Choosing an LED product means reading numbers like lumens, lux and candela. They all describe light, but they do not describe the same thing. Mixing them up is one of the most common mistakes in lighting procurement, and it leads to comparisons between products that were never meant to be compared. This article explains what each unit measures and how the three connect.
The Candela: Luminous Intensity in One Direction
The candela (cd) is the SI base unit of luminous intensity. It describes how much visible light a source emits in a specific direction, weighted by the sensitivity of the human eye. A bare LED die emits light in many directions, but a lens or reflector concentrates part of that output into a beam. Two sources with identical total output can have very different candela values if one concentrates its light and the other spreads it.
Candela is the right unit when the question is: how strong is the beam in a given direction? Spotlights, signaling lamps and streetlight peak intensity are all specified this way.
The Lumen: Total Luminous Flux
The lumen (lm) measures luminous flux, the total quantity of visible light emitted in all directions, again weighted by the eye response curve. If you wrapped the source in an imaginary sphere and collected every ray, the sum would be the lumen value.
Lumen is the right unit when the question is: how much light does this source produce in total? LED package datasheets list typical luminous flux per die, and luminaire datasheets list initial flux for the whole fixture.
The Lux: Illuminance on a Surface
The lux (lx) measures illuminance, the luminous flux arriving on a surface of one square meter. One lux equals one lumen per square meter. Unlike lumens, lux depends on distance and beam shape: the same 1000 lm source can deliver 500 lx on a workbench one meter below it, or only 50 lx three meters below, because the light spreads over a larger area.
Lux is the right unit when the question is: how bright is this surface? Workplace standards, greenhouse light targets and museum conservation limits are all written in lux.
How the Three Units Connect
The chain is simple once you see the geometry:
| Unit | Quantity | Question it answers | Typical context |
|---|---|---|---|
| Candela (cd) | Luminous intensity | How strong is the beam this way? | Spotlights, signal lamps |
| Lumen (lm) | Luminous flux | How much light in total? | LED die and luminaire datasheets |
| Lux (lx) | Illuminance | How bright is the surface? | Workplace and crop lighting standards |
Candela times the solid angle gives lumens. Lumens divided by area gives lux. Distance and optics sit between them, which is why a luminaire rated 20,000 lm can be either comfortable or harsh depending on the beam it produces.
Common Conversion Pitfalls
Three mistakes account for most confusion. First, comparing peak candela of a narrow spot against total lumens of a wide flood; the numbers describe different things. Second, quoting lux at one distance while the reader assumes another; always state the measurement distance. Third, forgetting that lumens are already weighted for human vision, so radiometric power in watts is never directly convertible without the spectrum.
Key Takeaways
Candela describes directional intensity, lumens describe total output, and lux describes what actually lands on the surface. Specify intensity with candela, source output with lumens, and task brightness with lux. When a datasheet gives one of the three, ask what geometry and distance it assumes before comparing it to another product.
Related reading
Spectral curves behind eye-weighted units: Spectral Power Distribution: Reading the SPD Curve.
More optical fundamentals in the LED Academy archive.
