Chapter 5. Spectral Power Distribution

The visible part of the spectrum ranges in wavelength from about 400 to about 700 nm. For any light, we can construct a graph that depicts the light's spectral power distribution (SPD)—the intensity (power) of the light at each wavelength in the visible spectrum. The SPD is a crucial determinant of the perceived color of the light. Most light sources emit heterochromatic light, consisting of a wide range of different wavelengths. Figure 1 compares the SPDs of three heterochromatic lights: sunlight, light from an incandescent bulb, and light from a fluorescent bulb. All three of these light sources emit what could be called white light, heterochromatic light that (1) contains wavelengths from across the entire visible spectrum and (2) has no really dominant wavelengths.

The SPD of an idealized white light would be a horizontal line: a light in which all wavelengths across the visible spectrum have exactly equal power. We perceive white light as more or less colorless. The SPD of sunlight is quite close to that of an idealized white light. In contrast, the SPD of light from an incandescent bulb is more intense toward the yellow and red end of the spectrum, while the SPD of light from a fluorescent bulb has "spikes" at short and medium wavelengths that make fluorescent light somewhat more bluish than sunlight.