Specifications

Version 1.1 rev Jan 2013
Page 102
iXon Ultra
Appendix
A.1.12.1.3 - Calculation of Total Pixel Noise
The total pixel noise is not simply the sum of the three main noise components (readout noise, shot noise from the dark
signal and shot noise from the signal). Rather, the rms gives a reasonable approximation - thus:
total = sqrt (readnoise² + darkshot² + sigshot²)
where:
•totalisthepixelnoise
•readnoise is the readout noise
•darkshotistheshotnoiseofthedarksignal
•sigshotistheshotnoiseofthesignal
A.1.12.2 - Fixed Pattern Noise
Fixed Pattern Noise (FPN) consists of the differences in count values read out from individual pixels, even if no light is
falling on the detector. These differences remain constant from read to read. The differences are due in part to a variation
in the dark signal produced by each pixel, and in part to small irregularities that arise during the fabrication of the
EMCCD and in part to settling time of the electronics. Since xed pattern noise is partly due to dark signal, it will change
if the temperature changes but, because it is xed, it can be completely removed from a measurement by background
subtraction.
A.1.13 - Quantum Efciency/Spectral Response
The glossary refers to signals as a number of electrons. Strictly speaking, these 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. Photons of different wavelengths have different probabilities of producing a photoelectron, and this
probability is usually expressed as Quantum Efciency (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 a EMCCD,
but the most signicant factor is the absorption coefcient of the silicon that serves as the bulk material of the device.
A.1.14 - Readout
Readout is the process by which data are taken from the pixels of the EMCCD and 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 analog to digital conversion and then storing the data in computer memory.
The time taken to perform this operation is known as the “read time”.
A.1.15 - Saturation
Saturation is the largest signal the EMCCD can measure. A signal is measured in 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.