6. EFFECT OF DISTANCE FROM LIGHT SOURCE

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

1. Point source: 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. Infinitely large source: 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

In real situations this means that the fall-off of light energy is close to an inverse square relationship for small light sources, and less rapid for very large light sources. This fall-off applies to linear (light energy) units, so you need to convert to nonlinear (perceived brightness) units if this is the kind of unit you are using. The table below uses nonlinear conversion used to calculate the nonlinear units in which RGB "brightnesses" are expressed in graphics programs such as Photoshop.

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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