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| Radiation Measurement Techniques | by Kipp&Zonen | |
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Planet Earth is receiving solar energy (UV+VIS), and transmitting IR radiation
1. Radiation Measurement
2. Requirements
3. Principle of Measurement
4. Attaining higher Accuracy
1. Radiation Measurement
There are various reasons for studying radiation. Looking
at it on a cosmic scale, planet earth is receiving solar energy
and at the same time losing energy to the universe in the far
infrared part of the spectrum, keeping the total budget at zero.
Solar and far infrared radiation are important climatological
parameters, because large flows of energy are involved. To keep
track of the budget (some pklaces on earth receive or lose more
energy than others) the radiation is measured. Often radiation
is studied as part of climatological studies. Another reason
for studying radiation comes up when one wishes to use it as
a source of energy. Particularly the solar radiation can be used
for this purpose. The measurement of ultraviolet radiation is
also a major issue nowadays.
As you have seen, there are three major regions in which measurements
are desired:
* UV (UV-A, around 386nm; UV-B, around 306nm)
* Solar (0.3 to 3.0 microns, also known as VISIBLE radiation)
* IR (Greater than 3 microns)
There are different ways in which to measure the incoming (UV
and solar) radiation.
You can measure global, direct or diffuse radiation. Diffuse
radiation measurement instruments cover the whole hemisphere
(180 degrees viewing angle) and are shaded from the sun. The
detector is placed horizontally. Direct radiation meters are
aimed directly at the sun, so the detector surface is at right
angles with the incoming radiation. When measuring global radiation,
the instrument receives both the diffuse and direct component
on a horizontally placed detector. For measuring outgoing (IR)
radiation, there is only need for a global measurement.
2. Requirements
The requirements for measuring solar or infrared radiation
are largely dependant on the accuracy that is wanted.
* For measuring global solar radiation, the standard instrument
is a pyranometer. Its equivalent for the far infrared range is
called a pyrgeometer. For the UV range the instruments are UV,
UV-A or UV-B radiometers.
* By 'shading' a pyrgeometer or UV radiometer from the direct
radiation (coming directly from the sun) one can measure the
diffuse radiation (coming form the hemisphere, not directly from
the sun).
* For the measurement of direct radiation one can either use
a calculation (global minus diffuse, corrected for the cosine
of the zenith angle of the sun) or a measurement with a tube
shaped detector that has ot be aimed at the sun.
The latter instrument is called a pyrheliometer. A measurement
with a pyrheliometer is more accurate, but at the same time it
demands more attention. There exists also a pyrheliometer version
of the UV radiometer. All the instruments mentioned are available
in different accuracy classes, varying from reference instruments
(+/-0.5%) to second class instruments (roughly +/- 10%).
3. Principle of Measurement
Three of the basic instruments, pyranometer, pyrheliometer
and pyrgeometer are all based on a thermal detector. This detector
has a flat spectral response. The differences are in a filter
material and in field of view. For a pyranometer and pyrheliometer
the filter material, which also protects the sensor against environmental
influences, is glass, only letting solar radiation (0.3 to 3
microns) pass. For a pyrgeometer, the material used is coated
silicon, only allowinf far infrared radiation (3-50 micron) to
pass. A pyrheliometer has a limited field of view (5degrees opening
angle). The thermal detectors have a small voltage output, linearly
proportional to the incoming radiation. Since 1993 pyranometers
and pyrheliometers have been characterized standard no 9060,
according to the ISO. This ISO standard can act as a guideline
in choosing the right detector specification. There is no ISO
standard defined for pyrgeometers or UV radiometers. The UV radiometers
are different in both detector type and filter specifications.
Designed in close cooperation with the Royal Dutch Meteorological
Institute originally the purpose of the design was to offer a
solution for routine meteorological monitoring of trends in UV
radiation. For analysing related atmospheric conditions, however,
it turned out that it was impossible to rely on broadband UV
measurements. To solve this problem, UV-B and UV-A radiometers
were designed to have a narrow band sensitivity. For the UV-B
radiometer, the central wavelength was chosen at 306nm because
the optimum effect is reached when the human skin (erythermal)
spectrum and the solar spectrum are multiplied. The UV-A radiometer
central wavelength, 386nm, was chosen because this particular
wavelength is also used in the WMO air pollution network.
4. Attaining higher accuracy in measurements 
This picture shows the solar spectrum, on which the spectral
ranges of the pyranom
eter is superimposed. In the ongoing research for more accurate
measurements two trends deserve a little extra attention:
* The first one is ventilation. It has been noticed, also in
ISO 9060, that ventilation of radition meters improves the accuracy
and the reliablity of the measurements. It minimizes certain
offsets and keeps the measuring instruments free from dew and
frost
* The second trend is combining the tracking of pyrheliometers
and the shading of pyranometers.
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