Signal to Noise Ratio is one of the main characteristics describing light detectors and indicating their statistical properties. SNR can be defined as the ratio between the received light signal and the noise associated with the acquisition of that signal. The SNR value increases with increasing signal size. The maximum SNR value is reached when the maximum signal is recorded (i.e. when the saturation level is reached). Before the saturation level is reached, the dominant noise source is the so-called Photon Shot Noise.
Mathematically, SNR is defined as the root mean square of the signal current to the sum of the variations of the individual noise sources - Dark Signal, Readout Noise and Photon Shot Noise.
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Dark Signal - a noise source generated by the flow of Dark Current during an exposure. Dark current is produced by all CCD sensors and is typically measurable in tens of mA per pixel. The dark signal increases the overall noise in the measured signal by increasing the photon noise contribution. Since the dark signal is temperature dependent, its value can be reduced by cooling the CCD sensor (a 7°C temperature reduction reduces the dark signal by half). It is usually expressed in electrons per second per pixel at a defined temperature.
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Readout Noise - this is a natural property of the sensor and, with the exception of EMCCD, is usually the sensitivity limit for most cameras. It is a combination of noise sources that come from the amplification process and the conversion of photoelectrons generated to voltage. It is expressed in electrons.
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Photon Shot Noise - incident photons have their own noise known as photon noise - this is noise from the signal itself. Photons are randomly distributed over time, usually described by Poisson statistics. The photon noise is a characteristic of the light radiation incident on the detector, so it is not removable - it can only be reduced by reducing the exposure time of the light radiation to a minimum or by cooling the detector, thus reducing the dark signal and thus the photon noise.