When you use photometric lights, 3ds Max provides physically based simulation of the propagation of light through an environment. The results are not only highly realistic renderings, but also accurate measurements of the distribution of light within the scene. The measurement of light is known as photometry.
For the purpose of rendering and measuring lighting, we consider light as radiant energy capable of producing a visual sensation in a human observer. When we design a lighting system, we’re interested in evaluating its effect on the human visual response system. Thus photometry was developed to measure light, taking into account the psychophysical aspects of the human eye/brain system.
To render a pixel that represents a surface, a photometric renderer must consider both the reflectance that is specified by the surface's material properties, and the luminance that results from the scene illumination.
Luminance is a function of the energy emitted by the lights in the scene. Each 3ds Max photometric light lets you specify this energy using one of three measurements:
To visualize these measurements, it helps to think of a computer monitor:
Because 3ds Max works with these physically based photometric values, it can accurately simulate real-world lighting and materials.