Candela

This article is about the unit of luminous intensity. For other uses, see Candela (disambiguation).

candela
Photopic (black) and scotopic[1] (green) luminous efficiency functions. The photopic includes the CIE 1931 standard[2] (solid), the Judd–Vos 1978 modified data[3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data[4] (dotted). The horizontal axis is wavelength in nm.
General information
Unit systemSI
Unit ofluminous intensity
Symbolcd
Conversions
1 cd in ...... is equal to ...
   international candles   ≈ 1.02 cp 
   Hefnerkerze   ≈ 1.11 HK 

The candela (symbol: cd) is the unit of luminous intensity in the International System of Units (SI).[5][6] It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the luminous efficiency function, the model of the sensitivity of the human eye to different wavelengths, standardized by the CIE and ISO.[7][4][8] A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.

The word candela is Latin for candle. The old name "candle" is still sometimes used, as in foot-candle and the modern definition of candlepower.[9]

  1. ^ "CIE Scotopic luminosity curve (1951)".
  2. ^ "CIE (1931) 2-deg color matching functions".
  3. ^ "Judd–Vos modified CIE 2-deg photopic luminosity curve (1978)".
  4. ^ a b Sharpe, Stockman, Jagla & Jägle (2005) 2-deg V*(l) luminous efficiency function Archived 27 September 2007 at the Wayback Machine
  5. ^ International Bureau of Weights and Measures (20 May 2019), The International System of Units (SI) (PDF) (9th ed.), ISBN 978-92-822-2272-0, archived from the original on 18 October 2021
  6. ^ CIE (2020). CIE S 017:2020 ILV: International Lighting Vocabulary, 2nd edition (2nd ed.). CIE.
  7. ^ ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry. ISO/CIE. 2023. doi:10.25039/IS0.CIE.23539.2023.
  8. ^ Wyszecki, G.; Stiles, W.S. (1982). Color Science: Concepts and Methods, Quantitative Data and Formulae (2nd ed.). Wiley-Interscience. ISBN 0-471-02106-7.
  9. ^ "Candlepower – Definition". Merriam-Webster Dictionary. Retrieved 15 February 2015.