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Light Filters are used to manipulate lighting results. In the real world this is often achieved through the use of stencils and gels to change the color, quality, and amount of light in a shot. RenderMan's light filters are a powerful and flexible way to do the same (and even more) in lighting your scenes.
Light Filters do not affect photon emission in the PxrVCM or PxrUPBP integrators.
PxrIntMultLightFilter is a light filter that allows you to multiply the intensity/exposure of the light. This is very useful when you want to isolate a particular asset(s) from the rest of the scene that has different intensity/exposure. This is via linking the objects to the PxrIntMultLightFilter.