In order to characterize fluorescent light, we present a decay time measurement method based on the use of a commercial photon counting module. This module, adapted for all fiber measurements, is well adapted for the developing field of optodes for chemical and biochemical sensing. Moreover, it improves the detection sensitivity to the femtowatt and picowatt range. It is thus possible to excite the fluorescence with very low power pump pulses, down to a few nanowatts, and to measure the temporal distribution of the fluorescent signal. As sources we can use LEDs, or white lamps plus a monochromator, avoiding unwanted phenomena such as saturation, amplified spontaneous emission (ASE), and excited state absorption (ESA). We obtain a measurement accuracy of 0.1% in a couple of minutes.