Candela (cd)

The candela is the SI base unit of luminous intensity: luminous flux emitted per unit solid angle in one specific direction. Where lumens describe how much light a source produces in total, candela describes how strongly that light is concentrated in a direction. It is the only one of the common photometric quantities that says anything about the shape of a beam.

Formally, one candela is one lumen per steradian, and the SI definition fixes the luminous efficacy of monochromatic radiation at 540 THz to 683 lumens per watt. In practice, the working relationships matter more than the definition.

Candela, Lumen, Lux: The Three-Way Distinction

Quantity	Unit	Measures	Property of
Lumen	lm	Total light emitted, all directions	The product
Candela	cd	Light intensity in one direction	The beam
Lux	lx	Light arriving per m² of surface	The installation
Luminance	cd/m²	Light leaving a surface toward the eye	The surface as seen

A source radiating equally in all directions produces 4π ≈ 12.57 lumens per candela. Focus the same source into a narrow cone and the lumens do not change while the candela on the beam axis rise sharply. That is exactly what a reflector or a lens does, and it is why two flood lights with identical lumen ratings can light completely different scenes.

Why Candela Is the Number to Ask For

For any product with a defined beam angle (flood, spot, street, high bay, mast head), the lumen figure alone cannot tell you what will happen on your site. Peak candela can, because it converts directly into delivered light:

lux = candela ÷ distance²

A fixture with 34,444 cd peak intensity delivers about 538 lx directly beneath it at 8 m, and 239 lx at 12 m. Off the beam axis, multiply by the cosine of the angle, which is why the centre of a light pool is always brighter than its edge.

Peak intensity	Lux at 6 m	Lux at 8 m	Lux at 10 m	Lux at 12 m
10,000 cd	278	156	100	69
20,000 cd	556	313	200	139
34,444 cd	957	538	344	239
50,000 cd	1,389	781	500	347

This arithmetic is also an audit. If a supplier claims 500 lx at 10 m, they are claiming 50,000 cd. Ask for the candela figure and check that the claim closes.

Where You Actually Meet Candela

In the photometric file. An IES or LDT file is essentially a table of candela values across vertical and horizontal angles. Lighting design software reads that table and computes illuminance point by point for your geometry. Our guide to reading an IES file covers how to interpret and sanity-check it.

In the polar curve. The familiar petal-shaped diagram on a datasheet plots candela against angle. A tall narrow petal is a concentrating optic; a broad flat one is a wide distribution.

In beam angle definitions. The beam angle is conventionally the cone within which intensity is at least 50% of peak, so it is defined in candela terms.

In glare and aiming work. Discomfort depends on intensity toward the eye, which is why outdoor luminaire classes limit candela at high angles rather than limiting lumens.

How Candela Is Measured

A goniophotometer rotates the complete fixture (or the sensor around it) and records intensity at each angle, producing the candela distribution that becomes the IES file. This is a different instrument from an integrating sphere, which measures total flux in lumens but says nothing about direction. A complete photometric report normally includes both.

One historical note worth knowing: the candela replaced the older candlepower, and one candela is very close to the light of one traditional wax candle, which is where the name comes from.