Page 59
New iStar ICCD
Pre-Acquisition Setup - CCD
5.3.1.6 - Photon Counting
Photon Counting can only be successfully carried out with very weak signals, because as the name suggests, it involves
counting only single photons per pixel. If more than one photon falls on a pixel during the exposure, an ICCD cannot
distinguish the resulting signal spike from that of a single photon event, and thus the dynamic range of a single frame
exposure is restricted to one photon.
Under such ultra-low light conditions, ‘photon counting mode’ imaging carries the key benefit that it is a means to
circumvent the Multiplicative Noise
, also known as ‘
Noise Factor
’. Multiplicative noise is a by-product of the Electron
Multiplication process and affects both EMCCDs and ICCDs. This gives the new ‘effective shot noise’ that has been
corrected for multiplicative noise.
Photon Counting mode does not measure the exact intensity of a single photon spike, but instead registers its presence
above a threshold value. It does this for a succession of exposures and combines the individual ‘binary’ images to create
the final image. As such, this mode of operation is not affected by the multiplication noise (which otherwise describes
the distribution of multiplication values around the mean multiplication factor chosen). The end result is that low light
images acquired through this mode of acquisition are improved by a factor of ~x2-2.5 Signal-to-Noise, compared to a
single integrated image with the same overall exposure time.
To successfully photon count with ICCDs, there has to be a significantly higher probability of seeing a ‘photon spike’
than seeing a darkcurrent/EBI ‘noise spike’. The lower the contribution of this dark noise sources to a single exposure
within the accumulated series, the lower the detection limit of photon counting and the cleaner the overall image will be.
