Version 1.4 rev 13 Oct 2017
33
Newton
6.3 p
Hoton
c
oUnting
Photon Counting is a technique used to record very low-level signals that might otherwise be lost in background noise.
One of the big advantages of an EMCCD is that, by setting the EM gain sufficiently high, the charge at the CCD output
amplifier due to a single photon can be made many times greater than the CCD readout noise. Applying an EM Gain
of several hundred can allow single photon events to be detected. These appear as single spikes that have amplitudes
significantly above the read noise. When the photon flux is low, Photon Counting can be utilized. By ‘low’ we mean that
there is little chance that two photons will arrive at the same pixel (or binned set of pixels) in the same readout period. In
photon counting mode the CCD is continuously readout without any gaps between scans (so that no signal is lost). If a
‘spike’, corresponding to a single photon, is detected above the set threshold level then it is counted as a single event
and a 1 is added in memory corresponding to that pixel position.
As the CCD is continually scanned, the signal builds up in memory and can be viewed live on the screen. The following
traces demonstrate the dramatic improvements that are possible from this measurement method. In order to select the
Photon Counting option, activate the
Enable
tick box in the
Photon Counting
area of the Setup Acquisition dialog box.
A suitable threshold level must be chosen, above which any spikes arising can be counted as a single event.
Figure 6: (Left) Normal Accumulation of 100 Scans, (Right) Photon counting with accumulation of 100 scans
