Our knowledge of ozone and UV is very much based on instruments
measuring the thickness of the ozone layer and the amount of UV radiation
falling on the earth's surface. Their principles are explained below.
Ozone Measuring Instruments
Ozone can be measured from the ground and from satellites.
Ground-based instruments point to the sun and measure UV
radiation (at least) at two wavelengths. One wavelength is in
the UV-B where the radiation
is partly absorbed by the ozone layer. The other wavelength is in the
UV-A where no ozone
absorption takes place. By comparing the measurements at the two
wavelengths, the total ozone amount above the instrument can be
calculated and reported, for example in
Satellite instruments, like NASA's Total OzoneMapping Spectrometer
measure the Sun's UV radiation that is reflected from the earth's
atmosphere. Like for ground-based instruments, measurements are performed
at several wavelengths, some of which are affected by the absorption by ozone
and some are not. By applying a rather complicated algorithm the total ozone
amount in the satellite's angle of view can be retrieved. For more information
and actual data, see
http://toms.gsfc.nasa.gov, the official TOMS website.
One advantage of satellites compared to ground-based instruments is that they
can gather ozone information from over the entire globe. However, the method
of retrieving the thickness of the ozone layer from space is a more indirect one than
the method of calculating the thickness from the ground. Ground-based instruments
are therefore very important for checking the measurements of the satellites.
UV Measuring Instruments
Instruments measuring UV can be built in a variety of ways. Some mimic the
action spectrum of sunburn.
Therefore the reading of those instruments is directly related to the ability of
UV radiation to cause sunburn. Unfortunately, every biological effect
(for example cancer, snow blindness, etc.) is differently affected by wavelengths
in the UV-B
So this type of instrument is not suitable to cover all effects.
employed in the National Science Foundation's UV monitoring network therefore
work differently. They measure the whole UV
spectrum in tiny wavelength
steps. The result is a spectrum rather than a single value. The instruments are
therefore called "spectroradiometers". By multiplying spectra with the
action spectrum of sunburn, the
intensity of sun burning radiation can be determined. So in a way the first type
of instruments has an action spectrum built-in, whereas in the case of
spectroradiometers, the action spectrum has to be applied afterwards by
mathematics. That's more complicated, but also more flexible, as different
action spectra can be applied.
How do spectroradiometers work?
As so often in science, the principle of spectroradiometers is rather simple,
though the details may be complicated. Spectroradiometers consists of several components,
each of which serves a different purpose. The first component is a means of collecting
light (called fore-optics). This is done through the use of a diffusing material on top
of the instrument. After gathering the light, it is then delivered to a so-called
"monochromator", which is responsible for separating the
light into its varying wavelengths. It works in a fashion similar to the way in
which a prism separates white light into its anticipated rainbow of colors.
Once the light is separated, light of only one color (or only one specific
wavelength) is directed toward a detector,
where the radiation is converted into a electrical signal. The detector deployed in
the NSF spectroradiometers is a so-called "photomultiplier tube". Light hitting
the surface of the tube knocks away electrons from the surface material. These electrons are
amplified within the photomultiplier tube, and the result is an electrical current that is
dependant upon the strength of the radiation falling on the instrument's collector.
The current is finally logged and stored with a computer.
An important part of spectroradiometric measurements is
relates the output of the instrument (i.e. the electrical current of the
photomultiplier tube) to the physical unit. (In the case of the NSF
irradiance"). The calibration source of the NSF instruments are special
lamps, that are periodically mounted above the instrument's collector.
The irradiance that these lamps produce is known and stated in certificates that come
with the lamp. During calibration, the lamps are energized and the signal reported by the
spectroradiometer is compared with the irradiance values in the lamp
certificates. This gives the relation between instrument output and physical
units, which then can be applied to the instrument output when measuring sun
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