1. A down-conversion method for analyzing light emissions from a luminescent sample comprising the following steps: a. generating excitation light from an excitation signal; b. passing the excitation light through a sample; c. detecting the luminescent light after it passes through the sample to provide a detector signal; d. reducing the frequencies of the excitation and detector signals by mixing those signals with at least one other signal; e. filtering those mixed signals with linear second-order IIR filters applying at least a portion of a modified Goertzel algorithm to the mixed signals; f. determining a phase shift between the excitation signal and the detector signal.