A sun photometer is a type of

photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ph ...

conceived in such a way that it points at the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. Recent sun photometers are automated instruments incorporating a sun-tracking unit, an appropriate optical system, a spectrally filtering device, a

photodetector Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as Photoelectric effect, photoelectric or photoc ...

, and a

data acquisition system Data acquisition is the process of sampling signals that measure real-world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems, abbreviated by the acro ...

. The measured quantity is called ''direct-sun

radiance In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiatio ...

''. When a sun-photometer is placed somewhere within the earth's atmosphere, the measured radiance is not equal to the radiance emitted by the sun (i.e. the ''solar extraterrestrial radiance''), because the solar flux is reduced by atmospheric absorption and

scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...

. Therefore, the measured radiant flux is due to a combination of what is emitted by the sun and the effect of the atmosphere; the link between these quantities is given by Beer's law. The atmospheric effect can be removed with

Langley extrapolation Langley extrapolation is a method for determining the Sun's irradiance at the top of the atmosphere with ground-based instrumentation, and is often used to remove the effect of the atmosphere from measurements of, for example, aerosol optical thick ...

; this method therefore allows measuring the solar extraterrestrial radiance with ground-based measurements. Once the extraterrestrial radiance is known, one can use the sun photometer for studying the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

, and in particular for determining the atmospheric

optical depth In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material. Thus, the larger the optical depth, the smaller the amount of transmitted radiant power through ...

. Also, if the signal at two or more suitably selected spectral intervals is measured, one can use the information derived for calculating the vertically integrated concentration of selected atmospheric gases, such as

water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

, etc.

References

* Glenn E. Shaw, "Sun photometry", ''Bulletin of the American Meteorological Society'' 64, 4-10, 1983. Optical devices Electromagnetic radiation meters {{Optics-stub

See also

References