Light

This material is generally used for producing self-illuminated surfaces.

Color - This parameter controls the color of the light material.

Color Multiplier - A multiplier for the Color parameter.

Transparency - A texture to use as transparency for the material. Note that making the material less opaque does not affect the intensity of the self-illumination color. This is so that you can create perfectly transparent materials that nevertheless still emit light.

Double sided - Checking this option makes the object emit light from its back side as well. If this is off, the material is rendered as black on the back sides. 

Emit on back side - Checking this option makes the object emit light from its back side as well. If this is off, the material is rendered as black on the back sides. 

Compensate exposure - This option is used when rendering with the V-Ray Physical Camera. When enabled the intensity of the material will be adjusted to compensate for the camera exposure

Multiply by opacity - When enabled, the color of the light material is multiplied by the transparency texture. Otherwise, the color and opacity act independently (so-called additive transparency).