Time correlated single photon counting (TCPSC) is a technique to measure light intensities with picosecond resolution. Its main application is the detection of fluorescent light. A pulsed light source excites a fluorescent sample. A single photon detector records the emitted fluorescence photons. Thus, per excitation cycle, only a single photon is detected. Fast detection electronics records the time between the excitation pulse and the detection of the fluorescence photon. A histogram accumulates multiple detected photons to yield a time-resolved fluorescence intensity decay.

Time-resolved anisotropy

Time-resolved anisotropies measure the depolarization of fluorescence light. Parallel polarized light is used to excite the sample. The fluorescence of this samples is the detected polarization resolved by a vertical and parallel detection channel. A normalized difference of the time-resolved fluorescence intensities of these detection channels measures the depolarization of the sample independently of absolute fluorescence intensities.

The time-resolved anisotropy, r(t), is obtained by fluorescence intensity of the sample excited by parallel polarized light.  A normalized difference of the fluorescence light detected by a parallel, VV, and perpendicular, VH, detector defines the time-resolved anisotropy.
r(t) = \frac{f_{VV}(t) - f_{VH}(t)}{f_{VV}(t)+2\cdot G \cdot f_{VH}(t)}
Here G is the G-factor of the instrument, which corrects for the detection efficiency for the parallel and perpendicular light.
f_{VV}(t) = \frac{1}{3} f(t) \cdot \left(1 + 2 \cdot r(t)\right)
f_{VH}(t) = \frac{1}{3} f(t) \cdot \left(1 - r(t)\right)



TTTR stands for time tagged time-resolved data or experiments. In TTTR-datasets the events, e.g., the detection of a photon, are tagged by a detection channel number. Moreover, the recording clock usually registers the events with a high time resolution of a few picoseconds. For long recording times of the detected events, a coarse and a fine clock are combined. The fine clock measures the time of the events relative to the coarse clock with a high time resolution. The time of the coarse and the fine clock is usually called macro and micro time, respectively.