Spectral Power Distributions
by Nanette Salvaggio
All forms of light can be identified by their spectral power distributions. If we accept that white light can be dispersed into the individual wavelengths of visible spectrum as illustrated in Figure 1-5, then it follows that it can also be measured at each of these wavelengths. When this is done the resulting measurement is called a spectral power distribution (SPD). An SPD is measured with a spectrophotometer, which measures a light source as a function of wavelength. Figure 1-3, seen previously, provides the SPD for a tungsten and a daylight light source. Notice that the tungsten light source is smooth and is referred to as a continuous light source. It simulates a black-body radiator at any temperature. As can be seen in the equation, all the terms are constant except for wavelength.
