Spectacle Prescriptions For VDU Operators

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An emmetropic pre-presbyope should not have any problem in focusing the range of distances required for VDU work. The VDU screen may be situated at a distance of 50-70cm while the keyboard and often the script is usually at a closer distance of approximately 33-40cm.

Whereas 30-year-old can focus at all the necessary distances without any problem, an operator in their mid-forties will be able to read the VDU screen easily but may have difficulty reading scripts if they are positioned at a closer distance. A 50-year-old operator may have difficulty in focusing on both the VDU screen and the scripts without a spectacle prescription.

                   

Problems occur for the presbyopic operator in that a conventional near correction for 40cm is often too restrictive. For a 60-year-old, the keyboard and script will be in focus but the VDU screen will be blurred. To enable the older operator to read both the screen and the script, spectacles specially for use with a VDU may have to be prescribed.

The straight line bifocals (i.e. so-called executive (E) or D segment) are superior to round or crescent-top bifocals. They provide a greater area of useful vision and therefore minimize head and eye movements. Also, there is a reduction of 'jump'(displacement of objects) which occurs when viewing is changed from one section of the lens to the other. However, operators up to the age of 50 years may find conventional bifocals suitable, although the bifocal segment height may need to be increased to correspond to the position of the VDU screen. It is suggested that the ideal situation for viewing the VDU screen is with the eyes depressed at an angle of 15-20 degrees from the horizontal.

Older operators (over 50 years of age) may require an intermediate and near prescription. This can be provided by bifocals in which the upper part of the lens provides an intermediate prescription to focus the script if it is positioned nearer than the screen. However, if the screen and script are placed at the same distance then operators often cope well with an intermediate single vision prescription. Trifocal lenses can be used if clear distance vision is required. Particularly useful are trifocals with distance and intermediate Executive lenses with a D segment for the near prescription.

The use of progressive lenses is controversial. Some authors consider them to be less suitable than bifocals, especially when a script to be copied is placed at the side of the VDU. They have smaller reading area and can produce unwanted peripheral distortion. They also require considerable head movement for VDU work. When viewing the screen the operator usually looks through the intermediate zone of the lens, which is relatively narrow and set low, requiring the head to be tilted back. While the width of the zone may be adequate for most visual needs of daily life it is not entirely satisfactory for VDU work. Therefore some reject progressive lenses because of the forced head posture, resultant neck cramp and limitations of the intermediate and near portions size upon vision.

However, more recent progressive lenses have been designed specifically for VDU operators. These have larger intermediate areas which are set slightly higher than conventional progressive lenses and allow the centre of the intermediate vision to be in the middle of the area situated between 10 and 20 degrees below the horizontal.

Contact lenses have an advantage over spectacles in that they give rise to fewer reflections and distortions. However, problem may occur due to the fact that concentration on the VDU or any task for that matter, often results in a reduced blink rate, which will cause symptoms resembling those of a 'dry eye'. The tear film can also be affected by high temperatures and low humidity which may result in similar symptoms.

For tinted lenses, it would be best to try to control/eliminate the glare sources wherever possible before prescribing tinted VDU spectacles.