The fluorescence of LDS 798 dye in aqueous solution has a very short lifetime of 24ps, independent of excitation wavelength. The time response of common photon counting detectors depends on the wavelength of the registered photon. In lifetime measurements, the instrument response function (IRF) is usually approximated by the temporal profile of the scattered excitation light. Because λExc is typically much shorter than λEm, a systematic error may be present in these measurements. We demonstrate that the fluorescence decay of LDS 798 is a better approximation of IRF, in particular, for avalanche photodiodes used in the near infrared spectral region.