Clara
37
Version 2.0 rev 24 Mar 2015
Scan Types: Keep Clean & Acquired
The CCD is continually being ‘
scanned
’ to prevent its becoming saturated with dark current (see dark signal). If the
Scan is being used simply to ‘
clean
’ the CCD (i.e. it is a keep-clean scan), the charge from the CCD is discarded.
In an acquired scan, however, the charge undergoes A/D conversion and is acquired into computer memory so that it
can be used for subsequent processing and display: it is ‘
read out
’ (see
Readout
above). In this User’s Guide ‘
scan
’
generally refers to an acquired scan - unless the context specifically indicates otherwise.
Signal to Noise Ratio
The
S
ignal to
N
oise
R
atio (commonly abbreviated as
S/N
or
SNR
) is the ratio between a given signal and the noise
associated with that signal. Noise has a fixed component, and a variable component (shot noise) which is the square
root of the signal. Thus, the SNR usually increases (improves) as the signal increases.
The maximum SNR is the ratio between the maximum signal (i.e. the saturation level) and the noise associated with that
signal. At near saturation levels the dominant source of noise is the shot noise of the signal.
GLOSSARY
