Introduction to EMCCDs: General Characteristics
- EMCCDs are standard CCDs plus an electron multiplication stage.
- EMCCDs may be read out at high pixel rates (up to 30 MHz for E2V EMCCDs, probably up to 60 MHz for TI EMCCDs).
- The gain mechanism increases the variance in the output signal so that the signal-to-noise ratio goes as √(2N) rather than √(N).
- Equivalent to halving the detective quantum efficiency.
- Photon counting can substantially restore this effective loss in quantum efficiency.
- Clock induced charge (CIC) affects all CCDs, but you will only really notice it with high gain EMCCDs.
When Should You Use EMCCDs?
- Any time that you are really limited by readout noise.
- This is more likely to be the case when running with high pixel rates. Conventional CCDs give excellent read noise but only at low read-out rates.
- Recent developments in sCMOS technology are changing this by offering low read-out noise (~1-2 electrons) and 100 Hz frame rates, though best in rolling shutter mode.
- However, do not forget the equivalent loss in detector quantum efficiency using an EMCCD in analog mode.
- At the lowest signal levels, photon counting gives close to the theoretical full DQE at high frame rates.
EMCCD Ageing
- Illuminating the image area to saturation and running the device at a gain of 1000x will cause the device to fail within a few hours.
- In practice these devices will be used at much lower illumination levels.
- With reasonable care in system design, many years of operation will be obtained.
- The ageing effect is seen as an increase in the high-voltage (multiplication) clock level needed to achieve a specific gain.
- At low gains (gains of a few) no ageing is seen.
- Increasing the gain from 100x to 1000x roughly doubles the short-term ageing rate with little effect on the longterm ageing.
- The ageing is principally caused by excessive signal levels in the multiplication register.
- An increase of 5 V over the life of the device is about the limit before failure occurs.
