Lighting
Illuminance from light reaching a surface
- where
- average illuminance (lx)
- received luminous flux (lm)
- area (m²)
Explain this equation
Gives the average illuminance on a surface from the light actually reaching it. Use it to compare a lumen figure with a lux target.
Φ_received is the luminous flux in lumens that actually lands on the area, not what the lamps emit. A is the area in square metres. E_avg is the average illuminance in lux, which is lumens per square metre.
One lux is one lumen spread over one square metre, so spreading the same light over a larger area lowers the average in proportion.
10,000 lm landing on 20 m² gives 500 lx, a common office target. Double the area with the same light and you get 250 lx.
Watch out: don't use lamp-rated lumens directly, because some light is absorbed or misses the surface; utilisation and maintenance factors deal with that. An average says nothing about uniformity or glare.
Illuminance describes light arriving on a surface. Dividing the total received lumens by the area gives average illuminance, even though individual positions may be brighter or darker.
Start by identifying whether a question gives emitted or received light. A luminaire's total emitted lumens do not all reach the chosen working plane.
Enter your values to see the result.
Common mistake
Do not divide a lamp's rated emitted lumens by floor area without accounting for utilisation and maintenance.
Limits
Use received flux over the stated area. Average illuminance alone does not establish uniformity, glare, task suitability or compliance.
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.