A unit measuring the relative power output of a light source. Calculating the power delivered in the form of visible light is rather complicated. For a monochromatic (single frequency) light source such as a laser, the power in light watts equals 683V(l), where l is the wavelength of the light and V(l) is the relative power in watts per lumen (W/lm) required to produce a constant brightness sensation in the eye at wavelength l. Values of V(l) are defined by the International Commission on Illumination (CIE). The maximum value of V(l) is 1/683 W/lm = 1.464 mW/lm at the wavelength l = 555 nanometers (nm), the wavelength to which the eye is most sensitive. When the source delivers light over a range of frequencies (as a light bulb does), it is necessary to compute 683 times the integral of V(l) multiplied by the fraction of energy delivered at wavelength l. See also candela and talbot