Any light visible to the camera lens has the potential to produce an optical effect. You can use the Optical FX utility to create glows, halos, or lens flares on any light source that shines directly into the camera. See for details.
Especially in animations, light sources may become blocked (occluded) by moving objects. In the real world, the gradual movement of the objects cause the optical effect of a light glow to diminish gradually. However, in computer graphics, the light glow may suddenly disappear, which seems unnatural to human eyes.
In Maya, when you add a glow to a light, a geometry object is automatically created to compute the percentage of visibility of the light to help minimize the unnatural disappearance of computer-generated glow.
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