Dobson spectrometer

The Dobson Spectrometer, also known as the Dobson Ozone Spectrometer, is an instrument used to measure the amount of ozone in the atmosphere. Ozone are O₃ molecules that absorb the specific wavelength of dangerous UV light in the atmosphere. Ideally, no UVC light would penetrate through the atmosphere to the ground because it would all be absorbed in the ozone-oxygen cycle. Some UVB light and most UVA light, however, is not as readily absorbed because they exist at different wavelengths, so they penetrate to the ground level of Earth in higher quantities.

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The Dobson Spectrometer measures the intensity of the dangerous UV radiation (UVC) that reaches the Earth and compares it to that of UV radiation of a different wavelength (UVA) on the ground. If all of the ozone were removed from the atmosphere, the amount of UVC radiation would equal the amount of UVA radiation on the ground. Thus, because ozone does exist in the atmosphere, the Dobson Spectrometer can calculate the ratio between UVA and UVC radiation on the ground to determine how much ozone is present in the upper atmosphere to absorb the UVC radiation.

This ratio is determined by turning the R-dial on the instrument, a dial that can be turned a full 360^o. The spectrometer compares two different wavelength intensities, UVC and UVA, in order to calculate the amount of ozone. When turned, this R-dial filters and blocks out the light of the UVC wavelength until the intensity of the two wavelengths of light are equal. The ratio of the two wavelengths can be deduced once the intensities are the same. The results are measured in Dobson Units. One Dobson Unit is equal to .01mm thickness of compressed ozone in the column. If all of the ozone in the atmospheric column one was measuring were compressed, the thickness of the compressed atmosphere would equal a an answer in Dobson Units.

Though the instrument was invented over 80 years ago, it still has a resonating impact in today's world. For instance, the Dobson Spectrometer was a key instrument in discovering the hole in the ozone above Antarctica in 1985; this discovery in turn sparked the beginning of the important issue of global warming.

Sources:

http://ozone.gi.alaska.edu/dobson.htm

http://jwocky.gsfc.nasa.gov/dobson.html

http://www.sciencemuseum.org.uk/objects/meteorology/1950-159.aspx