 thumb|a photometer a photometer, generally, is an instrument that measures light intensity or optical properties of solutions or surfaces. photometers measure: *illuminance *irradiance *light absorption *scattering of light *reflection of light *fluorescence *phosphorescence *luminescence ==history== before electronic light sensitive elements were developed, photometry was done by estimation by the eye. the relative luminous flux of a source was compared with a standard source. the photometer is placed such that the illuminance from the source being investigated is equal to the standard source, as the human eye can judge equal illuminance. the relative luminous fluxes can then be calculated as the illuminance decreases proportionally to the inverse square of distance. a standard example of such a photometer consists of a piece of paper with an oil spot on it that makes the paper slightly more transparent. when the spot is not visible from either side, the illuminance from the two sides is equal. by 1861, three types were in common use. these were rumford's photometer, ritchie's photometer, and photometers that used the extinction of shadows, which was considered to be the most precise. ==principle of photometers== most photometers detect the light with photoresistors, photodiodes or photomultipliers. to analyze the light, the photometer may measure the light after it has passed through a filter or through a monochromator for determination at defined wavelengths or for analysis of the spectral distribution of the light. ==photon counting==  some photometers measure light by counting individual photons rather than incoming flux. the operating principles are the same but the results are given in units such as photons/cm2 or photonsÂ·cmâ2Â·srâ1 rather than w/cm2 or wÂ·cmâ2Â·srâ1. due to their individual photon counting nature, these instruments are limited to observations where the irradiance is low. the irradiance is limited by the time resolution of its associated detector readout electronics. with current technology this is in the megahertz range. the maximum irradiance is also limited by the throughput and gain parameters of the detector itself. the light sensing element in photon counting devices in nir, visible and ultraviolet wavelengths is a photomultiplier to achieve sufficient sensitivity. in airborne and space-based remote sensing such photon counters are used at the upper reaches of the electromagnetic spectrum such as the x-ray to far ultraviolet. this is usually due to the lower radiant intensity o
Electromagnetic_radiation_meters Optical_devices Photometry