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Glossary
Candela
A candela (symbol cd) is the SI base unit of luminous intensity—basically, how bright a light source appears in a particular direction (brightness per solid angle, not “total light output”). In industrial terms, it’s the unit you use when you care about directional intensity—think indicator lamps, beacons, machine vision illuminators, vehicle headlights, warning strobes, and photometric testing.
Formally, the candela is defined by fixing the luminous efficacy constant 𝐾𝑐𝑑. The definition (one-line version) is: 1 cd is the luminous intensity, in a given direction, of a source emitting monochromatic radiation at 540×10^12 Hz with radiant intensity 1/683 W/sr.
A useful practical relationship is that 1 cd = 1 lumen per steradian (1 lm/sr), so candelas relate to how lumens are concentrated into a beam. If you keep lumens the same but narrow the beam, the candela value goes up (it looks brighter in that direction).