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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.