Different sources of noise in EM-CCD cameras • Shot noise • Readout noise • Dark current • Spurious noise (also called Clock Induced Charge) • Noise Factor (also called Amplification noise)
Shot noise Shot noise is due to the particle nature of photons. Adjacent pixels of a CCD under uniform illumination will be unlikely to receive the same number of photons during a given exposure time. The same assumption can be made for the same pixel from frame to frame – shot noise quantifies this uncertanty! The number of photons that are collected by a given detector varies, and follows a Poisson distribution, depicted here for averages of 1, 4, and 10. Standard statistics state that Shot Noise =√Signal
Readout noise Noise in analog-to-digital converter The fundamental advantage of EM CCD technology is that EM gains can be sufficient to effectively eliminate readout noise, therefore eliminating this primary contribution to the instrumental detection limit!
Dark Current Due to the effective cooling, dark current is minimized and even during use of moderately long exposures it may often be considered practically negligible relative to the readout noise. The extent of contribution is dependent on exposure time, dark current generally being quoted in electrons/pixel/sec. Deep cooled CCDs can allow for exposure times up to several hours.
Clock Induced Charge (CIC) or Spurious Noise It originates from within the CCD camera. When charge is shifted pixel to pixel towards the output amplifier there is a very small but finite probability that charges can knock off additional charges by a process called impact ionization. These unwanted charges have the effect of generating an additional noise component. In a CCD camera this noise component is so low, it typically is hidden by the readout noise or dark current components. In an EMCCD however, spurious charges that occur in the image section are amplified by the EMCCD gain register and therefore are very detectable.
Amplification noise (Noise Factor) This noise component originates from the amplification process and is represented by the Noise factor. Due to the stochastic nature of the multiplication process there is no one-to-one mapping between the number of the input and output electrons. If our camera amplifies our signal by a gain of M we can define our Noise Factor (F) by the following expression: For EM-CCD technology F = 1.41.
The total camera noise If we consider P photons falling on a pixel in our camera with a quantum efficiency DQE and a Gain M, the signal output from our camera in electrons would be Substituting for Photon noise, the resulting equation for signal to noise is as follows: Dividing throughout by M