What is a photometer and what is it used for? A photometer is a device used to measure diverse aspects of the intensity of light. The instrument is used in photography, soil testing, and the water industry. It is used to compare light transmitted from two sources, when the specific characteristics from one source are known and are standard and the other unknown. The photometer gives an accurate comparison of light from different sources. . An example of how a photometer works is by putting a piece of paper with an oil stain in the middle of it that makes that part of the paper transparent.