IRF stands for instrument response function. In time-resolved fluorescence measurements the IRF is the temporal response of the fluorescence spectrometer to a delta-pulse. Suppose a initially sharp pulse defines the time of excitation / triggers the laser, then recorded response of the fluorescence spectrometer is broadened due to: (1) the temporal response of the exciting light source, (2) the temporal dispersion due to the optics of the instrument, (3) the delay of the light within the sample, and (4) the response of the detector. As the most intuitive contribution to the IRF is the excitation profile, the IRF is sometimes called ‘lamp function’. The IRF is typically recorded by minimising the contribution of (3), e.g., by measuring the response of the instrument using a scattering sample, or a short lived dye.