When doing indoor lighting level calculations, a coefficient of utilization (CU) is calculated to determine the amount of light in the room.
If an IES file is attached to the fixture, it is used to calculate the CU if possible. Not all IES files has sufficient information to calculate the CU. This calculation depends upon the wall, ceiling, and floor cavity reflectance values specified in the room definition.
If the IES does not have sufficient information to calculate the CU, or no IES file is attached to the fixture, a default value of 0.7 is used.
The CU that is displayed also includes the dirt depreciation factor set in the room and the lamp depreciation set in the light fixture schedule.