A client asks how many luminaires a room needs. The answer comes from the lumen method. It turns a target illuminance into a count and a grid. This page shows each step and the checks that follow.
What the method does
The method works on the whole room, not on one point. It asks how much light the luminaires put on the working plane. It then asks how many luminaires give the target.
Four numbers decide the count.
- The target illuminance, in lux (lx). It depends on the use of the room.
- The floor area, in m².
- The luminous flux of one luminaire, in lumens (lm).
- Two losses: the utilisation factor and the maintenance factor.
The count is N = E × A / (Φ × UF × MF). E is the target. A is the area. Φ is the flux of one luminaire.
The working plane
Light is measured on a plane, not on the floor. For an office the working plane is a desk. The tool reads the plane height and the target for each room use. The sources are Tables 3.1 and 3.2 of AS/NZS 1680.1:2006.
The height that matters is the distance from the luminaire to the plane. A ceiling luminaire in a 2.7 m room sits 2 m above a 0.7 m desk. A taller room needs more light for the same target.
The two losses
The utilisation factor (UF) is the share of the lamp light that reaches the plane. Walls and ceiling take the rest. The tool finds UF from the room index and the surface reflectances. Pale walls give a higher UF.
The room index is K = L × W / (Hm × (L + W)). Hm is the height above the plane. A long, low room has a high K and a high UF.
The maintenance factor (MF) covers dirt and lamp ageing. New luminaires lose output over time. The design must still meet the target at the end of the cleaning cycle. The default is 0.8. A maker data sheet and Table 14.4 of AS/NZS 1158.1.2:2010 give better values.
A worked room
Take an office 8 m long and 6 m wide. The ceiling is 2.7 m. Each luminaire gives 3600 lm and uses 36 W. The beam angle is 90°. Reflectances are 70/50/20 %.
- Floor area: 48 m²
- Target illuminance for an office: 320 lx
- Height above the working plane: 2 m
- Room index K: 1.714
- Utilisation factor: 0.676
- Maintenance factor: 0.8
The count is 320 × 48 / (3600 × 0.676 × 0.8). The result is 7.89 luminaires.
You cannot fit 7.89 luminaires in an even pattern. The tool rounds up to a grid of 3 by 3, which is 9 luminaires. The spacing is 2.67 m along the room and 2 m across it.
The layout gives 365 lx. That is above the 320 lx target. The lighting power density is 6.75 W/m².
Open the lighting calculator with these inputs.
The checks on the result
The tool applies three checks.
Target reached. The layout must give at least the target. Here it gives 365 lx of 320 lx.
Spacing for even light. The spacing divided by the height must stay under a typical limit. Here the ratio is 1.33 against a limit of 1.5. A wider spacing leaves dark patches between luminaires.
Luminaire above the working plane. A luminaire at desk height gives no result. The tool says so and asks for the mounting height.
Where it goes wrong
A typical UF stands in for the maker's table. The tool states when it uses a typical value. Replace it with the data for the luminaire you specify.
The maintenance factor is a guess. A dusty workshop needs a lower factor than a clean office. A lower factor means more luminaires.
The count is rounded down. A count of 7.89 is 8 at the least. The grid then decides the final number.
The target comes from the wrong room use. A corridor and an office have different targets. Select the use that matches the room.
Spacing is ignored. A count that meets the lux can still leave dark bands if the grid is wide.
Use the result as a first layout. A lighting design for a tender or a building consent needs the maker's photometric data. This page is a design aid. Check every value against the current edition of the standard.
See also: Obtrusive light and the AS 4282 zones, Maximum demand and diversity and Shop lighting circuit loop impedance on a B curve 10 A device.