This chapter presents a number of basic optical and spectroscopic tools, both mathematical and technical, with particular emphasis on those appropriate to Fabry–Perot interferometry. With a faster detector one may analyze the fluctuations of intensity themselves to derive information, giving intensity-fluctuation spectroscopy. In older spectroscopic methods the field is manipulated prior to detection. The 'products' of these spectroscopic methods — fluctuating intensity, visibility curve and spectrum — are obviously all related. In any spectroscopic experiment the light passing through the equipment has to be detected — either visually, or by emission of photoelectrons from a photocathode, or by direct interaction of the photons with a variety of recording mechanisms. For spectroscopic purposes one of the most important precautions relates quite simply to drying the plate. Practical photomultiplier operation, particularly for use with photon correlation spectroscopy, has been usefully discussed by C. J. Oliver.