A quantitative fluorescence technique is presented as 'differential real-time fluorescence spectroscopy' (DRFS), pursuant to which one can simultaneously make time-resolved measurement of fluorescence changes in two cuvette samples (10 and 11) (e.g., experimental and control), wherein the fluorescence is induced by excitation light (25), and wherein both samples have been identically prepared with a selected ratiometric dye. The ratiometric dye may either be of excitation-shifted variety or of emission-shifted variety. With DRFS, it is possible to monitor (41) the response of specimen cells (12 and 13) to a chemical, biological or physical agent, in the absence and in the presence of a selected experimental variable, and to determine both of these responses in real time, i.e., at the same time as the measurements are being made.
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