APPENDIX
iKon-L APPENDIX
Page 160
A1.1.11 - Noise
Noise is a complex topic, the full exploration of which is beyond the scope of this glossary. Noise may,
however, be broken down into two broad categories:
1. Pixel Noise
2. Fixed Pattern Noise
A1.1.11.1 - Pixel Noise
Let us first attempt to define pixel noise. Assume that a light signal is falling on a pixel of the CCD. If the charge
on the pixel is read, and the read process is repeated many times, the noise may be taken as the variation in
the values read. The Root Mean Square (rms) of these variations is often used to express a value for noise.
As a rule of thumb, the r.m.s. is four to six times smaller than the peak to peak variations in the count values
read from the pixel. Pixel noise has three main constituents:
• Readout noise
• Shot noise from the dark signal
• 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.
A1.1.11.2 - 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.
A1.1.11.3- Readout Noise
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 falls 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
• sigshot is the shot noise of the signal
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