photometer

thumb|A photometer A photometer is an instrument for measuring photometric quantities such as luminous flux, illuminance, or luminance.

Historically, photometry was done by estimation, comparing the luminous flux of a source with a standard source. By the 19th century, common photometers included Rumford's photometer, which compared the depths of shadows cast by different light sources, and Ritchie's photometer, which relied on equal illumination of surfaces. Another type was based on the extinction of shadows.