Counting Loss in TCSPC Systems

There are several reasons why a TCSPC system may lose photons. The most obvious one is that the detector delivers an output pulse for only a fraction of the photons that reach its active area. Moreover, not all of the detected photons deliver a useful output pulse with an amplitude above the CFD threshold. 

Loss of photons inside the TCSPC module results from the faets that a single TCSPC channel can record only one photon per signal period, and that the module is blind during the dead time, i.e. the time during which a detected photon is processed. The detection and consequent loss of a second photon in one signal period is usually called pile-up effect. The term counting loss covers both pile-up-related and dead-time-related loss. 

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For the discussion above it was assumed that the detected light signal was continuous. However, signals measured by TCSPC are mostly pulsed signals. Moreover, the detection and therefore the dead time is synchronised with the signal period. This synchronisation can lead to a different behaviour than predicted by (7.33). Dead-time-related counting loss in nonreversed start-stop systems is illustrated in Fig. 7.82. 

Of course, the multidetector technique does not increase the maximum throughput rate of a TCSPC system. In any TCSPC device there is a small but noticeable loss of photons due to the dead time" of the processing electronics. The dead time of advanced TCSPC devices is of the order of 100 ns, and for count rates above 1 MHz the counting loss becomes noticeable (see Sect 7.9, page 332). The counting loss for a multidetector TCSPC system is the same as for a single detector system operated at the total count rate of the detectors of a multidetector system. 

Compared with classic systems, the dead time of advanced TCSPC systems has been considerably reduced. It is however, still on the order of 100 to 150 ns. The fraction of photons lost in the dead time - the counting loss - becomes noticeable at detector count rates higher than 10% of the reciprocal dead time (see Sect. 7.9.2, page 338). The counting loss can be compensated for by a dead-time-compensated acquisition time. Therefore, often a relatively high loss can be tolerated. The practical limit is the maximum useful" count rate, which is defined as the recorded rate at which 50% of the photons are lost. For currently available TCSPC modules, the maximum useful count rate ranges from 3 to 5 MHz, corresponding to a detector count rate from 6 to 10 MHz. 

For the results shown below, a Becker Hickl BHL-600 laser module was used, with a wavelength of 650 nm, 80 ps pulse duration, and 50 MHz repetition rate. The incident power density at the surface of the leaf was approximately 1 mW/mm. The measurement wavelength was selected by a 700 15 nm bandpass filter. The fluoreseenee deeay curves were recorded in one TCSPC channel of a Beeker Hickl SPC-134 system. One fluorescence decay curve was recorded eaeh 2 seconds, at a count rate of about 2-10 s Dead time compensation was used to avoid the influenee of counting loss on the recorded intensity. Typical results are shown in Fig. 5.32. 

The efficiency versus the count rate of a single TCSPC channel and a four-module TCSPC system is shown in Fig. 5.94. The efficiency of the single-channel system remains better than 0.9 and the figure of merit better than 1.05 for count rates up to 1 MHz detector count rate. This is better than for any other lifetime imaging technique. For a detector count rate of 10 MHz, the values are 0.5 and 1.4, respectively. Higher count rates not only result in a substantial loss in efficiency but also increase lifetime errors by pile-up-effect (see Sect. 7.9.1, page 332). For detector count rates above 10 MHz the solution is multimodule systems see Sect. 5.7.5, page 146. 

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