It is outside the scope of this book to describe in any great detail the basic technical concepts behind lighting principles. It is sufficient here to state that the unit used for the intensity of illumination at a particular surface—that is, the quantity of light falling on a surface from different light sources—is the lux (lumen per square metre or lumen/m-2).
An ordinary candle at a distance of 1 metre gives an intensity of about 1 lux; at 2 m distance, 1/4 lux; at 3 m distance, 1/9 lux; and so forth. In other words, the light
intensity is inversely proportional to the square of the distance. Sunlight can give light intensities of up to 100,000 lux. The luminance is the ‘lightness’ of a surface. This may either be a surface that lights up itself, for example, a VDU screen, a light, or a surface that reflects light from other light sources. The usual unit for luminance is the candela per square metre (cd/m2). Table 7.2 summarises a number of technical terms, units, and relationships.
There are three main areas of human requirements that must be considered when designing lighting installations:
1. Injuries (safety)
2. Performance effects
3. Comfort
These requirements are usually considered in other types of ergonomic dimensioning work. Both quantitative and qualitative aspects of these three demands will be examined in relation to the design of lighting installations.
