The measurement of total column ozone with spectrometers is an analytical technique with a long historical background. The first measurements with a Dobson Ozone Spectrophotometer were conducted in the mid-twenties. A global network of Dobson instruments was established during the years following the first International Geophysical Year in 1957. A modern instrument, the Brewer Ozone Spectrophotometer, was developed in the nineteen seventies and introduced into the global network in 1982 with the delivery of instruments to Greece, Sweden, Germany and Canada.
The basic measurement principle of both types of spectrometers is the same. The thickness of the ozone layer is determined by comparing the intensity of solar radiation that has passed through the atmosphere at wavelengths in the ultraviolet that are strongly and weakly absorbed by ozone. The Dobson utilizes a kind of internal virtual 'ozone layer' (a variable attenuator called an 'optical wedge') to measure the intensity ratio of two wavelengths, the Brewer directly measures the intensity of light at a number of different wavelengths in the ultraviolet. The physical principles and instrumental characteristics of these methods are well known and acknowledged and therefore do not need any further justification as a primary technique for the NDACC. It is, however, important, to characterize the individual instruments of both spectrometer types and to determine their specific calibration constants. NDACC has about 20 NDACC affiliated Dobson and Brewer instruments.