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SI Units – Luminous Intensity

SI Units candela banner
Credit: NIST
cd - Candela - Luminous Intensity - 2018

The candela (cd) is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1, which is equal to cd sr W−1, or cd sr kg−1 m−2 s3, where the kilogram, meter and second are defined in terms of h, c and ∆νCs.

Radiation at frequencies other than 540 x 1012 Hz is also measured in candelas in accordance with the standard luminous efficiency, V (λ), curve that peaks at 540 x 1012 Hz (yellow-green). Learn more about the realization of the candela and luminance (candela per square meter).

Resources for Students and Teachers

graphic image of SI Superhero, Candela
Credit: J. Wang and B. Hayes/NIST

League of SI Superheroes – Candela

This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. Don't let her small size fool you, Candela's power over light helps to brighten the whole world. A candela is a measure of brightness about equal to the light given off by a single candle.

Navigate to more SI base unit information:



Created June 21, 2011, Updated January 16, 2024