Andor Technology iXon Ultra 897 User Manual page 171

IMAGING GLOSSARY
The glossary refers to signals as a number of electrons. More strictly speaking these Quantum
Efficiency;
are 'photoelectrons', created when a photon is absorbed. When a UV or visible
photon is absorbed by the detector it can at best produce only one photoelectron.
Spectral
Photons of different wavelengths have different probabilities of producing a
Response
photoelectron and this probability is usually expressed as quantum efficiency (QE)
or spectral response. QE is a percentage measure of the probability of a single
photon producing a photoelectron, while spectral response is the number of electrons
that will be produced per unit photon energy. Many factors contribute to the QE of
an EMCCD, but the most significant factor is the absorption coefficient of the
silicon that serves as the bulk material of the device.
Readout is the process by which data are taken from the pixels of the EMCCD and Readout
stored in computer memory. The pixels, which are arranged in a single row, are read
out individually in sequence. Readout involves amplifying the charge on each pixel
into a voltage, performing an A/D conversion, and storing the data in computer
memory. The time taken to perform this operation is known as the 'read time'.
Saturation is the largest signal the EMCCD can measure. A signal is measured in
Saturation
terms of the amount of charge that has built up in the individual pixels on the
EMCCD-chip. A number of factors determine the maximum amount of charge that
the EMCCD can handle.
Each pixel is ultimately limited in the amount of charge it can hold - its 'well depth',
in other words, is finite. This maximum charge, or well depth, is generally quoted in
-19
electrons or coulombs (where 1 coulomb = 1.6 x 10 times the number of
electrons).
Scan;
The EMCCD is continually being 'scanned' to prevent its becoming saturated with
Acquired Scan;
dark current (see dark signal). If the Scan is being used simply to 'clean' the
Keep Clean Scan
EMCCD (i.e. it is a keep-clean scan), the charge from the EMCCD 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). In the User's Guide 'scan' generally refers to an
acquired scan - unless the context specifically indicates otherwise.
Section 8 . 3
Page 171