EMCCD Vs ICCD

ICCDs edge out electron-multiplying CCDs in low light? (We don’t think so!)

A recent article has been published in Laser Focus World in September 2004 by David Dussault and Paul Hoess - David is an application engineer at Paul Hoess (Munich, Germany) and Paul is president of Stanford Computer Optics (Berkeley, CA) and general partner of Paul Hoess KG. Read laser focus world article

The article claims that EMCCDs perform better than ICCDs only at light levels above those that require cooling or slow scan. Andor Technology, as the pioneers of EMCCD technology would like to set the record straight.

David and Paul claim “ICCDs edge out EMCCDs in low light” because “charge-transfer noise greatly reduces their noise performance” (Laser Focus World, September 2004). They present theory to show this should be so and also have a series of pictures to confirm the theory; all seems very compelling. The trouble is that the numbers used in the theory are erroneous and the pictures for the EMCCD do not look like low light level images should from our cameras, strongly indicating that something is seriously wrong with the EMCCD images. Without access to the original measured data we can only guess that either there was no gain, or that the signal was lost in darkcurrent because the camera was not cooled, or the camera was faulty. We know from our own measurements, and those of independent users, that BI EMCCDs outperform ICCDs.

The theory mistake is easy to spot, it is because they use a “charge–transfer noise of 5.4e”, we were not involved with any of this testing and cannot even guess were they get this number from, the true value for the camera model quoted is 0.02 (0.05 at worst). When this number is used in their theory the camera model quoted will outperform the ICCD for all signal ranges shown, and this is without considering their other assumptions. (For reference: the charge-transfer noise arises from the Clock Induced Charge or Spurious Charge.) They choose the peak QEs so that the EMCCD is only 1.8 times better, but this does not do justice to the high and broad nature of BI EMCCD QE, something not matched by any photocathode.

They appear to have other misunderstandings of EM technology; they state “very slow pixel rates” are required “to minimize the impact of charge-transfer noise”. Again we do not know where this comes from because the opposite is in fact true. (They may be confusing some old assumptions from non-EMCCDs.)

They make a curious statement “Hence, we consider chip cooling of an ICCD camera as being unnecessary and a useless cost increase of the system”. They make a valid point but this statement goes over the edge; they mention the dark-field subtraction method required by the ICCD, but they have forgotten that for this to work best the CCD needs to be temperature stabilized; and this is best done by cooling. Modest cooling also reduces the ICCD fixed pattern noise and blemishes making the process more accurate.