EMCCD Tutorial

Q10 What effect does EMCCD have on signal to noise or dynamic range?

Both Signal to Noise (S/N) ratio and Dynamic Range vary with the extent of EM gain applied. There are strong arguments for not applying excessively high EM gain multiplication factors. ‘Unravelling Sensitivity, Signal to Noise and Dynamic Range’.


Plots of Signal to Noise vs Signal Intensity for Andor iXon DV885LC

Plots of Signal to Noise vs Signal Intensity for Andor iXon DV885LC at both x300 gain and gain-off settings vs leading Sony ICX285 based CCD camera.


The above figure shows Signal to Noise of the Andor iXon DV885LC camera vs a leading camera incorporating the Sony ICX285 CCD sensor (conventional CCD) with low readout noise performance (5.5 electrons/pixel), corrected for pixel size difference. The Sony ICX 285 sensor is currently used in a large variety of microscopy cameras. At high EM Gain the DV885LC notably outperforms the Sony ICX. The S/N noise plots cross over at ~ 300 photons/pixel after which the DV885LC still shows highest S/N performance when operated with EM Gain turned off. Only at high light levels would the Sony ICX285 sensor give a marginally more favorable microscopy image resolution due to the slightly smaller pixel size (6.45 μm) than the DV885LC (8 μm). On balance, the iXon DV885LC is by far the more flexible 1MegaPixel ‘workhorse’ camera, due to its superior S/N at all light levels, faster frame rate performance (~double that of the Sony-based camera), coupled with more than ample resolution capabilities.


Plots of Signal to Noise vs Signal Intensity for Andor iXon DV885LC

The above plot shows Dynamic Range vs EMCCD Gain for iXonEM+ DU-897. Shown for EM amplifier @ 10, 5 and 1MHz readout speed and for Conventional amplifier at 1MHz readout speed. Well capacities used in DR calculation are characteristic of the CCD97 512x512 back-illuminated L3 sensor from E2V. Dynamic range only exceeds 14-bits max @ 1MHz, through either amplifier.

There are a number of interesting points to note from the above plots:

1. The rationale behind offering readout speeds slower than 10MHz through the EM-amplifier is so that frame rate can be traded of against dynamic range. You can see that the highest dynamic range through an EM amplifier comes from the slowest 1MHz readout speed.

2. At any readout speed through the EM-amplifier, the best combination of dynamic range and sensitivity can be obtained at a EM gain setting equal to the readout noise at that speed. At this point the DR is at maximum and the effective readout noise is 1 electron (i.e. just on the verge of single photon sensitivity).

3. At x1000 EM gain the dynamic range is only 400:1. Excessively high EM gain can also accelerate EM gain ageing in back-illuminated EMCCDs (see section 7). EM gains of x300 or less are more than sufficient to optimize sensitivity, while ensuring dynamic range is not excessively compromised. The only occasions when Andor recommend extending EM gain to x1000, is for single photon counting experiments.

4. The highest dynamic range is through the conventional amplifier at 1MHz.

5. It is clear that the actual sensor dynamic range only exceeds 14-bits @ 1MHz, through either EM or conventional amplifier. Therefore, it is at 1MHz that we require an option to match this higher dynamic range output with a scientific grade, noise free 16-bit A/D digitization. The iXonEM+ is uniquely designed to do just that, making use of a real scientific grade A/D that is optimized for 1MHz readout.