A detailed analysis of the Dobson spectrophotometer and its accuracy has been made. The spectrophotometer is the principal instrument used for measuring the atmosphere's ozone overburden. Emphasis has been put on the physics of the measurement method. Among topics considered are: the theoretical basis of the measurement and its limitations, such as those due to aerosol scattering and interfering gaseous absorption; the optical design and its limitations, such as due to stray light) and the techniques of operation and calibration and their limitations. With some topics the available literature is very limited and a number of original contributions have been made by the author. Summarising the effects of the various error sources is difficult owing to the great variety of error sources, their large variation from instrument to instrument in some cases, and their dependence on external parameters which are often unknown. Three broad generalisations can be made; firstly, that instrument-related errors form the greatest aggregate source of error; secondly, that the largest instrument-related errors arise from faulty extraterrestrial constants, stray light and defective optical adjustment; and thirdly, that atmosphere-related errors are small except at sites affected by severe air pollution. It seems that perhaps two thirds of recent years' standard AD direct sun method ozone estimations are accurate to 3% or better, relative to the current absolute scale, and that this absolute scale is itself accurate to about 3%. Some error sources which change with time can cause erroneous ozone trends of 10% per decade, and erroneous trends equivalent to a few percent per decade may be common in individual instruments. The review makes recommendations for further research into error sources and for improvements in the operation of the world's Dobson instrument network.

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