Andor Technology iXon Ultra 897 User Manual page 170

IMAGING GLOSSARY
signal; and shot noise from the light signal itself. Shot noise cannot be removed
because it is due to basic physical laws. Most simply defined, shot noise is the
square root of the signal (or dark signal) measured in electrons.
Readout noise (which in our detectors is, in any case, low) is due to the amplifier and
electronics: it is independent of dark signal and signal levels; it is only very slightly
dependent on temperature; and it is present on every read, as a result of which it sets
a limit on the best achievable noise performance. Shot noise from the dark signal is
dependent on the exposure time and is very dependent on the temperature; shot noise
from the signal is additionally dependent on the signal level itself. If either the
signal or the dark signal fall to zero, their respective shot noise also falls to zero.
The total pixel noise is not, however, simply the sum of the three main noise
components (readout noise, shot noise from the dark signal, and shot noise from the
signal). Rather, the Root Sum Square (r.s.s.) gives a reasonable approximation -
thus:
total = sqrt (readnoise² + darkshot² + sigshot²)
where total is the pixel noise, readnoise is the readout noise, darkshot is the shot
noise of the dark signal, and sigshot is the shot noise of the signal.
Fixed Pattern Noise
Fixed pattern noise (FPN) consists of the differences in count values read out from
individual pixels, even if no light is falling on the detector. These differences remain
constant from read to read. The differences are due in part to a variation in the dark
signal produced by each pixel, and in part to small irregularities that arise during the
fabrication of the CCD. Since fixed pattern noise is partly due to dark signal, it will
change if the temperature changes. But because it is fixed, it can be completely
removed from a measurement by background subtraction.
Section 8 . 3
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