An EMCCD detector (Electron Multiplying Charge-Coupled Device) is a standard CCD with optional gain. These are often used for low-light applications, including biological applications and single-molecule work, where the option of integrating the signal for longer periods of time does not exist due to the transient nature of the phenomenon or risk of damaging the sample from extended exposure.
EMCCDs have on-chip gain, which is derived from a cascade register arranged between the serial register and a second output node. As the charge from each pixel is clocked out, the magnitude of this charge is increased as it steps through the gain register. Typically, the gain can be adjusted over a range of values, reaching as high as 1000.
While useful, there are some caveats in using this gain mode for quantitative work. To alleviate this concern, many EMCCD sensors have both a ‘standard’ as well as a ‘multiplier’ output, giving the researcher the option of using the most appropriate mode for their particular experiment.
The benefit of an EMCCD is that the amplification occurs before readout of the signal. This means that the signal is not readout noise limited. In other words, through amplification, the signal is raised well above the noise floor, which is largely determined by the noise of the readout electronics (pre-amplifier and A/D convertor).
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