Andor Technology iXon Ultra 897 User Manual page 168

IMAGING GLOSSARY
The system uses the two main binning variants, vertical and horizontal binning. Binning Patterns
Imaging: The default imaging mode is Full Resolution Image, which causes a
value to be read out for each pixel on the CCD-chip. However, the Image Dialog
Box, accessible from the Setup Acquisition Dialog Box, lets you define superpixels
which in effect represent the charge binned (by means of a combination of vertical
and horizontal binning) from groups of pixels (each superpixel being a rectangle of a
given height and width in pixels). Viewed in Image Display Mode, data acquired in
this way will appear either as a 'high' definition image (where data have been read
out from individual pixels or from small superpixels) or as a more coarsely defined
image, where data have been read out from larger superpixels. The advantage of
defining superpixels is increased sensitivity and smaller data files. The Image
Dialog Box also lets you define a sub-area of the chip from which to acquire data.
Counts refer to the digitization by the A/D and are the basic unit in which data are Counts
displayed and processed. Depending on the particular version of the detection
device, one count may, for example, be equated with a charge of 10photoelectrons
on a pixel of the EMCCD.
Dark signal, a charge usually expressed as a number of electrons, is produced by the Dark Signal
flow of dark current during the exposure time. All EMCCDs 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 7
degrees Celsius that temperature falls, dark signal halves. EMCCD 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 0 degrees Celsius will
Section 8 . 3
Page 168