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Theoretical considerations

In principal, UV-B measuring instruments (Tevini, 1993) may broadly be classified into two classes: broad band detectors and spectroradiometers. Broadband detectors are made sensitive to a certain l range in the UV-B by combinations of special filters. However, no broadband detector has an ideal l independent sensitivity over the UV-B range.

Spectroradiometers measure the radiation flux at individual l s with a relative narrow spectral band. The most precise spectral measurements are made with double monochromator spectroradiometers. The absolute calibration of UV-B detectors remains to be optimised: an uncertainty of 3-5% is currently considered as a good result.

One of the most commonly used spectroradiometers is the Dobson O₃ Spectrophotometer. Total O₃ observations are made with this instrument by measuring the relative intensities of selected pairs of UV l s (l ), called A, C, C’ and D l s respectively (Komhyr, 1980). The general data reduction procedure is described in Komhyr (1980). The basic principle is based on difference (N) of logarithmic fractional transmittance of ultraviolet radiation, between the top of the atmosphere (Lo) and at the surface the earth (L)

Observational procedure of total O₃ provides for the determination of N values for each l pair.