Instrument function of the Fabry-Perot spectrometer

A scanning spectrometer may be described either by its transmission as a function of an actual spectral variable (e.g., wavelength) or by its response to a monochromatic source as a function of an instrumental scanning parameter. The distinction between these two descriptions has not always been recognized in previous developments of the theory of the parallel-plate Fabry-Perot spectrometer, and errors and ambiguities have often resulted. An outline and review of the theory is presented, including the effects of etalon surface irregularities and the finite solid angle of radiation admitted through the etalon. The order of interference is used as the argument of the functions involved, careful attention being given to the definitions and physical meanings of all functions and quantities. Optimization of the operating parameters of a spectrometer is considered very briefly.