Cornelius DPC 230 Specifications download pdf (Page 16)

10 Operation Modes

result is the waveform of the light signal. Because three input channels are available in the

TCSPC mode three signals from three detectors can be recorded simultaneously.

Reference

Accumulation over many

Reference

(not recorded) (not recorded)

Reference Reference

(not recorded) (not recorded)

Reference

(recorded)

Photon

Reference Reference

(not recorded) (not recorded)

Photon Reference

(recorded)

Time

Reference

(not recorded)

Ref.

(not rec.)

Photon Reference

(recorded)

Time

excitation periods

Photon time

Time in signal period

Photon density vs. time in signal period

Ref.

(not rec.)

Fig. 13: Build-up of the signal waveform in the TCSPC mode

In contrast to TCSPC devices using the TAC/ADC principle the DPC is able to record several

photons per signal period. This ‘multi-hit’ capability of a TDC is often praised as a panacea

against pile-up distortions. However, in the typical fluorescence lifetime applications the life-

time is on the order of a few nanoseconds. The dead time of the detector/CFD/TDC combina-

tion is at least at the same order, if not longer (about 5 ns for PMTs and 50 ns for SPADs).

Thus, there is little chance for a single detector to record several photons within the fluores-

cence decay. In standard fluorescence applications with lifetimes on the order of a few ns the

multi-hit capability is therefore of little benefit. It is, however, important when fluorescence

decay functions in the 100 ns range or above are recorded, i.e. from quantum dots, semicon-

ductors, or rare-earth chelates.

As all bh TCSPC devices, the DPC has a selectable frequency divider in the reference chan-

nel. The divider ratio determines the number of signal periods over which the signal is re-

corded (see Fig. 4 through Fig. 6). It should be noted here that a result containing several sig-

nal periods normally cannot be fully exploited by standard data analysis programs. Neverthe-

less, recording several signal periods can be useful when the time scale is to be checked by a

signal of known period, or when a TCSPC device is used as an optical oscilloscope [1, 2].

TCSPC with Absolute Timing

The time-tag data of the TCSPC mode still contain the absolute times of the detected photons.

Thus, a TCSPC measurement can be used to simultaneously obtain the waveform of the opti-

cal signal and the autocorrelation or cross-correlation within one or between different detector

channels [1, 2]. The waveforms of the detector signals are obtained as shown in Fig. 13, the

correlation curves as shown in Fig. 10. The result is the same as for the FIFO (or time-tag)

mode of the bh TCSPC mode where the fluorescence decay curves are obtained from the rela-

tive (‘micro’) time and the FCS curves from the absolute (‘macro’) times [2]. The results can

be built up online from the incoming data stream, or off-line from the time-tag data files.

Please see [2] and ‘Configuring the Runtime Display’ page 45.

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Cornelius DPC 230 Specifications Page 16

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