May 29 2014
A photometer is an instrument for measuring the light intensity, or optical properties, of optical sources or detectors. It operates more like a transducer, which converts electric current into mechanical signal.
The current source may be a selenium cell, which provides current when light strikes its surface, or a permanent source, such as a battery.
Most of the photometers detect light using photomultipliers, photodiodes, and photoresistors. Light measurements can be carried out after the light passes through a filter, or monochromator, for analysis of the spectral distribution, or determination of light intensity at defined wavelengths.
The principal photometric scales include luminous flux, measured in lumens, luminous intensity, luminance, and illuminance. Luminous flux measurements are carried out for characterizing the output of sources, such as lamps, LEDs, and fiber-optic illuminators, using a luminous flux meter.
Luminance measurements involve the measurement of light emitted along a particular direction on the surface of an extended source, using a luminance meter.
Luminous intensity measurements are carried out for characterizing the output of a relatively small source in a particular direction.
Illuminance measurements involve measurements of the amount of luminous flux incident on a surface.
Some photometers perform light measurements by counting individual incoming photons, rather than using flux. Such devices are limited to applications in conditions where the irradiance is low.
Photometers are directly used in photography for measuring reflectance, color variation, glare, and other properties. They can also be incorporated into devices such as telescopes, spectrographs, and densitometers.