One of the factors to be taken into account when determining the effective area of the EPIC cameras is their quantum efficiency (QE). The QE of both types of EPIC CCD chip as a function of photon energy is displayed in Fig. 27 and 28. It is the QE of the EPIC MOS chips that in practice limits the energy passband at its high end, while the pn camera can detect photons with high efficiency up to 12 keV. The event selection strategies can be important in modifying the overall detection efficiency.