Coefficient of utilization and its factors

The coefficient of utilization (CU) or utilisation factor is the ratio of the lumens that actually reach the working plane to the total lumens emitted by the lamps:

CU = lumens received on the working plane / total lumens emitted by the lamps

It is always less than 1 (typically 0.4–0.8), because some light is absorbed by the luminaire, walls and ceiling or falls outside the working plane. It is used in the lumen method: N = E × A / (φ × CU × MF).

Factors considered while selecting CU

1. Type of luminaire and light distribution: direct luminaires send most light downward and give a high CU; indirect lighting depends on ceiling reflection and gives a low CU. Luminaire efficiency (light output ratio) also matters.
2. Room index (room dimensions and mounting height): RI = L × W / [Hm (L + W)]. Large, low rooms (high RI) give high CU; narrow, tall rooms lose more light on walls (low CU).
3. Reflection factors of ceiling, walls and floor: light-coloured, brightly painted surfaces reflect more light back and raise CU; dark surfaces lower it.
4. Mounting height: higher mounting spreads more light onto walls, lowering CU.
5. Colour and condition of surroundings and obstructions: machinery and partitions block light.

CU is read from manufacturers' tables for the chosen luminaire, entering with room index and reflectances.