Computer Vision Syndrome - Visual Requirements of VDT Users

VDT's (Visual Display Terminals) place a significant level of strain on our eyes. Anyone who uses one often can attest to this fact. In some studies, up to 80% of VDT users report 'Computer Vision Syndrome' symptoms of eyestrain. The reason for this prevalence comes from both ergonomic and physiologic factors.

Ergonomic factors are now fairly well understood by most VDT users: have a comfortable chair, good lighting (free of glare), quality monitor with high refresh rate, etc. Computer Vision Syndrome occurs due to the limits of our ocular physiology which are not yet fully understood by many users, which I wish to address briefly. Humans have evolved from hunter - gatherers. In the past we've hunted, fished and farmed - tasks requiring mainly distance vision. Sure we have pretty good focusing skills for near work, BUT our focusing is inherently of low endurance. Never before have we placed so much demand upon our visual system as we have in the past 30 years. We simply were not meant to perform demanding near point tasks all day long. Think of the time we spend in front of computers, video games, and television - much of it under artificial light! The stress from computers essentially comes from 1). relatively poor contrast and, 2). reduced endurance.

Reading any type of printed material will eventually cause our eyes to feel tired, but compared to staring at a computer monitor, reading isn't as hard on our eyes. Our retina (containing rods and cones) responds only to colour, motion, and borders. These are the building blocks for even the most complex visual tasks. A printed page is comprised of dark letters placed upon a white background. The contrast is high, and the "borders" of the letters (black on white) are very well defined. With a VDT, the adjacent pixels give the illusion of a line or border, but our eyes can still tell the difference. The borders of letters are not well defined on a monitor. Our brain interprets a poor border as an out of focus image. This "blur" then triggers an adjustment to our focusing muscles. Therefore, with a VDT (especially one with large dot pitch or low refresh rate) our eyes are constantly working to fine-tune the clarity of the image. Since the image never becomes perfectly clear, the focusing muscles are in a constant state of flux. They're over worked, and therefore become tired quite quickly, hence the term Computer Vision Syndrome.

Our eyes have to work more to read a VDT, than a printed page. Many people with "OK" focusing skills can read comfortably for long periods without experiencing eyestrain. However, place them in front of a VDT and suddenly, "OK" focusing skills aren't enough, and symptoms of eyestrain are noted resulting in Computer Vision Syndrome. As an eye doctor, I see many people who don't experience eyestrain except in front of the computer. It's as if the VDT is the straw that broke the camel's back of our focusing. These are the people that require specialized VDT glasses. Focusing is sufficient for most tasks, but is insufficient for long term use with a VDT. While I won't go so far as to say VDT's cause our vision to decline, they most certainly take someone who is borderline for getting glasses and pushes them over the line into the glasses. There.. did this long explanation give you eyestrain?!

More about different lens designs to help those who wear bifocals or progressives, in another article.