Spectral X-ray imaging with single photon processingdetectors gains substantial interest for many applications. In this paper wediscuss fundamental parameters as contrast to noise ratio (CNR) and spectralresponse as a function of the material in the object. Image properties havebeen simulated for different photon energies using MCNP5, assuming an idealdetector with 32 × 32 pixels. Simulations are supported by experimentalresults obtained with detectors from the MEDIPIX family.The CNR is strongly dependent on the number of incident photons and thenumber of photons absorbed in the object. The requirement for substantialabsorption in the object limits the range of useful photon energies. In mostcases the CNR is improved when high energy photons are removed from thespectrum.Materials can be uniquely identified or layers of different materials can beseparated provided that there is a substantial difference in their spectralX-ray absorption. In most cases an absorption edge in the spectrum is neededto obtain good results. Several examples of material identification andmaterial separation are discussed.