Photometric lighting related info in Maya 2011

mia_photometric_light - new in 2011
This shader takes all the guesswork out of balancing the photon energy with the light intensity by performing a numerical integration of the chosen intensity distribution to calculate the proper photon energy, as well as taking care of adapting the photon density to the distribution. This is all done automatically.

To use this shader, simply use the same instance of the shader as both light and photon shader on a light source. The light must have an origin (in other words, the shader does not work on an infinite light source). The light must be set to emit photons, and must be given a non-zero energy value (although the actual value will be overriden by the shader, a non-zero energy value is required; otherwise mental ray for Maya will not emit any photons at all for the light). The exponent should always be 2. If the light is a spotlight, a spread value should be used.

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A nice application example by Jakub Sporek
http://www.fushal.net/2010/12/true-photometric-lights-in-maya-and.html