[citation needed] However some longer-wave and less harmful UVB and most of the UVA are not absorbed as ozone is less opaque to these frequencies, so they penetrate to the ground level of Earth in higher quantities.

The results are measured in Dobson Units, equal to 10 μm thickness of ozone compressed to Standard conditions for temperature and pressure (STP) in the column.

Ozone distribution is derived from the change in the ratio of the same UV-pair frequencies with time as the sun sets.

Some modernized versions of Dobson spectrophotometer exist and continue to provide data.

31 and 51 with which Joe Farman of the British Antarctic Survey discovered the Ozone Hole above the South Pole in 1984.

The oldest instrument still in use is No.8 located at the roof of the Norwegian Polar Institute at Ny-Ålesund, Svalbard.

The history of the stations and instruments can be found at the World Ozone and UV Data Centre.