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  • The "Dobson spectrophotometer", also known as "Dobsonmeter", "Dobson spectrometer", or just "Dobson" is the earliest instrument used to measure atmospheric ozone. It was invented in 1924 by Gordon Dobson. A history of the development of the instrument is here and an example of one of Dobson's own instruments remains on display in the University of Oxford Department of Physics. Dobson spectrophotometers can be used to measure both total column ozone and profiles of ozone in the atmosphere. Ozone is tri-atomic oxygen, O₃; ozone molecules absorb harmful UV light in the atmosphere before it reaches the surface of the earth. No UVC radiation penetrates to the ground as it is absorbed in the ozone-oxygen cycle. However some longer-wave and less harmful UVB and most of the UVA is not absorbed as ozone is less opaque to these frequencies, so they penetrate to the ground level of Earth in higher quantities. The sources of light used may vary. Beside the direct sun light, the light from the clear sky, moon or stars may be used.