It’s probably stating the obvious but anyone engaged In lighting out of doors should be as familiar with the ‘natural’ light sources as with artificial sources used indoors. On a clear day the basic light source is of course the sun, plus skylight, with skylight being blue. Figure 17.1 Illustrates the lighting levels typical of a fine sunny day. It is worth asking the question ‘why Is the sky blue?’ Figure 17.2 shows that when direct sunlight enters the Earth’s atmosphere the shorter wavelengths (blue light) are scattered by the particles In the Earth’s atmosphere. Hence we see the ‘sky’ as blue.