mental ray supports use of IES and Eulumdat light profiles. Light profiles are made available by vendors of physical lamps to describe precisely how much light is emitted in which direction. Light profiles are generated by measuring emitted light at the grid points of a grid placed around the light. For any light direction, this allows determining the light energy by interpolating the nearest grid points. Eulumdat is a European format while IES is an international standard.
mental ray can parse IES and Eulumdat light profiles when used and store them as an approximated grid for efficient lookups. Light profiles are a new shader parameter type that a shader can access like any other parameter. Specialized shader interface functions allow the shader to extract the brightness for a given direction from the profile parameter.
Light profiles assume that the light is an exact point light, and guarantee only that the IES/Eulumdat computation is correct at a sufficient distance. However, mental ray also allows assigning standard area light source definitions to the light source in order to give a credible approximation at close distances, such as soft shadows which are not achievable with light profiles alone.
mental ray supports light profiles directly specified on any light source in the scene description. There is no need to use a special shader knowing about light profiles.
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