Candela (cd)
cd is the candela, the SI unit of luminous intensity: how much light a source gives out in a given direction.
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How to read it
The BIPM’s SI Brochure defines the candela by fixing the luminous efficacy of monochromatic radiation at a frequency of 540 × 10¹² hertz, K_cd, at exactly 683 lumens per watt. It is the base unit the other light units are built from. NIST’s Guide for the Use of the SI gives the lumen, lm, the unit of luminous flux, as a candela steradian, cd · sr, and luminance in candelas per square metre, cd/m². NIST converts the footlambert, an older unit of luminance, at 3.426 259 cd/m².
Also searched as: candela symbol, cd, cd/m2, cd unit, luminous intensity unit.
Look-alikes
Case matters in unit and element symbols. The candela is cd, in lowercase; Cd, with a capital C, is the element cadmium, number 48 in the Green Book’s table of elements. See chemical element symbols.
Read next
- Next in Physics and chemistry symbolsAmpere (A)A is the ampere, or amp, the SI unit of electric current. One ampere is one coulomb of charge flowing past a point each second.
- The same shape in Certification marksKof-K kosher symbolThe kosher symbol of Kof-K Kosher Supervision, a US certification agency: a K set inside the Hebrew letter kof.
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The same shape elsewhere
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Sources
- The International System of Units (SI), 9th edition, SI Brochure, 9th edition (2019), updated 2024, Bureau International des Poids et Mesures (BIPM), Sec. 2.3.1 The candela; Appendix 1 (CIPM 1946; list of decisions)
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811 (2008), National Institute of Standards and Technology, Table 3; Table 2; Sec. B.8
- Chemistry 2e, 1.4 Measurements, OpenStax (Rice University), Table 1.2
- Quantities, Units and Symbols in Physical Chemistry (the Green Book), IUPAC Green Book, 3rd edition, 2nd printing (2008), International Union of Pure and Applied Chemistry (IUPAC), Sec. 6.2 Symbols for the elements
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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