Definition: The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10^{12}Hz, K_{cd}, 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} s^{3}, where the kilogram, metre and second are defined in terms of h, c and Δν_{Cs}.

Encoding: The symbol is the Latin small letters 'cd', Unicode/ASCII character decimal codes 99 and 100.

This definition implies the exact relation \(K_{\rm{cd}} = 683\ {\rm{cd\:sr\:kg^{-1}\: m^{-2}\:s^{3}}}\) for monochromatic radiation of frequency \(\nu = 540 \times 10^{12}\ {\rm{Hz}}\). Inverting this relation gives an exact expression for the candela in terms of the defining constants \(K_{\rm{cd}}\), \(h\) and \(\Delta\nu_{\rm{Cs}}\):
$$1\ {\rm{cd}} = \left( \frac{K_{\rm{cd}}}{683} \right)\ {\rm{kg\:m^{2}\:s^{-3}\:sr^{-1}}}$$
which is equal to
$$1\ {\rm{cd}} = \frac{1}{(6.626\,070\,15 \times 10^{-34})(9\,192\,631\,770)^{2}\,683} (\Delta\nu_{\rm{Cs}})^{2}\,h\:K_{\rm{cd}}$$
$$\approx 2.614\,830 \times 10^{10} (\Delta\nu_{\rm{Cs}})^{2}\:h\:K_{\rm{cd}}$$

The effect of this definition is that one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency \(540 \times 10^{12}\ {\rm{Hz}}\) and has a radiant intensity in that direction of \((1/683)\ {\rm{W/sr}}.\) The definition of the steradian is given below Table 4.