In apparatus for detecting the emission of fluorescent radiation from a sample in response to excitation by two or more different radiation sources (145, 160) over the same time period, each radiation source (145, 160) is imprinted with a modulation regime which can then be separately detected in the emitted fluorescent radiation. For instance in techniques based on fluorescent resonant energy transfer ("FRET"), the drive current to two or more different LED sources (145, 160) may be frequency modulated, or pulse width modulated, according to different modulation regimes. Responses of the sample, for instance of different donor/acceptor probes contained in the sample, to each of the sources (145, 160) can then be separately detected by means of the different modulation regimes, even where the wavelengths or wavelength ranges of the responses are the same or overlapping.