Quantum Efficiency (QE)
The quantum efficiency (QE) is the fraction of incident photons registered by a detector. For an ideal detector, this is 100% (every incoming photon results in a single count). In reality, however, no detector is 100% efficient. If, for instance, the detector is 70% efficient, then every 100 photons would result in 70 counts. The QE is a strong, highly structured, function of energy for both Chandra detectors, ACIS and HRC.
In the Chandra calibration database (CALDB) the QE of the detectors is divided into a mean curve (vs. energy), the QE file, and a map of the deviations across the face of the detector, the quantum efficiency uniformity (QEU) file.