Instrumentation

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 Dobson Units.

Satellite instruments, like NASA’s Total OzoneMapping Spectrometer (TOMS), 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 and UV-A. So this type of instrument is not suitable to cover all effects. 

The instruments 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.

Calibration

An important part of spectroradiometric measurements is calibration. Calibration 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 spectroradiometers "spectral 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 light.