Sun photometer
Encyclopedia
A sun photometer is a type of photometer
conceived in such a way that it points at the Sun
.
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
.
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
.
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.
Once the extraterrestrial radiance is known, one can use the sun photometer for studying the
atmosphere
, and in particular for determining the atmospheric optical depth
.
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.
Photometer
In its widest sense, a photometer is an instrument for measuring light intensity or optical properties of solutions or surfaces. Photometers are used to measure:*Illuminance*Irradiance*Light absorption*Scattering of light*Reflection of light*Fluorescence...
conceived in such a way that it points at the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
Recent sun photometers are automated instruments incorporating a Sun-tracking unit, an appropriate optical
system, a spectrally
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
filtering device, a photodetector
Photodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
, and a
data acquisition system.
The measured quantity is called direct-Sun radiance
Radiance
Radiance and spectral radiance are radiometric measures that describe the amount of radiation such as light or radiant heat that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from...
.
When a sun-photometer is placed somewhere within the Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
, 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
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
and scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
.
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
Langley extrapolation is a method for measuring the Sun's radiance with ground-based instrumentation,thereby removing the effect of the atmosphere. It is based on repeated measurements with a sun photometer operated at a given location for a cloudless morning or afternoon, as the Sun moves across...
; 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
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
, and in particular for determining the atmospheric optical depth
Optical depth
Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...
.
Also, if the signal at two or more suitably selected spectral intervals
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
is measured, one can use the information derived for calculating the vertically integrated concentration
of selected atmospheric gases, such as water vapour, ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
, etc.