What is a candela?
The unit of luminous intensity: how much light a source sends into one direction.
The candela (cd) is the SI unit of luminous intensity: how much light a source sends into one particular direction. One candela is one lumen per steradian, where the steradian is the unit of solid angle, the three-dimensional spread of a beam.
Flux and candela are not the same thing. Two lights can give out the same total lumens, but the one that squeezes that light into a narrower cone has a higher candela value in its beam direction. This is why AGL fixtures are specified in candela: a pilot sees the light along a set direction, so what counts is how much is sent that way, not how much is emitted in all.
Frequently Asked Questions
Q1What is a candela?
The SI unit of luminous intensity. One candela is one lumen per steradian, the amount of light a source sends into one direction per unit of solid angle.
Q2What is a steradian?
The unit of solid angle, the three-dimensional equivalent of a flat angle. It measures how wide a cone of light is, so it lets intensity be defined for a beam and not just a line.
Q3Why can two lights with the same lumens have different candela values?
Lumens count the total light emitted. Candela counts how much goes in one direction. A light that focuses the same lumens into a narrower beam has a higher candela value along its axis.
Q4Why are AGL fixtures specified in candela?
A pilot sees an AGL light along set directions and from a distance. The candela value in those directions describes how bright it appears, which is what the standards need to control.
Q5Is candela the same in every direction?
No. It depends on direction. A fixture has a peak candela value on its beam axis, and lower values away from it, so it is measured at many angles.
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