Coefficient of utilization

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A coefficient of utilization (CU) is a measure of the efficiency of a luminaire in transferring luminous energy to the working plane in a particular area.

The CU is the ratio of lumens from a luminaire incident upon a work plane relative to the lumens emitted by the lamps within the luminaire.

A CU (usually expressed as a percentage) is the luminous flux received on working plane. For example, some of the light emitted by a luminaire may exit away from the desired plane and is therefore wasted. A CU measures the light actually reaching the desired plane as a percentage of the total light produced by the fixture.