The gloss of paint surface is assessed either by visual comparison between-a test panel and a series of panels presenting a range of standard gloss surfaces or by the use of instruments depending on photoelectric devices which measure the amount of light reflected from the surface of a painted panel which a beam of light is directed on it.

A common assembly consists of a selenium cell connected to a galvanometer; light falling on the selenium cell generates an electric current which is measured by the swing of the galvanometer. The gloss, as determined by this instrument, is expressed as a percentage of the gloss of a standard glass surface determined under the same conditions. The angle at which the beam of light meets the paint surface is defined, and instruments are available which enable measurements at different angles are available. 60° and “0° are commonly used angles of measurement of gloss.