An analysis is presented which allows the spectral linewidth of alight source with a pure Gaussian or a pure Lorentzian spectral profile to be calculated from the measured visibility of the transmission fringes of a scanning Fabry–Perot interferometer. The linewidth is expressed in terms of the free spectral range of the interferometer and hence the result is general and may be applied to any interferometer. The relationship between fringe visibility and spectral linewidth has an analytic form for complete profiles. In addition, the method is applied to truncated spectral profiles, which may provide a more appropriate model for the spectral profile of some lasers, and the differences are characterized. Linewidths up to 2 times the free spectral range for Lorentzian and 1.3 times for Gaussian envelopes are measurable using these techniques.