The oldest type of ozone spectrophotometer is the Dobson instrument, which uses quartz prisms to select the wavelengths of the incoming light. Dobson designed the instrument in the 1920s. During the following decades, it has undergone upgrades with respect to the electronics, but the basic optical parts are unchanged. After the International Geophysical year (1958), and especially when the impact of human activities on the ozone layer became clear (1970's), many instruments were installed to form a worldwide network. Although other instruments were developed, the Dobson instrument has long been considered as the reference instrument for total ozone observations. At Uccle Dobson instrument no 40 was operational since mid-1971 until spring 2009, when it was put at the disposal of WMO for use in Ukraine. To assure the quality of the measurements, the instrument participated in different intercomparison campaigns organized under supervision of WMO. More detailed information on these calibrations can be found on the Dobson calibration web pages. The data are stored at the World Ozone and Ultraviolet radiation Data Centre of WMO.