Illuminance from Lumens and Area

Illuminance from luminous flux and area

Illuminance comes from E = Φ / A. Here E is in lux (lm/m²), Φ is the total luminous flux in lumens, and A is the lit area in m². So 1000 lm spread across 5 m² works out to 200 lux. For reference, NBR ISO 8995 lists living room 150–300 lx, kitchen 300–500 lx, office task 500 lx, and surgery 1000+ lx.

Applications

Lighting design for homes and commercial spaces, checking workstations against NR-17 ergonomics, industrial lighting projects under NBR ISO 8995, sizing LED fixtures from their rated lumen output, and a fast sanity check against what the luxmeter reads on site.

FAQ

What is the difference between lumen and lux? A lumen counts all the light a source puts out. Lux tells you how much of that light actually lands on a surface, in lm per m².

Does this formula assume uniform light? It does, so what you get is the average illuminance. The real spread depends on the fixture's polar curve and the shape of the room.

How much light does an office need? For everyday office tasks, NBR ISO 8995 puts it around 500 lux on the work surface, and you'll want more than that for fine visual work.