1. A down-conversion method for analyzing light emissions from a luminescent sample comprising the following steps: a. providing an excitation signal to an excitation light source; b. providing excitation light to the luminescent sample; c. detecting luminescent light emitted from the luminescent sample; d. converting the luminescent light into a detector signal; e. mixing the detector signal with a mixer signal to create a mixed detector signal; f. mixing the excitation signal with the mixer signal to create a mixed excitation signal; g. low-pass filtering the mixed detector signal and the mixed excitation signal to remove the high-frequency components of each signal; h. processing the low-pass filtered mixed detector signal with a first linear second-order digital IIR filter and processing the low-pass filtered mixed excitation signal with a second linear second-order digital IIR filter; and i. transforming each of the linearly-filtered signals using control logic to compute real portions and imaginary portions to determine the phase shift between the excitation light and the emitted luminescence.