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CCD Detectors

01CCD Detectors

Quantum efficiency

This section covers quantum efficiency as the proportion of incident photons detected and compares the efficiency of CCDs with the human eye.

Fraction of photons detected

Quantum efficiency is the percentage of incident photons that cause an electron to be released and therefore contribute to the detected signal.

quantum efficiency=number of electrons produced per secondnumber of incident photons per second×100%\text{quantum efficiency} = \frac{\text{number of electrons produced per second}} {\text{number of incident photons per second}} \times100\%

A high quantum efficiency means that relatively little of the incoming photon signal is lost. This gives a CCD a major advantage when observing faint astronomical sources.

Detector Typical quantum efficiency Consequence
Human eye Approximately 11–4%4\%. Most incident photons do not contribute to a detected response.
CCD Approximately 7070–90%90\%. A far greater fraction of the incoming signal is recorded, helping reveal much fainter sources.
If a CCD has a quantum efficiency of 80%80\%, approximately 8080 of every 100100 photons arriving at it cause electrons to be released.

Exam Tip: Define quantum efficiency using the fraction or percentage of incoming photons that cause electrons to be released. A vague statement about the detector being “efficient” is not enough.

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