Dr Gordon Dobson, a University of Oxford physicist, became interested in measuring ozone in the stratosphere in the 1920s. His first spectrograph used a photographic plate technique, which was inconvenient as the photographic plates were returned to Oxford from around the world for development and measurement. Around 1927, he built his photoelectric spectrophotometer which enabled the relative intensity at two wavelengths to be measured directly. This ground-based instrument measures column ozone by the technique of differential absorption of ultraviolet (UV) light with the sun (or moon) as a light source. By comparing the UV light intensity at wavelengths that are strongly absorbed and weakly absorbed by ozone, the column ozone content of the atmosphere is accurately determined.
New Zealand agreed to have a Dobson Spectrophotometer in 1937, but Dobson No 17 did not arrive in New Zealand until 1950. When Dobson No 72 was installed in New Zealand in 1987, No 17 was sent to the Antarctic, where it made contributions to the ozone-hole studies. It is still in operation taking daily ozone readings with its periscope through the roof of the observatory. The 2018 WMO/UNEP Scientific Assessment of Ozone Depletion, has confirmed that the ozone “hole” is beginning to close as a result of measures taken under the Montreal Protocol to phase-out ozone depleting substances.
More modern Dobson Spectrophotometers are remarkably similar to the original which is shown in the picture. The Dobson Ozone Spectrometer is still in use world-wide with a network of over 100 active instruments. ‘Number One’ is now in the London Science Museum.
Picture reproduced with kind permission of the University of Oxford Department of Physics
For further information:
Gordon Dobson see https://www2.physics.ox.ac.uk/research/atmospheric-oceanic-and-planetary-physics/history/biography-dobson
Dobson No17 see https://www.stuff.co.nz/science/110213327/the-ups-and-downs-of-nzs-first-ozonemeasuring-dobson-spectrophotometer
The Dobson Spectrophotometer see https://www.ozonelayer.noaa.gov/action/dobson.htm