Appendix
QUANTUM EFFICIENCY/SPECTRAL RESPONSE
The glossary refers to signals as a number of electrons. More 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 Efciency (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 CCD, but
the most signicant factor is the absorption coefcient of the silicon that serves as the bulk material of the device.
READOUT
Readout is the process by which data are taken from the pixels of the CCD 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 A/D conversion, and storing the data in computer memory. The time taken to
perform this operation is known as the Read Time.
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