Quantum Efficiency (QE) is a fundamental parameter in the field of optical engineering, describing the efficiency with which a photodetector or photosensitive device converts incident photons into electrons. In a charge-coupled device (CCD) or other photodetector, quantum efficiency is the ratio between the number of charge. Quantum efficiency (QE) is a measure of the number of charge carriers created per the number of incident photons of a given wavelength. As this measurement is wavelength-dependent, it yields a graph of QE by wavelength, such as Figure 1. For example, if a sensor had 75% QE and was exposed to 100 photons, it would be able to convert to 75 electrons of signal. This measure really matters for detector performance.