What does candela mean?
The candela (symbol cd) is the International System of Units' base unit of luminous intensity — the measure of how bright a light source appears when viewed from a particular direction. Since 1979 it has been defined precisely in terms of physical constants: the intensity, in a given direction, of a source emitting green light of frequency 540 × 10¹² hertz with a radiant intensity of 1/683 watt per steradian. Unlike everyday words for brightness, the candela is directional, distinguishing it from the lumen, which measures total light output regardless of direction. Engineers, lighting designers, and regulators use candelas to specify headlights, signals, and display brightness. The word descends from Latin candela, meaning candle, reflecting its historical origin in the measurable glow of an actual candle flame. It remains essential wherever precise, standardized measurement of visible light matters.
nounThe SI base unit of luminous intensity, defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and has a radiant intensity in that direction of 1/683 watt per steradian.
- The SI base unit of luminous intensity, equal to the intensity of a source emitting monochromatic radiation of frequency 540 × 10¹² hertz with a radiant intensity of 1/683 watt per steradian in that direction.
- Formerly, a unit based on the luminous intensity of a standard candle flame (the historical 'international candle').
"The new LED panel produces roughly 800 candelas at its center."
"Headlight regulations specify the maximum candela the beam may project at any angle."
"Photographers comparing flash units often look at the guide number rather than raw candela figures."
The regular English plural "candelas" is standard in most contexts; in strict SI usage the unit name is sometimes left uninflected after a numeral (e.g., "5000 candela").
"The beacon emits about 10,000 candelas in its brightest sector."
Every lightbulb's brightness is ultimately measured against a definition fixed to a single frequency of green light — the candela has been the SI's 'glowing' base unit since 1979.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of candela
The word candela comes directly from Latin candela, meaning "candle" or "a taper," itself derived from candere, "to shine or glow white." English borrowed it as a scientific term in the 19th century when photometry needed a standard unit of light intensity, initially tied to actual candle flames. Its Latin root also underlies related English words such as candle, incandescent, candid, and chandelier, all sharing the idea of glowing whiteness.
How candela is actually used
A technical term used almost exclusively in physics, optics, engineering, lighting design, and regulatory contexts; rare in everyday speech. The plural is normally "candelas", though the uninflected form "candela" is sometimes retained in strict SI usage.
Easily confused with candela
A candle is an actual light source made of wax and wick, while the candela is the standard scientific unit used to measure luminous intensity.
The lumen measures total luminous flux (overall light output), whereas the candela measures luminous intensity in a specific direction.