Note: The fundamental SI unit of luminous intensity is the candela.* The candela is the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. The candela so defined is the base unit applicable to photopic quantities, scotopic quantities, and quantities to be defined in the mesopic domain.
From 1909 until January 1, 1948, the unit of luminous intensity in the United States, as well as in France and Great Britain, was the international candle, which was maintained by a group of carbon-filament vacuum lamps. For the present unit as defined above, the internationally accepted term is candela. The difference between the candela and the old international candle is so small that only measurements of high precision are affected.**
From 1948 to 1979, the unit of luminous intensity was defined in terms of a complete (blackbody) radiator. From this relation Km and K’m, and consequently the lumen, were determined (see luminous flux). One candela was defined as the luminous intensity of 1/600,000 of one square meter of projected area of a blackbody radiator operating at the temperature of solidification of platinum and at a pressure of 101,325 newtons per square meter (N/m2, or Pa).
* Adopted at the Sixteenth Conference on Weights Measures on October 1, 1979.
** NBS Circular 459, Announcement of Changes in Electrical and Photometric Units. Washington DC: U.S. Department of Commerce; May 15, 1947.« Back to Definitions Index