Specifications

Version 1.1 rev Jan 2013
Page 99
iXon Ultra
Appendix
A.1.2 - Accumulation
Accumulation is the process by which data that have been acquired from a number of similar scans are added together
in computer memory. This results in improved signal to noise ratio.
A.1.3 - Acquisition
An Acquisition is taken to be the complete data capture process.
A.1.4 - A/D Conversion
Charge from the EMCCD is initially read as an analogue signal, ranging from zero to the saturation value. A/D conversion
changes the analogue signal to a binary (digital) number, which can then be manipulated by the computer.
A.1.5 - Background
Background is a data acquisition made in darkness. It is made up of xed pattern noise, and any signal due to dark
current.
A.1.6 - Binning
Binning is a process that allows charge from two or more pixels to be combined on the EMCCD-chip prior to readout.
A.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 EMCCD.
A.1.8 - Dark Signal
Dark signal, a charge usually expressed as a number of electrons, is produced by the ow of dark current during the
exposure time. All CCDs 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 EMCCD reduces dark signal (typically, for every 7ºC that
temperature falls, dark signal halves). EMCCD readout noise is low, and in order not to compromise this by shot noise
from the dark signal, it is important to reduce the dark signal by cooling the detector. 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.