Counting loss dead-time compensation

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. 

The recorded intensities may also be changed by possible counting loss in the TCSPC module. If and are measured consecutively, a count rate of a few percent of the reciprocal dead time should not be exceeded, or dead-time compensation should be used (see Sect. 7.9.2, page 338). Moreover, often a different IRF of both channels has to be taken into account. 

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. 

Dead time correction is made by empirical measurement of observed count rates as a function of increasing concentrations of activity. From these data, the dead time loss is calculated and a correction is applied to the measured data to compensate for the dead-time loss. Other techniques, such as use of buffers, in which overlapping events are held off during the dead time, use of pulse pile-up rejection circuits, and use of high-speed electronics, have been applied to improve the dead time correction. 

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