Sun photometer
A sun photometer is a photometer that is pointed directly at the sun. Current sun photometers consist of optics, spectral filters, a photodetector and a data logger. Automated sun photometers have a control unit with which the device follows the sun throughout the day. The measured quantity is the direct sunlight.
The radiation on the ground or in the atmosphere is lower than in space, as it is weakened by absorption and scattering . Therefore the measured radiation flux is a combination of the solar radiation and the atmospheric effects. The relationship between these two quantities is described by the Lambert-Beer law .
The atmospheric effects can be removed by a so-called Langley extrapolation. Measurements taken at different positions of the sun with different path lengths through the atmosphere but constant atmospheric conditions are used to deduce the irradiation outside the atmosphere. If the extra-atmospheric radiation is known, the sun photometer can be used to study atmospheric conditions, particularly atmospheric optical thickness . If the radiation is measured in several suitable spectral channels, the aerosol concentration or the vertically integrated concentration of some atmospheric gases, for example water vapor or ozone, can be measured with the sun photometer .
literature
- Glenn E. Shaw, "Sun photometry", Bulletin of the American Meteorological Society 64 , 4-10, 1983.