The Dobson instrument
The Dobson instrument is based to the UV absorbing properties of ozone between 280 nm and 330 nm, it is a differential instrument where the absorptions at several wavelengths are compared to yield the ozone columns by a simple algorithm.
Since 1925, the Dobson instrument is the standard instrument for the determination of ozone columns. The Royal Meteorological Institute operates instrument n°40 as well as the more recent Brewer instrument; both are described in details on RMI website where links to the instrument calibration and validation sites can be found.
Current series of Arosa (Switzeland) data beginning in 1926, the series are the most reliable for long term trends at mid-latitude. The absence of Antarctic data before 1957 still constitutes a handicap for the estimate of trends (METEOSWISS document).