Spectral Instruments Series 800, Руководство пользователя

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Spectral Instruments
Pt # 2500- 34
signal. An image with 16 electrons of dark signal contributes 4 electrons to the
total system noise for that image. If you look only at readout noise and dark
noise without considering image noise (reasonable for measuring the dark areas
between bright areas) it doesn’t take a lot of dark noise to make it difficult to
measure background-level image signals.
To reduce the impact of dark signal, S800 cameras utilize a thermo-electric
cooling system that allows the CCD to be operated at as low a temperature as is
consistent with that CCD and the camera configuration.
Dark signal noise combines with readout noise as the square root of the sum of
the squares (this is called quadrature). For a camera with a readout noise of 4
electrons and a dark signal of 4 electrons, the combined noise is 5.6 electrons.
For a 4-electron camera that is running at one thermal electron per 10 minutes,
this means that it is possible to integrate for some tens of minutes before the
noise from a dark image significantly degrades the total noise figure for the
system.
Dark signal is not uniform in its distribution over an image. Variations in dark
signal generation rate are all (but one - preamp glow) related to inhomogeneities
in the sensor or in the substrate upon which it is built. The variation to be
expected - the dark signal non-uniformity (DSNU) - can be as high as 25% for
some CCDs. It is rarely less than 10%. Because the stable dark image patterns
are visible and because the dc level is significant to low-light-level imaging it is
important to correct for dark before quantitative analysis is performed.
4.1.3 Light Images
Light images are what you are after. They all offer their own individual “quirks”
when it comes to making quantitative measurements. The most important of
these “quirks” relates to non-uniform illumination. The second relates to non-
uniform quantum efficiency in the sensor.
If you want to know how much signal is contributed by an event in one area
compared to a similar event in another area you need to be assured that there is
no instrumental effect affecting the measurements. There usually is!
The basic process for correcting light images is called “shading correction” in
some literature, it is called “flat fielding” in other literature. If you can get a
measure of the shading effect then you can compensate for it - although you can
never recover from the reduced signal in the shaded areas. This reduced signal
means that the signal-to-noise ratio (SNR) is ever poorer in a shaded region than
it is in a non-shaded region. The only fix for this problem is preventative - it is
not recuperative.