It is possible to determine fluorescence lifetimes from the frequency response of a fluorophore as well as from its time response. Various schemes for measuring such a response curve are illustrated and include the use of sources whose intensity is modulated at single, swept frequencies, at several frequencies simultaneously, or over a broad, continuous band of frequencies. Detection methods that are considered include a single broad-frequency-response device, one that is swept, and another that hops from one modulated frequency to another. Signal-to-noise (S/N) expressions are derived for each combination of source and detector and conclusions are drawn from comparison of the S/N expressions. From these considerations and the availability of existing sources for use in time-resolved fluorometry, the most attractive approaches employ a source modulated at several frequencies simultaneously, each of which is monitored by its own tuned detector, or a sinuosidally modulated source that is swept synchronously with a tuned detector.