The QE is a measure, usually given in %, that describes the ability of the CCD to convert the photons that are collected by the photodiodes of the chip into electronic charges, in other words, the fraction of photons that are actually detected by the device. The QE is a function of the wavelength. Current high resolution chips peak around 550 nm (yellow-green) with about 65% QE. Back-illuminated chips even top this with more than 80%.