SI Units

Candela

Definition

The SI base unit of luminous intensity, defined by fixing the luminous efficacy of 540 THz monochromatic radiation to 683 lm/W.

Explanation

One candela approximates the luminous intensity of a common wax candle, from which the unit takes its name. A 60-watt incandescent bulb emits roughly 60–80 candela in all directions. The candela specifically measures intensity in a given direction, making it the foundation for both lumen (total flux) and lux (illuminance) calculations.

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