8
With this understanding in place, the remainder of this section will look at how aspects of this
measurement are specified in the CC1.
2.2 Coin Window
If pulses are simultaneously detected within some short time window, then they are said to be
coincident. The CC1 refers to this time window as the Coincidence Window, or “Coin Window”
for short. Technically, the Coin Window is the maximum allowed time between the rising edge of
a pulse on Channel 1 and a pulse on Channel 2 before two pulses are no longer considered to
be coincident.
The Coin Window can be set on the CC1 to be any integer value between 1 and 8
nanoseconds.
2.3 Dwell Time
The Dwell Time is the amount of time to accumulate pulses. For example, if the Dwell Time is
20 seconds, then all single pulses detected on Channel 1 during a 20 second measurement
period would be totaled and displayed. This would also be true for Channel 2 and the coincident
channel.
The Dwell Time can be set on the CC1 to values ranging between 0.1 second and infinity. When
a measurement is initiated, the CC1 will accumulate counts for an amount of time equal to the
Dwell Time. If the Dwell Time is infinity, the CC1 will accumulate counts indefinitely or until the
ON/OFF button is pressed and the count process stopped.
2.4 Gate Chan.
The Gate Chan. setting determines whether or not the GATE channel performs any function. If
the Gate Chan. value is set to “ENABLED,” single and coincidence counts will only be detected
when the GATE signal is high. If the Gate Chan. value is set to “DISABLED,” the GATE signal
will be ignored.
2.5 Subtract Acc.?
In some experiments, the CC1 may be used to measure coincidence counts to identify
correlations between signals. Coincidence counts can often indicate the presence of some
shared source of photon generation or optical phenomena. However, coincidence counts can
also be the results of random chance.
Statistically, there is some probability that a random stream of pulses on Channel 1 will
occasionally produce a pulse that is coincident with another random stream of pulses on
Channel 2. This statistical likelihood is called the “accidentals” and it can be calculated using the
following formula:
(sec) timedwell
Singles #2 Channel Singles #1 Channel(sec)Window eCoincidenc2
sAccidental
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