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V-Ray's extra attributes for the Surface Luminance node allow control of luminance calculation.

The most common workflow for the Surface Luminance node attributes is to connect the node to a floating texture input slot of

Luminance Mode – Controls how luminance is calculated. 

Disabled – No luminance is calculated.
Direct Only – Calculates luminance from direct lighting. It creates a grayscale mask over the points of objects lit by direct lighting.
Diffuse Only – Calculates luminance from indirect lighting (GI). It creates a grayscale mask over the points of objects lit by indirect lighting. 
Direct+Diffuse – Calculates luminance from direct and indirect lighting. This creates a grayscale mask over the points of objects by direct and indirect lighting.
Matte Shadow – Creates a grayscale mask over all shadowed points of objects. Unlike the standard Matte Shadow render element, this mode creates a two-sided mask covering both the lit and unlit points of the objects.

Subdivs – Controls the subdivisions for the diffuse lighting, when the Luminance Mode is set to Diffuse only or Direct+Diffuse.

Contribute to GI – When enabled, the direct light contributes to the GI calculations. The effect is visible only when the texture is used for diffuse calculation with either