Instruments used for the measurement of light intensity, called photometers, make possible a comparison between an unknown intensity and a standard or known intensity. They are based on the inverse-square law, which states that as a light source is moved away from a surface it illuminates, the illumination decreases in an amount inversely proportional to the square of the distance. Thus the illumination of a surface by a source of light 2 ft away is 1/4 of the illumination at 1 ft from the source. Conversely, for two light sources, one at 1 ft from a surface and the other at 2 ft, to give the same illumination to the surface, it would be necessary for the source at 2 ft to have an intensity 4 times that of the source at 1 ft.
A photometer measures relative rather than absolute intensity. The Bunsen photometer (named for R. W. Bunsen) determines the light intensity of a source by comparison with a known, or standard, intensity. The two light sources (one of known, one of unknown intensity) are placed on opposite sides of the surface (a disk of paper) to be illuminated. In the center of this surface is a grease spot that, when illuminated equally from both sides, will appear neither lighter nor darker than the paper but will become almost invisible. Using the inverse-square law, the intensity of the unknown light source can be easily determined when the relative distances at which the two sources produce equal illumination are known. The Rumford photometer (named for Count Rumford), or shadow photometer, compares intensities of light sources by the density of the shadows produced. In the Lummer-Brodhun photometer, an opaque screen is placed between the two sources, and a comparison is made possible by an ingenious arrangement of prisms.