems too. (See myopia for optics detail)

Hyperopia (farsightedness)

When a hyperope views an object at distance she needs to use her focusing mechanism to converge the rays of light from behind the retina to focus on the retina, in contrast to the person with no refractive error whose accommodative system is completely at rest. Hyperopia causes an added work load which can tax the focusing mechanism. When this person views an object at near she has to exert additional effort to see the object. Depending on the magnitude of the hyperopia, this burden can manifest as simple eyestrain to headaches to complete blur. Even small amounts of hyperopia can cause eyestrain in people whose accommodative systems are not up to the task, or who are in their late thirties, with presbyopia around the corner. (See hyperopia for optics detail)

Astigmatism

Astigmatism is like having two prescriptions in one eye. One meridian will have a distinct refractive error while 90 degrees away the same eye will have a completely different refractive error. This is usually caused by the corneal curvatures. The astigmatic cornea (clear dome of the eye) looks like a profile of a football, which has two distinct curvatures. The non astigmatic eye looks like a basketball: all meridians have equal curvature (See astigmatism for optics detail). Astigmatism causes blur at both distance and near since the eye can only focus at one plane or meridian at a time. Even slight amounts of astigmatism can cause enough blur to make the accommodative system work hard to "lock" onto its target.

compare spherical vs. astigmatic eye

Binocular vision problems

There are big thick books written about this topic as two eyes can get into a lot of trouble trying to work together. The ciliary muscles, all twelve extraocular muscles that "point" the eyes, and the brain (at numerous levels) all have to work together in perfect unison. If not, eye strain or double vision results. In this brief passage we will introduce you to a very cursory introduction to binocular vision.

Ninety five percent of people use both eyes together to "fuse" onto a single target. Five percent will have one weak eye that crosses inwards (esotropia) or outwards (exotropia). Usually this occurs at a young age as the brain adapts to the situation by "suppressing" (turning off) varying degrees of central vision in the crossing eye. This prevents double vision and lets the good eye view the object of regard clearly. Some people with slight tropias overcome their situation by increasing demand on the binocular system and not allowing the eyes to cross or turn out. These people may still "trope" (one eye going in or out) intermittently, especially when they are tired or when the visual system comes under strain, like prolonged near work at a VDT.

Most people's eyes have slight misalignments that must be overcome to focus on a single point in space. These imperfections are called "phorias" (who names these things anyway?), which involve both eyes working together. In their position of rest, for instance when the eyes close, the eyes may turn inwards (esophoria) or outwards (exophoria). The higher the phoria, the harder the binocular system must work to overcome it to align the eyes.

Phoria's are easy to test: Simply have a friend focus at the tip of a pen while you cover one eye, then quickly shift the cover to the other eye. When you do this, observe the eye that has just been uncovered. In most people you will notice a slight "jump" of the eye to focus back on the pen (the "patient" will note that the pen "moves" also). This happens very fast and takes practice to see.

Now for complication: Everything you have read so far, from accommodation to phoria, are linked together. For instance, when the eye increases accommodative amplitude (focuses for near), the eyes automatically turn inward or converge. This makes sense, because when you look at near you need your eyes to converge AND focus in order to see an object. But what about the hyperope who has to over focus for everything? Her brain tells her eyes to turn in too much, which can cause a high esophoria (inwards tendency) or even (accommodative) esotropia (crossed eyes). Correcting the hyperopia with glasses will help realign the eyes by relaxing the accommodative system and moving the eyes more outward towards alignment. But then the convergence system may be weak because the accommodation was always stimulating it, so now the she has eye strain again. Another example: If the accommodative system is weak reading glasses may help - but what if the person is an exophore (eyes want to go outward) also? The reading glasses reduce the extra amount accommodation demand used due to hyperopia. This can make the eyes go out even more (due to the linkage of accommodation and convergence - less accommodation with glasses, less convergence), causing even more problems. Prisms can be used to allow the eyes to continue turning outward when reading, but the brain sometimes adapts to the prism and makes the eyes turn out more.

As you can see, treating these problems can be quite challenging. This is not an exact science either, so some guess work is involved. Again, it takes time and patience to address these issues in an eye exam, and many doctors are forced to forgo in depth analysis in a routine exam.

VDT specific problems

Dry eyes

When a person looks at a VDT screen the eyelid is much more open than when looking down and reading a book. Since about twice as much eye surface is exposed when looking at a VDT, the eyes are more likely to dry out (see diagram). Also, people blink about half as much when looking at a computer screen as when reading a book. Both of these factors combine to make dry eyes a very common complaint of VDT users.

dry eyes are affected by the angle of your eye when reading books, computer or mobile devices

Lighting

Direct sources of lighting should not be directly in view when looking at the VDT. The Illuminating Engineering Society has established that illumination between the screen and immediate surroundings should not exceed a threefold difference in intensity and a tenfold difference should not be exceeded between the screen and the entire field of view.

These rules make sense; as everyone knows what it is like to walk out of a dimly lit room into bright sunlight ... it is uncomfortable. The same holds, on a smaller scale, for gazing at different intensities within a room.

Helpful hints

Indirect light whenever possible, try to cover windows or a least don't have one facing you or directly in back of you. If lighting is too bright overhead wear a visor or put a dark tint on the top third of the lenses on a pair of computer reading glasses. Change the brightness of the screen to match the surrounding environment. Try to keep the monitor brightness to the brightness in the room. Winning the lottery and stop looking at the computer screen all day are also things to consider.

Reflections can also cause eye strain. Your computer screen is a major source of reflections from other light sources in the room. Many high end computer monitors have reflective coatings on the front of the screen, decreasing some of the reflections. Unfortunately, most of the reflections come from the phosphorus layer on the BACK of the screen, and the glare coat does nothing for this. These reflections can decrease contrast and increase demand on the visual system.

Glasses with an antireflective coat do not help much for reflection off the screen. Glare screens, on the other hand, can help quite a bit. There are several types of glare screens available.

Glare screens

Looking much like a screen door, mesh glare screens do not allow oblique light from light sources in the room to hit the monitor. This greatly reduces reflection. The drawback is that you have to look through the mesh to see your monitor screen! These mesh screens are less expensive as glass, they work and are worthwhile in of themselves, but the extra cost of a quality glass screen is probably worth it. Taking the amount of hours spent in front of the computer, the extra cost becomes insignificant if every day for years you will have less eye strain.

Glass/plastic glare screens filter out reflection by cutting down on light hitting the monitor from external sources, and the again cutting down on the light that does get in and reflect off the monitor back to the user. Light from the monitor only travels through the screen once, while reflections need to travel through twice (it "absorbs" reflections). The one drawback is that the monitor screen is darkened somewhat. This can be overcome by most good monitors by turning up the illumination setting. Some glare screens have a Polaroid screen. This is a good addition, especially if you have a few "hot spots" on your screen where you can actually see the reflection of a light source (i.e.: a window, or bulb).

Plastic thin Film glare screens that stick onto your screen are very handy, such as for laptops. Many are high tech and include "privacy filters" so people next to you cannot see your screen. Some people complain the privacy filter aspect may increase eyestrain by decreasing the luminance. Also, true anti-reflective qualities may be harder to transfer onto a thin film than a hard piece of glass or resin (plastic).

These screens can significantly improve contrast on your computer monitor, and therefore make your eyes work less. A good way to see if reflections are bothering you is to take a file folder and hold it between your computer screen and light sources in the room, see if this makes viewing the screen more comfortable.

Anti-Glare Coatings for your glasses

A Crizal anti-glare coating will greatly improve your comfort at the computer. The same reflections that will bother you from your computer screen will bother you from your glasses! Untreated lenses will reflect light from behind you and will internally reflect (scatter) light from in front of you. In fact you can lose up to 15% of light transmission (scatter) with some lenses uncoated versus less than 1% with a coating! A high quality coating will put very hard zirconium (artificial diamond) scratch coating on the lens to reduce scratches that produce glare and distortion.

VDT screens, helpful facts

  • Better resolution equal less eye strain. Smaller dot pitch is better (< 0.28)
  • Rule of thumb: Letters on screen should be large enough that you can just barely see them at 3 times your usual viewing distance. Example: if you usually view your screen at 18 inches, you should just be able to barely read your monitor at 54 inches.
  • Screen Flicker: Most people can see flicker up to about 50Hz. Better monitors have less detectable flicker of 70Hz and above. Bright light sources will increase sensitivity to flicker. Therefore if you are noticing your screen flickering, then LOWER your screen brightness. Even if you do not see the screen flickering it may bother you, it really pays off in the long run to have the fastest flickering monitor affordable (72Hz+).
  • Fluorescent lights may also cause headaches if a person is sensitive enough to see them flicker. A pink tint on reading glasses may help this as it cuts out some of the blue light, decreasing brightness and thereby decreasing the flicker sensitivity. Also, blue light causes blur as it focuses in front of the retina.
  • Black on white is probably the best color combination for screen viewing.

Work Environment, hints

  • Distance from eyes to computer: Over 50 cm the best, make sure that you can (at least) just barely read the text at 150 cm.
  • The top of the screen should be around nose level. It is easier to look down than up, and those that need progressives or bifocals will find considerable relief with this.
  • Your head and body should be at a facing forward and not twisted to see hard copy etc.
  • Hard copy is best located directly below the screen inclined at a slight angle. Worst is flat on the desk to the side.

See Also

Essilor Computer and Visual Fatigue Lenses

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