Model of the CCD response, or charge collection efficiency (CCE), which is the probability for the collected energy, illustrating the key components. Also shown is the 95% window which is the default data selection (vertical lines). For completeness we also show the typical read-out noise (system) which gives the typical output for a pixel without absorbed X-ray. The electronic bias level of the CCD has been subtracted from the energy axis, causing the read-out noise system peak to extend below the zero energy level.