In Revit, photometrics are parameters for creating realistic lighting fixture families. Photometrics help to define the visible light that displays in a rendered image of a building model.
The photometrics that are available for a particular lighting fixture depend on its light source definition. They include parameters such as Light Loss Factor, Initial Intensity, and Initial Color Control.
When you specify that the light distribution of a light source is Specifying an IES File for a Light Source, you can specify an IES file. An IES file is a text file provided by a lighting manufacturer. It describes the intensity of a light source at points on a spherical grid. It also describes the geometry of how the light comes out of the lighting fixture (the photometric web).
A photometric web for a light source
Revit uses the IES file to make a photometric web to represent the light source. In general, IES files result in more accurate lighting results in rendered images. For information about the IES file format, go to http://www.iesna.org. See Specifying an IES File for a Light Source.