Serious visual problems can easily arise in work on VDUs, and this can cause a lack of visual comfort, subjective eye troubles, and reduced levels of work performance. The problems can be reduced by having a suitable distribution of luminances in the room and at the workplace, examples of which are shown in Figure 7.5 and
Figure 7.7. When working with VDU screens with light text on a dark background, the lighting levels in the room must be kept relatively low. However, this must not be so low that operators cannot do other visual tasks (for example, reading printed or handwritten text). A suitable compromise is a general lighting level of 200 to 300 lux with table lighting available which can give a local light level of about 600 lux for reading difficult handwritten text or smaller printed text.
Reflections on the keyboard must be avoided. This can be achieved by leaving a light-free area over the keyboard. The size of this area is shown in Figure 7.7 and depends on the height of the light above the table.
The ceiling lights must also be placed in such a way that they do not give rise to reflections in the VDU screen. This can best be done by placing the light close enough to the screen so that the reflection occurs below eye height. Practical experience has shown that there is a limited ‘wedge-shaped’ area (see Figure 7.7) within which the lights should be placed. If the lights are installed behind this limit line, there is a risk of reflections occurring. This, however, reduces with distance, and the risk of reflection is even less if the lamps are shaded (for example, with a Paracube) so that they appear dark if viewed from the side. Many lamps suitable for use in connection with VDU work have a negligible luminance when viewed from an angle greater than 20° from the light axis.
If the limitations regarding reflections on the keyboard and in the screen are combined with suitable angles for the light fittings, the result is as shown in Figure 7.7. Lights should not be positioned within the hatched areas.
In the terminal control room, one can either choose lights that give a mainly downwards-directed light, together with sharp edge lines, or ones with ‘softer’ edge lines. If the light in the first case is viewed from a sufficiently acute angle, it will
be seen as being relatively dark and the risk of reflection is relatively small. The luminance should be a maximum of 100 cd/m2 at a viewing angle of up to 20°. The lighting efficiency of such a lamp is relatively good, but the limiting edge lines are very sharp and a small mispositioning of the screen in relation to the light fitting can cause severe reflections. If it is known that more flexible furnishing is necessary, it is better to choose a light fitting that gives softer shadows and a certain degree of indirect lighting. Table lighting should be arranged so that it does not cause reflections on the screen. This means that the table lamp must not be able to be turned in such a way that the light source can be seen. The light should instead be able to be adjusted by means of movable reflectors placed within the shade. These reflectors can then be used to direct the light towards different areas of the table.
It may be necessary to obtain special spectacles for those with visual deficiencies in order to provide them with good visual conditions. Ordinary spectacles are often designed to allow for reading at a normal distance, ca. 50 cm. Spectacles therefore need to be specially ground for reading a VDU screen at a closer distance. If there are also overview panels that have to be read at several metres’ distance, a third ground area in the spectacles is required for the trifocal or multifocal glasses. When ordering spectacles of this type, an optician must be contacted who has a special interest in this type of problem. Information on the type and nature of the work should be provided in order for the correct grinding to be carried out.