The watt per steradian (W · sr ^-1) is the standard unit of radiant intensity. Reduced to base units in the International System of Units (SI), it is the equivalent of a kilogram meter squared per second cubed per steradian (kg · m ² · s ^-3 · sr ^-1). A point source of electromagnetic (EM) power that radiates equally well in all directions, and whose output is 1 W · sr ^-1, has a total output power of 4  (approximately 12.5664)watt s (W). This is because there are 4  steradians in three-dimensional space with respect to a point of reference. The watt per steradian can be used to define the radiant intensity at any EM wavelength, from low-frequency radio waves through the gamma-ray spectrum. For visible light, in the wavelength range of approximately 390 to 770 nanometers (nm), the lumen is the preferred unit. Suppose an electromagnetic field having P watts of total power is radiated from an isotropic emitter (that is, a point source that radiates equally well in all directions). Then the radiant intensity, P', in watts per steradian is given by the following formula: P' = P / (4 ). Also see: Electromagnetic Field, Lumen, Power, International System Of Units (SI), Steradian, And Watt