U

UGR

Unified Glare Rating. The stipulated value of UGRL states the glare limitation which must not be exceeded in an application. See also glare.

Uniformity

(of illuminance). Uniformity of illuminance is often required by photometric regulations for interior lighting to ensure a harmonious lighting situation. The formula symbol U0 defines the quotient of spatial minimum value and spatial average value of illuminance (see also average illuminance) in the visual task area to be analysed.

Utilance

(U) Ratio of the effective luminous flux ΦU incident on the effective area of a lighting installation to the sum of the luminous flux of all luminaires  in the room ΣΦL. It is also the quotient of Utilisation Factor FU and light output ratio RLO:

Utilisation factor

The Utilisation factor FU is the ratio of the effective luminous flux ΦU incident on the effective area of a lighting installation to the sum of the combined luminous flux of all lamps in operation within the lighting installation  Σ Φ0. It is also the product of Utilance U and light output ratio RLO.