Coefficient of utilization

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 luminous flux from a luminaire incident upon a work plane to that emitted by the lamps within the luminaire. Luminous flux is usually measured in lumens.

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.

See also

• Luminous efficacy
• Luminous efficiency
• Wall-plug efficiency

SI photometry unitsv · d · e

Quantity	Symbol[nb 1]	SI unit	Symbol	Dimension	Notes
Luminous energy	Qv [nb 2]	lumen second	lm⋅s	T⋅J	units are sometimes called talbots
Luminous flux	Φv [nb 2]	lumen (= cd⋅sr)	lm	J	also called luminous power
Luminous intensity	Iv	candela (= lm/sr)	cd	J [nb 3]	an SI base unit, luminous flux per unit solid angle
Luminance	Lv	candela per square metre	cd/m2	L−2⋅J	units are sometimes called nits
Illuminance	Ev	lux (= lm/m2)	lx	L−2⋅J	used for light incident on a surface
Luminous emittance	Mv	lux (= lm/m2)	lx	L−2⋅J	used for light emitted from a surface
Luminous exposure	Hv	lux second	lx⋅s	L−2⋅T⋅J
Luminous energy density	ωv	lumen second per metre3	lm⋅s⋅m−3	L−3⋅T⋅J
Luminous efficacy	η [nb 2]	lumen per watt	lm/W	M−1⋅L−2⋅T3⋅J	ratio of luminous flux to radiant flux
Luminous efficiency	V			1	also called luminous coefficient
See also: SI · Photometry · Radiometry

1. ^ Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual") to avoid confusion with radiometric or photon quantities.
2. ^ ^{a b c} Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ or K for luminous efficacy.
3. ^ "J" is the recommended symbol for the dimension of luminous intensity in the International System of Units.