Last reviewed: Sep 15, 2026
Lighting specification goes wrong early when the wrong quantity is chosen. Three terms cover almost every confusion, and each one answers a genuinely different question.
The three quantities
Lumens measure the total light leaving a source. Lux, or foot-candles, measure the light arriving at a surface per unit area. Luminance measures how bright a surface appears to the eye or a camera, in candelas per square metre.
| Quantity | Unit | Answers the question |
|---|---|---|
| Luminous flux | Lumen (lm) | How much light does the product make? |
| Illuminance | Lux (lx) or foot-candle (fc) | How much light arrives here? |
| Luminance | Candela per square metre | How bright does this surface look? |
| Luminous efficacy | Lumens per watt | How efficient is it? |
| Colour temperature | Kelvin (K) | What colour is the light? |
Why a lumen figure cannot be specified as a target
The same 800-lumen lamp can produce 500 lux on one desk and 120 on another, because illuminance depends on distance and on distribution. A specification written in lumens is a specification of the product; a specification written in lux is a specification of the result.
One foot-candle is about ten lux
US specifications often use foot-candles while international data sheets use lux. The conversion is roughly ten to one, and rounding errors from mixing the two are a frequent source of specifications that are out by a factor of ten.
Luminance is what the eye and the camera react to
Glare is a luminance problem, not an illuminance problem. A surface can be lit to a comfortable level and still be glaring if its luminance is far above its surroundings, which is why a white glossy desk in a dark room feels uncomfortable even when the meter reads correctly.
How to write a specification that works
Specify the product in lumens and colour temperature, and the result in lux on a named plane at a named position. Then state the surface reflectances if they are unusual, because a specification that ignores a dark ceiling is a specification that will be met on paper and fail in the room.
Related reading
- How Many Lumens Does a Room Need? A Working Method
- Three Layers of Light, and Why One Fixture Cannot Do All Three
Frequently asked questions
What is the difference between lumens and lux?
Lumens measure total light a source emits; lux measures how much of it lands on a surface (lumens per square metre). A bright bulb in a big room can still read dim in lux.
What lux level do I need?
Offices and task areas commonly target several hundred lux; residential living spaces are far lower. Convert your lumen plan to lux by dividing by the lit floor area.
What is luminance?
Luminance is the brightness you see from a surface or the lit bulb itself; high luminance in view causes glare. Control it with shades and diffusers rather than only lowering lumens.
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