The radiation measuring devices, globally called radiometers, are generally
based on the measurement of the differential heating of an instrument surface
exposed to radiation, compared to the part of the instrument that is not exposed
(Paw 1995). Another option is based on the temperature difference between two
irradiated surfaces, one black and the other white (Foken 2017).
The use of thermopiles by multijunction of thermocouples in series or parallel, in
several types of radiometers (pyranometers, pyrradiometers, etc.) is a possibility of
transduction or conversion of the radiative flow, in a thermal response and hence in a
voltage signal suitable for readable electronics (Oke 1992). The receiving surface of the
thermopile is usually coated with a glass, quartz, or polyethylene coating, which works
as a protective agent against atmospheric conditions, as spectral filters for the separation
of long and short wavelength radiation and as a means of normalizing the transfer of
convective heat to the surface of the thermopile, to minimize the effects of wind speed
on the energy balance of the instrument. Quartz coverings are only permeable to
short-wavelength radiation in the range between 0.3 and 3 lm. Special polyethylene
covers (Lupolen) are used for measurements of short and long wavelength radiation
(ranges between 0.29 and 100 lm) and silicon covers are used for measurements in the
ranges of long wavelengths (between 4 and 100 lm) (Foken 2017).
Radiometers can also be used to measure radiation incident hemispheric
radiation at very narrow angles. Radiometers can measure the period in which direct
sunlight occurs, for example by differential reading, between a photocell exposed to
direct and diffuse solar radiation and another exposed to diffuse solar radiation.
When the differences between the readings are significant, a switch is activated that
closes a circuit, indicating the presence of direct solar radiation (Paw 1995).
Radiometers, which operate according to the heating principle, are based on short
and long-wavelength radiation energy balances to which they are subjected (Paw
1995). Basically, the energy balance of the sensor surface, Ri, is:
R i À e r T
4
s ¼ h c ðT s À T a Þ þ kðT s À T est Þ
ð A1:23Þ
where T s is the sensor temperature, T a the air temperature, h c the convective heat
transfer coefficient, r the Stefan-Boltzmann constant, e the emissivity of the
surface, k the thermal conductivity and T est the body temperature of the device. If
the sensor has two exposed zones (as in the case of the net radiometer) or two zones
on the same surface, with different absorbances, then the energy balance of the
Eq. (A1.23) is modified, to obtain the differential temperature of the two different
surfaces or zones.
Pyranometers, (Fig. A1.8) measure the total radiation of short wavelength
incident on a surface. Pyranometers are devices used to measure solar radiation of
short wavelength, incident on a horizontal surface. The respective thermopile is
covered by double glass coatings, whose radiative properties are such that, as
mentioned above, they only allow the passage of radiation from the range 0.3–3
lm, to the incidence surface.
The incidence surface may be painted with black paint for increasing
absorbance. Half of the thermal junctions are connected to small bands whose
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Annex A1: Instrumentation in Environmental Physics
based on the measurement of the differential heating of an instrument surface
exposed to radiation, compared to the part of the instrument that is not exposed
(Paw 1995). Another option is based on the temperature difference between two
irradiated surfaces, one black and the other white (Foken 2017).
The use of thermopiles by multijunction of thermocouples in series or parallel, in
several types of radiometers (pyranometers, pyrradiometers, etc.) is a possibility of
transduction or conversion of the radiative flow, in a thermal response and hence in a
voltage signal suitable for readable electronics (Oke 1992). The receiving surface of the
thermopile is usually coated with a glass, quartz, or polyethylene coating, which works
as a protective agent against atmospheric conditions, as spectral filters for the separation
of long and short wavelength radiation and as a means of normalizing the transfer of
convective heat to the surface of the thermopile, to minimize the effects of wind speed
on the energy balance of the instrument. Quartz coverings are only permeable to
short-wavelength radiation in the range between 0.3 and 3 lm. Special polyethylene
covers (Lupolen) are used for measurements of short and long wavelength radiation
(ranges between 0.29 and 100 lm) and silicon covers are used for measurements in the
ranges of long wavelengths (between 4 and 100 lm) (Foken 2017).
Radiometers can also be used to measure radiation incident hemispheric
radiation at very narrow angles. Radiometers can measure the period in which direct
sunlight occurs, for example by differential reading, between a photocell exposed to
direct and diffuse solar radiation and another exposed to diffuse solar radiation.
When the differences between the readings are significant, a switch is activated that
closes a circuit, indicating the presence of direct solar radiation (Paw 1995).
Radiometers, which operate according to the heating principle, are based on short
and long-wavelength radiation energy balances to which they are subjected (Paw
1995). Basically, the energy balance of the sensor surface, Ri, is:
R i À e r T
4
s ¼ h c ðT s À T a Þ þ kðT s À T est Þ
ð A1:23Þ
where T s is the sensor temperature, T a the air temperature, h c the convective heat
transfer coefficient, r the Stefan-Boltzmann constant, e the emissivity of the
surface, k the thermal conductivity and T est the body temperature of the device. If
the sensor has two exposed zones (as in the case of the net radiometer) or two zones
on the same surface, with different absorbances, then the energy balance of the
Eq. (A1.23) is modified, to obtain the differential temperature of the two different
surfaces or zones.
Pyranometers, (Fig. A1.8) measure the total radiation of short wavelength
incident on a surface. Pyranometers are devices used to measure solar radiation of
short wavelength, incident on a horizontal surface. The respective thermopile is
covered by double glass coatings, whose radiative properties are such that, as
mentioned above, they only allow the passage of radiation from the range 0.3–3
lm, to the incidence surface.
The incidence surface may be painted with black paint for increasing
absorbance. Half of the thermal junctions are connected to small bands whose
324
Annex A1: Instrumentation in Environmental Physics