Sun photometer

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

The measured quantity is called direct-sun radiance.

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; this method therefore allows measuring the solar extraterrestrial radiance with ground-based measurements.

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, ozone, etc.