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iXon 3

Features & Functionality

Figure 14 belowshowsS/Nplotsderivedfromthespecicationsoftheback-illuminatediXon3EMCCDsat1MHz

(slowerframerateoperation),readouteitherwithEMGainONoralternativelythroughtheconventionalamplier

(i.e. standard CCD operation). Again, this plot assumes a photon wavelength at which the QE of the sensor is 90%.

Specically this gure applies to 897 and 888 models where the user has the choice of either EMCCD or conventional

ampliers.

At these slower speed operationswhenonehasthechoicetoreadoutasa“conventional”CCDitcanoftenbe

advantageous to do so in order to achieve better signal to noise. Indeed the plots show that the cross-over point is at

~42photons/pixel,belowwhichitisstilladvisedtoreadoutthroughtheEMamplierwithGainapplied.

3.1.4 - Multiplicative Noise Factor & Photon Counting

It is impossible to know the exact gain a detected signal charge traversing the EM gain register will acquire due to the

stochastic nature of the processes which produce EM gain. However it is possible to calculate the probability distribution

function of output charge for a given input charge.

Atreasonablyhighgainlevels(>30)thisuncertaintyintroducesanadditionalnoisecomponentcalledMultiplicative

Noise. This noise source is only present in signal amplifying technologies and is a measure of the uncertainty inherent

to the signal multiplying process. For example, during each transfer of electrons from element to element along the gain

register of the EMCCD, only a small probability exists that the process of impact ionization will produce an extra electron

duringthatstep.Thishappenstobeasmallprobabilitybutwhenexecutedover>590steps,averylargeoverallEM

gain results. However, the downside to this process results from the probabilities! Due to this, there is a statistical

variation in the overall number of electrons generated by the gain register from an initial charge packet. This uncertainty

isquantiedbyaparametercalled“NoiseFactor”anddetailedtheoreticalandmeasuredanalysishasplacedthisNoise

Factor at a value of (or 1.41:1) for EMCCD technology. Note: This noise source is signicantly greater for the Multi

Channel Plate (MCP) of ICCDs than for the gain register of the EMCCD. ICCDs have noise factors typically ranging from

1.5to>2.

So,thisisanadditionalformofnoisethatmustbetakenintoaccountwhencalculatingSignal/Noiseforthesedetectors.

However, one way to better understand the effects of this noise source is in terms of an addition to the shot noise of the

system. Extra multiplicative noise has the same form as shot noise in that each noise type results in an increase in the

variation of number of electrons that are read out of the sensor (under constant uniform illumination).

Figure 14: EM Gain ON vs. Conventional

Amplier signal to noise plots for back-

illuminated iXon3 EMCCDs at 1MHz readout

speed (applies to 897 and 888 models).

Andor Technology iXon3 Series User Manual

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