In the broadest sense, a photometer is any instrument used to measure illuminance or irradiance. As applied in industrial photometry, a "photometer" is the general term covering instruments for detecting: Illuminance is the total luminous flux incident per unit area. ... Irradiance, in physics, is the density of radiant flux incident on a given area, typically measured as a unit of power per unit area (for example watts per square metre). ... Photometry is a term used in physics, optics, and illuminating engineering for the measurement of quantities associated with light. ...

• scattered light intensity
• absorption
• fluorescence

Most photometers are based on a photoresistor or photodiode. Either exhibits a change in electrical properties when exposed to light, which can be detected with a suitable electronic circuit. In particle physics, scattering is a class of phenomena by which particles are deflected by collisions with other particles. ... Absorption, in optics, is the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make a transition between two electronic energy levels. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized Cadmium selenide (CdSe) quantum dots. ... A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. ... A photodiode A photodiode is an electronic component and a type of photodetector. ... An electrical network or electrical circuit is an interconnection of analog electrical elements such as resistors, inductors, capacitors, diodes, switches and transistors. ...

See also

• Photodetector A transducer capable of accepting an optical signal and producing an electrical signal containing the same information as in the optical signal. The two main types of semiconductor photodetectors are the photodiode (PD) and the avalanche photodiode (APD).