IlluminationThe Illumination shader performs a simple but important task: providing a physically accurate lighting model for light sources. This can mean gentle and gradual falloff curves for dim lighting, or bright and overexposed central areas in bright lighting. Many times CG artists avoid realistic lighting because of its tendency to create areas of overexposure, which often look unnatural due to their hard edges. The Glare shader corrects this problem, creating overexposures that look realistic; by using Illumination in combination with Glare, natural lighting effects can be easily achieved.
Brightness Multiplier allows the artist to turn-up the light's brightness well past 1, likely causing overexposure in some parts of the image. Falloff With Falloff the artist can set how he wants the light to behave as it falls off from the source. None and Linear: None tells the light to illuminate all objects with the same brightness regardless of distance, while Linear provides a linear fall off. Realistic uses the r-squared law for fall off. Unlike the Linear falloff, Realistic fall off causes nearby objects to be extremely bright, while at the same time distant objects never quite become completely black. Custom let's the artist specify a custom curve for the light falloff. Start specifies the distance at which the light begins to diminish, Mid specifies the distance at which the light is at half intensity, and End specifies the distance at which the light becomes completely black. (See the discussion on Custom Curves at the end of this chapter.)
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