Lighting of Dial and Metre Types of Instruments

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.

TABLE 7.3 Recommendations of Illumination of Instruments Recommended Use Lighting Technique Luminance of Markings (cd/m) Adjustability Meter reading, dark adaptation necessary Red directed light, direct or indirect, operator’s choice 0.07-0.2 Continuous Meter reading, dark adaptation not necessary but desirable Red or white light with low colour temp., direct or indirect, operator’s choice 0.07-3.4 Continuous Meter reading, dark adaptation not necessary White directed light 3.69 Fixed or continuous Reading of text on panel, dark adaptation not necessary Red built-in light or directed light or both, operator’s choice 0.07-0.3 Continuous Reading of text on control panel, dark adaptation not necessary White directed light 3-69 Fixed or continuous Operator may be exposed to sudden bright light sources White directed light 34-69 Fixed Very great heights, minimal daylight due to design of cockpit White directed light 34-69 Fixed Diagrams and chart reading, dark adaptation necessary Red or white directed light, operator’s choice 0.3-3.4 Continuous Diagrams and chart reading, dark adaptation not necessary White directed light 17-69 Fixed or continuous

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.