| Noise
Sources
All electronic circuitry generates undesirable noise.
The effect of this noise on performance is described
by the signal-to-noise ratio (SNR). Photon noise, preamplifier
noise and dark current noise are the three primary sources
of noise in a CCD camera.
Photon Noise
Photon noise, also known as
photonic or photon shot noise, is a fundamental property
of the quantum nature of light. The total number of
photons emitted by a steady source over any time interval
varies according to a Poisson distribution. The charge
collected by a CCD exhibits the same Poisson distribution,
so that the noise is equal to the square root of the
signal. Photon noise is unavoidable and is always present
in imaging systems; it is simply the uncertainty in
the data.
Preamplifier Noise
Preamplifier noise,
also called read noise, is generated by the on-chip
output amplifier. This noise can be reduced to a few
electrons with the careful choice of operating conditions.
Dark Current
Dark current, or thermally
generated charge, can be measured and subtracted from
data, but its noise component cannot be isolated. Dark
current noise is of particular concern in low light
applications.
Spurious Charge
Spurious charge is generated
on the leading edge of the drive clock which is when
the phase assumes the non-inverted state and holes are
forced back to the channel stop regions. The falling
edge of the drive clock has no influence on spurious
charge generation in CCDs. Spurious charge increases
exponentially with clock rise time and voltage swing,
sending holes back to the channel stop. A fast moving,
high amplitude clock increases impact ionization.
|
