6. EFFECT OF DISTANCE FROM LIGHT SOURCE

The rate of decrease in the intensity of light with distance from the light source depends on the size of the light source. All real situations lie between two extremes:

1. Moving away from a point source of light, the amount of light energy diminishes according to the square of the distance (the inverse square law)
2. Moving away from a light source of infinite extent (i.e. an infinitely large wall of light), the amount of light energy does not change, irrespective of distance

This means that the actual fall-off of light is close to the inverse square relationship for small light sources, and less for very large light sources. This fall-off applies linear radiance, so you need to convert to nonlinear brightness (which you can manipulate in Photoshop) if you want to try to get this right quantitatively.

Table 10.1. Relative fall-off of radiance and brightness with distance from a point source of light.

Figure 10.13. Fall off of brightness with distance, calculated using the proportional reduction of brightness with distance according to the inverse square law given in Table 10.1.

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