An apparatus and method for the stimulation and collection of fluorescent signals from a target analyte. The apparatus comprises: i) a frequency modulated optical source which emits excitation light of a wavelength which will stimulate a target fluorophore to emit a fluorescent signal when illuminated by the excitation light, ii) a first optical filter interposed between the optical source and the target fluorophore, iii) a second optical filter interposed between the fluorophore and a photo-detector and positioned to receive the emitted fluorescent signal. The beam path of at least one, preferably both, of the excitation light and the fluorescent signal is transmitted substantially through free space. A data processor will normally be used to calculate and record the phase delay between the excitation light and the fluorescent signal.