Special Demands of VDU Work on Vision
Compared to traditional office work, there are several reasons that can make computer work more demanding on the eyes. As mentioned above, probably the most important reason, is the greater intensity of the work. Many more functions can be performed within the same amount of time, and there are fewer reasons to direct your attention elsewhere (e.g. go looking for an eraser, different coloured pencil etc.) that would simultaneously give the eyes a short break.
This would especially affect new computer users for several reasons. Firstly, it would take a certain period of time before one becomes familiar with new computer applications and this period may be quite intensive. A new job may be quite stressful for some operators. New computer operators also lack the knowledge of more experienced operators who generally learn to use their eyes more efficiently. They achieve this by learning habits such as regularly looking away from the computer screen at suitable moments, interspersing computer work with other duties that are less visually demanding etc.
Another reason why computer work can be more demanding on the eyes, relates to the slightly lower legibility of characters on a computer screen compared to printed material. Characters on a computer screen are formed by tiny dots called pixels and are not formed by continuous lines, as is printed material. For this reason, characters on a computer screen need to be slightly larger in order to make recognition equally as good as with printed characters.
If the contrast or brightness needs to be increased to high levels, this leads to an increase in the size of the pixels with light spreading around their edges. This is referred to as "blooming", and results in characters that appear blurred and out of focus. High levels of office elimination would reduce the apparent contrast of characters on the screen and may necessitate the operator to increase the contrast or brightness to such high levels that may cause "blooming".
Some operators are sensitive to the "flicker" which is produced by the way in which video display units work. In order for the fluorescent pixels to glow, they have to be constantly activated by an electron beam within the VDU. The rate at which the pixels are activated determines whether a given individual will sense a flicker of the image on the screen. Some individuals are more sensitive to flicker, especially when using positive image display screens (dark letters on a white background).
Other factors that can increase the demand on the visual system, and which are particular to computer screens, relate to the presence of a glass screen in front of the characters. In the presence of poorly laid out office illumination or inappropriate positioning of computer screens, reflected light from the glass screens can be distracting and can reduce the quality and contrast of the image on the screen. To overcome this problem of glare, the most effective way is to remove or minimise the source of the glare. This often involves modifying the relative positioning of the light sources and the computer screen. If this cannot be achieved, one solution is to use a black hood over the VDU. Using a light background with dark characters on the screen and wearing darker (less reflective) clothing also help to reduce screen glare. Another solution is to use anti-glare screens (filters).
There are several different anti-glare filters available on the market. The neutral density filter is the most common type. This reduces the amount of light that hits the screen to about 15% to 25%, and thus reduces the amount of reflected light. However, the disadvantages are that specular or mirror-like reflections are not effectively reduced, and also, the brightness of the screen is necessarily reduced. Directional type of filters act as small venetian blinds, blocking light that reaches the screen from the operators sides. However, the very presence of these directional changes in the anti-glare screens can affect the observed image quality. Circular polarizer filters can effectively reduce both specular and diffuse reflections, however, these types of filters are more expensive.
It must be pointed out that all types of filters add another interface between the observers eyes and the computer screen. Whilst they all help to reduce glare in their own way, they also introduce another source of glare albeit much reduced due to the use of anti-refection coatings which are applied on most filters nowadays. They also need to be kept particularly clean as any type of dirt marks on anti-glare filters tend to be distracting to computer users. This is a common complaint amongst users of anti-glare screens. Dirty computer screens in general, is another reason for reduced legibility of characters on computer screens.
For the various reasons given above, i.e. the form of the characters, and to a lesser extent, glare and potential for soiling of glass screens etc., characters on a computer screen are generally of lower legibility than printed material. This would generally place a greater demand on the focusing system of the eyes, as it would necessitate more accurate focusing and the highest possible performance of ones visual system.
When one considers both the intensity of computer work (as referred to above)
and the extra difficulty involved in the recognition of characters
on VDUs compared to printed material, one can understand why
eyestrain is considered to be the single most common health
problem in modern day offices. As explained above, the adverse effects
of VDU use on the eyes can be both temporary and permanent. It is
thus essential that all computer operators become aware of the likely
problems that they may be faced with, if the proper precautionary
measures are not taken. The most important of these preventive measures
is simply the regular shifting of the focus onto a distant object
every five to ten minutes. Please refer to recommendations below
for a more detailed explanation of proper use of the eyes to prevent