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Operation Modes

Absolute-Timing Modes

In the ‘Absolute Time’ mode every photon is characterised by its time from the start of the

measurement and its input channel number. The interpretation of the data is merely a matter of

the software. The different ways of analysing the data and the presenting the results are de-

scribed below. The results can be built up online from the incoming data stream, or off-line

from the time-tag data files. Please ‘Configuring the Runtime Display’, page 45.

Intensity Traces

The simplest way of interpreting the TDC data is to calculate intensity traces from the photon

times. The photon numbers within consecutive time intervals of the photon data stream are

determined, and displayed as functions of time, see Fig. 7. Although the procedure is trivial,

intensity traces can be used to determine on-off states of molecules, and to check whether an

experiment is running as expected.

Photons: Time-tag data

Intensity trace: Number of photons in consecutive time intervals

Fig. 7: Buildup of intensity traces from the time-tag data delivered by the TDCs

Photon Counting Histograms

The photon counting histogram (PCH) of an optical signal is obtained by recording the pho-

tons within consecutive sampling-time intervals and building up the distribution of the fre-

quency of the measured counts versus the count numbers, see Fig. 8. For a classic light signal

of constant intensity the PCH is a Poisson distribution. If the light fluctuates, e.g. by fluctua-

tions of the number of fluorescent molecules in the focus of a laser, the PCH is broader than

the Poisson distribution.

time

Number

photons

detected

Number of photons per

Number

containing

N photons

PCH

intervals

Fig. 8: Photon counting histogram. Left: Photons counted in successive sampling time intervals. Right: Histogram

of the number of time intervals containing N photons

A first characterisation of the PHC was given in 1990 by Qian and Elson [26]. The PCH de-

livers the average number of molecules in the focus and their molecular brightness. Several

molecules of different brightness can be distinguished by fitting a model containing the rela-

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Cornelius DPC 230 Especificaciones Pagina 11