APPENDIX
iKon-L APPENDIX
Page 159
A1.1.7 - Counts
Counts refer to the digitization by the A/D conversion and are the basic unit in which data are displayed and
processed. Depending on the particular version of the detection device, one count may, for example, be
equated with a charge of 10 photoelectrons on a pixel of the CCD.
A1.1.8 - Dark Signal
Dark signal, a charge usually expressed as a number of electrons, is produced by the flow of dark current
during the exposure time. All CCD’s produce a dark current, an actual current that is measurable in (typically
tenths of) milliamps per pixel.
The dark signal adds to your measured signal level, and increases the amount of noise in the measured signal.
Since the dark signal varies with temperature, it can cause background values to increase over time. It also
sets a limit on the useful exposure time.
Reducing the temperature of the CCD reduces dark signal (typically, for every 7ºC that temperature falls, dark
signal halves). CCD readout noise is low, and so as not to compromise this by shot noise from the dark signal, it
is important to cool the detector to reduce the dark signal. If you are using an exposure time of less than a few
seconds, cooling the detector below 0ºC will generally remove most of the shot noise caused by dark signal.
The Detection Limit is a measure of the smallest signal that can be detected in a single readout. The smallest
signal is defined as the signal whose level is equal to the noise accompanying that signal, i.e. a signal to
noise ratio (S/N) of unity.
Sources of noise are:
• Shot noise of the signal itself
• Shot noise of any dark signal
• Readout noise
If the signal is small, we can ignore its shot noise.
Furthermore, if a suitably low operating temperature and short exposure time can be achieved, the lowest
detection limit will equal the readout noise.
The Exposure Time is the period during which the CCD collects light prior to readout.
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