Unravelling Sensitivity, Signal to Noise and Dynamic Range – EMCCD vs CCD
1. Sensitivity
TWO Fundamental Parameters determine sensitivity, for ANY camera type:
- System Noise Floor –
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Defines the instrument Detection Limit to overcome. Lower noise floor means fewer
photons are required for signal detection.
- Quantum Efficiency -
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QE is a measure of the cameras ability to capture photons. Higher QE means that
less photons are required to overcome the noise, resulting in a higher signal level
for a given photon input.
There is a common misconception that it is only the QE specification that determines
the sensitivity of a camera. In many situations, the system noise can determine
to a much greater extent the overall signal quality.
The driving force behind any low-light detection experiment is to increase the Signal-to-Noise
(S/N) ratio as far as possible under any given set of experimental conditions. Since
sensitivity encompasses both noise AND signal (the latter dictated by photon intensity
and QE), then by maximizing the parameters of sensitivity, S/N can be optimized.
High-end back-illuminated EMCCD cameras are often considered to be the most sensitive
detectors in the world, BECAUSE they combine technologies that optimize these two
individual parameters of sensitivity:
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EMCCD technology – Eliminates the readout noise contribution to detection limit
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Effective TE cooling – Eliminates darkcurrent noise contribution to detection limit
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Back-illuminated QE – Highest possible QE for maximum photon conversion
However, where longer exposures are possible and the experiment does not require temporal resolution, it can sometimes be more beneficial to work with a conventional, high QE, deep cooled CCD. The underlying reason for this will be described in more detail later.
Section 2 - Signal To Noise