- Sun photometer
A sun photometer is a type of
photometer conceived in such a way that it points at theSun .Recent sun photometers are automated instruments incorporating a Sun-tracking unit, an appropriate opticalsystem, a spectrally filtering device, aphotodetector , and adata acquisition system .The measured quantity is called "direct-Sunradiance ".When a sun-photometer is placed somewhere within the Earth's atmosphere, the measuredradiance is not equal to the radiance emitted by the Sun (i.e. the "solar extraterrestrial radiance"),because the solar flux is reduced by atmospheric absorptionand
scattering .Therefore, the measured radiant flux is due to a combination of what is emitted by the Sun and theeffect of the atmosphere; the link between these quantities is given byBeer's law .The atmospheric effect can be removed with
Langley extrapolation ; this method therefore allowsmeasuring the solar extraterrestrial radiance with ground-based measurements.Once the extraterrestrial radiance is known, one can use the sun photometer for studying theatmosphere , and in particular for determining the atmosphericoptical depth .Also, if the signal at two or more suitably selected spectral intervalsis measured, one can use the information derived for calculating the vertically integrated concentrationof selected atmospheric gases, such aswater vapour ,ozone , etc.References
* Glenn E. Shaw, "Sun photometry", "Bulletin of the American Meteorological Society" 64, 4-10, 1983.
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