THE CANDELA
 

The candela, symbolized by the lowercase nonitalicized pair of English letters cd, is the unit of luminous intensity. It is equivalent to 1/683 of a watt of radiant energy emitted at a frequency of 5.4 × 10¹⁴ hertz (cycles per second) in a solid angle of one steradian. (The steradian will be defined shortly.) This is a sentence full of arcane terms! However, there is a simpler, albeit crude, definition: 1 cd is roughly the amount of light emitted by an ordinary candle.
Another definition, more precise than the candle reference, does not rely on the use of derived units, a practice to which purists legitimately can object. According to this definition, 1 cd represents the radiation from a surface area of 1.667 × 10^-6 square meter of a perfectly radiating object called a blackbody at the solidification temperature of pure platinum.