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Photometer
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==Photon counting== {{main|Photon counting}} Some photometers measure light by counting individual [[photon]]s rather than incoming [[radiant flux|flux]]. The operating principles are the same but the results are given in units such as photons/cm<sup>2</sup> or photons·cm<sup>−2</sup>·sr<sup>−1</sup> rather than W/cm<sup>2</sup> or W·cm<sup>−2</sup>·sr<sup>−1</sup>. 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 of the objects being measured as well as the difficulty of measuring light at higher energies using its particle-like nature as compared to the wavelike nature of light at lower frequencies. Conversely, radiometers are typically used for remote sensing from the [[visible light|visible]], [[infrared]] though [[radio frequency]] range.
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