**4.1 TCSPC**

TCSPC helps to measure the emission decay of a molecule in a fast and an accurate way, due to the high repetition rate of the laser (ps or fs lasers). The accuracy of the measurements depends on the arrival of randomly emitted photons to the detector at different time channels. To initiate the measurements, a reference signal from the laser source is registered at the electronics, and the arrival time of the laser signal is measured by a constant function discriminator (CFD). Then, a linear increase in the voltage starts when the signal passes through time to amplitude converter (TAC). In the meanwhile, an electrical signal is registered from the emitted photon at the CFD, and another signal is sent to the TAC to stop the voltage increase. The time difference between the start and stop corresponds to the time delay after examining signal by the rest of the electronics. Repeating these measurements many times gives the histogram plot at the end. **Figure 3** presents the components of TCSPC [8, 12, 13].
