Single-X-ray-photon counting pixel array detectors are now widely used on synchrotron beamlines for X-ray diffraction and imaging experiments. Efforts have been carried out in the recent years to extend X-ray detector system analysis to include charge-sharing effects and characteristic X-ray re-absorption on image quality parameters such as MTF, NPS and DQE. These efforts led to the formulation of an analytical model of single-X-ray-photon counting pixel array detectors which is presented in this contribution to the IWORID-2013 conference. This model links together imaging and spectroscopic performance of pixel array detectors. It provides a framework for optimising the design of single-energy threshold, multiple-energy thresholds and energy-sensitive pixel array detectors. This analytical model is applied to typical silicon- and CdTe-based pixel sensor geometries associated to single-X-ray processing read-out electronics.