Candela

Also called cd, candlepower, luminous intensity, peak beam intensity

The candela is the SI base unit of luminous intensity, meaning how much light a source sends in one particular direction, and it is the number that decides how far a light actually reaches.

What is a candela?

It is one of the seven SI base units, alongside the metre, the second and the kilogram, and it is the only one defined around human vision. The current definition fixes it by constant. In the words of the SI Brochure: the candela is the SI unit of luminous intensity in a given direction, defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 times ten to the twelfth hertz to be 683 when expressed in lumens per watt. That frequency is a green close to the peak of daytime human sensitivity, and the 683 has been the anchor since the 16th General Conference on Weights and Measures set it in 1979. The 2019 revision restated it as an exact defining constant rather than changing its value.

The Brochure then gives the consequence in a form you can picture: one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 times ten to the twelfth hertz and has a radiant intensity in that direction of one 683rd of a watt per steradian.

The everyday translation: a candela is roughly the light of one candle, in one direction. The old word candlepower meant close enough to the same thing that the two are still used interchangeably in the lighting trade.

Candela, lumens and lux

Three units, three different questions, and the whole confusion in off-road lighting marketing lives in the gap between them.

Lumens measure everything the source emits, added up over every direction. Candela measures how much of it goes one particular way. Lux measures how much lands on a surface once it gets there. A bare bulb and a searchlight can put out the same lumens and differ by four orders of magnitude in candela, because the searchlight has taken the same light and aimed it.

Lumens tell you what a lamp cost to build. Candela tell you what it does.

This is why off-road lights are advertised in lumens and headlamp standards are written in candela. Lumens are a bigger, friendlier number that goes up when you add emitters. Candela is the number that goes up only when the optics are good, and it is the one SAE J583 uses to say what a fog lamp may and may not do.

Turning candela into distance

Light spreads out with the square of distance, so illuminance falls off the same way. One candela of intensity produces one lux at one metre, a quarter of a lux at two metres, and so on. Run it backwards and you get the specification lamp makers actually publish: the one lux distance, meaning how far away the beam still delivers one lux, is the square root of the peak candela in metres.

So a lamp claiming a 500 metre one lux distance is claiming 250,000 candela at its centre. That is a useful sanity check, and a useful thing to notice when a manufacturer publishes lumens but not candela.