(Abbr. CU) Portion of rated bare-lamp lumens that exit the fixture and reach the work plane. The CU accounts for the light directly from the luminaire as well as light reflected directly from the luminaire as well as light reflected off the room surfaces. The CU value is used in lighting calculations to estimate light levels or the number of luminaires needed. The CU is determined from a photometric test and is typically published on product catalog sheets in tabular form.
A term used for lighting appliances; the ratio of lumens received on a flat surface to the light output, in lumens, from a lamp; used to evaluate the effectiveness of luminaries in delivering light.
Ratio of luminous flux (lumens) from a luminaire received on the "work plane" [the area where the light is needed] to the lumens emitted by the luminaire.