In certain situations a minimal level of lighting is required on older types of dial and meter instruments because, among other things, the eyes have to be dark – adapted while using them. Figure 7.6 shows the possible reading errors due to poor luminance on the instrument face. They all should have the same level of lighting. A ratio of 7:1 can be allowed between the most strongly and the most dimly lit instruments.
Apart from instrument lighting, all direct light towards the operator’s eyes must be avoided. Table 7.3 gives recommended values for instrument lighting.
There are two principles in the lighting of instruments, direct and indirect, and these will be discussed further.
7.3.1 Direct Lighting
This method of lighting is independent of the instrument itself. The lighting can be positioned over the instrument and in this way reduce reflections from it. The reflected light is directed downwards. The advantages of this method of lighting are:
1. Lighting is even.
2. Controls can also be lighted.
3. The space between the instruments is lit.
4. Broken bulbs can easily be changed.
The disadvantages with this method of lighting are:
1. It may be difficult to position the lamp so that it does not block the area of view.
2. Shadows may be produced by the edge or pointer of the instruments.