Photometry

Photometry, in physics, measurement of the luminous intensity of a light source, or the amount of luminous flux falling upon a surface from such a source (see Light). Photometry is important in photography, astronomy, and illumination engineering. Instruments used for photometry are called photometers. Light waves stimulate the human eye in different degrees, depending on the wavelength of the light. Because it is difficult to make an instrument with the same sensitivity for different wavelengths as the human eye, many photometers use a human observer. Photoelectric photometers need special colored filters to make them respond like the human eye (see Photoelectric Cell). Instruments that measure radiant energy instead of light are called radiometers, and must be made equally sensitive to all wavelengths.

The intensity of a light source is measured in candlepower, usually by comparing the source with a standard source provided by the National Institute of Standards and Technology. The known and unknown sources illuminate portions of a window surface side by side, and their distances are adjusted until the illumination on the surface is the same. The relative intensity is then calculated from the inverse square law.