Light Extra Attributes
The V-Ray Additional Attributes can be added to every Maya standard Light by selecting its shape node and selecting Light attributes from the Attributes>VRay menu. This will add several attributes to that light which allow you to control V-Ray specific options for the light.
Photon subdivs - controls the number of diffuse photons traced for the light. Larger values mean a more precise photon map, but will take more time and memory.
Diffuse Multiplier - a multiplier for the diffuse photons
Caustic subdivs - this option controls the amount of photons that V-Ray will trace to estimate caustics. Large numbers slow down the calculation of the caustics photon map and may take more memory.
Caustics Multiplier -this value is a multiplier for the generated caustics by the selected light. Note that this multiplier is cumulative - it does not override the multiplier in the Caustics render roll-out section.
Shadow bias - moves the shadow toward or away from the shadow-casting object (or objects). If the the value is too low, shadows can "leak" through places they shouldn't, produce moire patterns or make out-of-place dark areas on meshes. If the Shadow bias is too high, shadows can "detach" from an object. If the the value is too extreme in either direction, shadows might not be rendered at all.
Cutoff Threshold - this parameter specifies a threshold for the light intensity, below which the light will not be computed. This can be useful in scenes with many lights, where you want to limit the effect of the lights to some distance around them. Larger values cut away more from the light; lower values make the light range larger. If you specify 0.0, the light will be calculated for all surfaces.
Override motion blur samples - allows you to overide the default number of samples that will be used to sample the current light for motion blur.
Motion blur samples - when the Override motion blur samples is enabled this value will be used when sampling the motion blur created by the current light.