pho·tom·e·ter   (fō-tŏm'ĭ-tər)    n.  An instrument for measuring a property of light, especially luminous intensity or flux.

Main Entry: ¹pho·tom·e·ter
Pronunciation: fO-'täm-&t-&r
Function: noun
: an instrument for measuring theintensity of light, luminous flux, illumination or brightness by comparison of two unequal lights from different sources usually by reducing the illumination of one (as by varying the distance of thesource or using a polarizing device) until the two lights appear equal with the amount of adjustment serving as the basis of comparison and the equality of illumination being judged by various means

photometer

device that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Such devices are generally transducers that convert an electric current into a mechanical indication-e.g., a pointer moving across a dial. The source of the current may be a selenium cell, which generates a current when light falls on it, or it may be a permanent source, such as a battery, in which case the current passes through a cadmium sulfide cell whose resistance varies with the amount of light falling on it.

Learn more about photometer with a free trial on Britannica.com.