Lighting
Room index for utilisation data
- where
- room index
- room length (m)
- room width (m)
- luminaire height above the working plane (m)
Explain this equation
Gives a single number describing the shape of a rectangular room. It is used to look up the utilisation factor in a manufacturer's table for the lumen method.
L and W are the room length and width in metres. H_m is the height of the luminaires above the working plane (often desk height, about 0.75 to 0.85 m), not floor to ceiling. K_r is the room index, a plain number.
L × W is the floor area receiving light, and H_m × (L + W) is half the wall area between the luminaires and the working plane. Light striking the walls is partly absorbed, so the ratio shows how favourably the room delivers light to the work. Large, low rooms score high; small, tall rooms score low.
An 8 m by 6 m room with luminaires 2.5 m above the working plane gives about 1.37. Values typically run from below 1 for small tall rooms to 5 or more for large open areas. A higher index gives a higher UF.
Watch out: don't use ceiling height unless the working plane is the floor. The index alone gives no illuminance.
Room index summarises rectangular room geometry for selecting utilisation-factor data. A wide shallow space behaves differently from a narrow deep one.
Measure mounting height from the working plane, not automatically from the floor. Use the resulting index with the manufacturer's reflectance and utilisation tables.
Enter your values to see the result.
Common mistake
Do not substitute ceiling height unless the luminaires are at the ceiling and the working plane is at floor level.
Limits
Rectangular-room lumen-method geometry. Correct reflectances and manufacturer photometric data are still required; the index alone does not give illuminance.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.