Name: Andor Technology iXon3 Series
Brand: Andor Technology
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Page 21

iXon 3

Features & Functionality

3.1.2 - EM Gain & Read Noise

As explained on the previous pages, EMCCD sensors allow the detected signal to be amplied on the actual sensor

itself before being readout through the output amplier and digitized by the Analog to Digital (A/D) converter. The reason

that this on-chip-multiplication process gives such a spectacular improvement in low light detection is that it negates the

effect of any electronic noise that may be generated by the read out electronics.

AllCCDcamerashaveanassociatedminimumelectronicnoiseoorwhichisoftentermedtheRead Noise of the

system. Read noise is produced during the readout process mostly by the output amplier but also has contributions

from the digitization electronics. This sets the minimum signal level that can be detected by the camera, as any signal

level below the read noise level will be indistinguishable from the read noise itself.

Read noise has therefore been the major limiting factor for low light level detection in CCDs for many years until the

introduction of EMCCD cameras by Andor Technology in 2000. By applying EM gain, a weak signal that would otherwise

be indistinguishable from the read noise can be amplied above the read noise level and thus be read out as a useful

signal. This amplication of the signal before being read out effectively reduces the read noise level of the camera and

even at relatively modest EM gain settings the effective read noise can be reduced to less than 1 electron r.m.s.

OneotherpointtonoteisthatsincereadnoiseincreaseswithincreasedreadoutratetheapplicationofEMgainreally

comes into its own at higher readout rates as any increase in the read noise can be overcome simply by increasing the

EM gain. For example, an iXon3 897 typically has a read noise of 50 electrons rms when reading out at 10MHz. This can

easilybereducedto<1electronbyapplying>x50EMgain.

3.1.3 - EM Gain ON vs EM Gain OFF

Figure 13 below shows Signal to Noise (S/N) plots derived from the specications of the back-illuminated iXon3

EMCCDs, read out at 10MHz for a photon wavelength at which the Quantum Efciency (QE) of the sensor is assumed

to be 90%. Such plots are very useful to gauge at what signal intensity it becomes appropriate to use EM Gain to

increaseS/N.

Itisclearthatat10MHzreadout,oneneedstoencounterrelativelyintensesignalsof>2900photons/pixelbeforeit

becomesadvantageoustooperatewithEMGainoff.Notethatthe“ideal”curverepresentsapureSignaltoShotNoise

ratio and is shown for reference – if the camera had no sources of noise, this is what the curve would appear like. Even

withEMGainturnedonweencounteruniformlylowersignaltonoisethantheidealcurve.Thisisduetotheinuenceof

Multiplicative Noise, which has the effect of increasing the shot noise by a factor of

2 or ~1.41.

Figure 13: EM Gain ON vs. EM-

Gain OFF signal to noise plots for

back-illuminated iXon3 EMCCDs at

10MHz readout speed (applies to

897, 860 and 888 models).

Andor Technology iXon3 Series User Manual

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