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Unformatted text preview: led contrast to noise
ratio (CNR). Note, that in this case, the
variance (power) is not
used but the standard
deviation of the noise. In
log-scale this differs only
in a multiplicative factor of
2. 19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 40 Constant vs signal dependent noise
Video preamplifier noise originates from the random motion of electrons in the
transistors. This makes the noise level dependent on how the electronics are designed,
but not on the level of the signal being amplified. A typical CCD camera will have an
SNR of about 300 to 1000 (40 to 60 dB) caused by constant amplitude noise. 19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 41 Constant vs signal dependent noise (2)
Noise that increases with the signal level results when the image has been represented by a
small number of individual particles. For example, this might be the x-rays passing through a
patient, the light photons entering a camera, or the electrons in the well of a CCD. The
mathematics governing these variations are called counting statistics or Poisson statistics. 19.05.2009 Dr. Pierre Elbischger - MIP1/ISAP'SS09 42 Example: CCD element
CCD is uniformly illuminated such that an average of N electrons are generated in each
well. By sheer chance, some wells will have more electrons, while some will have less.
To be more exact, the number of electrons will be Poisson distributed with a mean of
μ=N (signal level), and a variance σ² = N that describes how much variation there is
from well-to-well. With increasing illumination (number of electrons per well is increased) the signal
becomes larger faster than the noise, resulting in an overall improvement in the SNR.
Consider a typical CCD camera with an CNR of 300. That is, the noise from the CCD
preamplifier is 1/300th of the full scale signal. An equivalent noise would be
produced if the quantum sink of the system contains 90,000 particles per pixel. If the
quantum sink has a larger number of particles, the preamplifier noise will be
predominant. Accordingly, most CCDs are designed with a full well capacity of
100,000 to 1,000,000 electrons, min...
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- Spring '09