
Light Functions are, in essence, a Material that can be applied to filter a light's intensity.
It should be noted, however, that you cannot change the color of lights using a Light Function, only the Light Color setting can do that. Also, Static Lights cannot use Light Functions, as this feature is only available for non-lightmapped lights, like Stationary and Movable lights.
Since you are primarily using Light Functions to change the overall shape of a light, it is possible to confuse them with IES profiles, which are shown in Example 3.1. However, IES profiles are significantly cheaper to calculate, and should be used instead of Light Functions whenever possible.