Three units for measuring light.
Try this
2
illuminance E = Φ / (4π·d²) — light thins out with the square of distance = 20 lux
Lumens to lux
1 lux = 1 lumen spread over 1 m². A point source spreads its lumens over a sphere of area 4π·d², so doubling the distance quarters the lux — the inverse-square law.
Which unit for which job?
- Candela — brightness in one direction (a torch beam spec).
- Lumen — total light a bulb pumps out in every direction (the number on the box).
- Lux — how brightly a surface is lit (an office wants ~500 lux on the desk).
Your turn
A 1600 lm bulb lights a 4 m² desktop evenly. Roughly how many lux?
The three
- Candela — luminous intensity in a direction.
- Lumen — total luminous flux output.
- Lux — lumens per square metre at a surface.