Alkaline earth sulfide phosphors doped with Eu(2+) or Ce(3+) are good candidates for wavelength conversion in LEDs for general lighting applications. A very broad emission spectrum can be obtained by preparing SrS:Eu-SrS:Ce core-shell particles. These microcrystals are rectangular cuboids and can also be efficiently excited with electrons in a scanning electron microscope. The broad emission spectrum, ranging from 450 nm to 650 nm, consists of the combined emission of Ce(3+) and Eu(2+) centers. Using cathodoluminescence (CL) in a scanning electron microscope, evaluation of the emission characteristics of individual cuboids was possible. Sharp peaks in the CL-spectra of individual particles could be observed and described as being resonance modes. The spectral positions and the line widths of resonance modes in these core-shell microcrystals are predicted by an extension of the plane wave model, taking into account the dispersion of the refractive index and the existence of open orbits. (C) 2010 Elsevier B.V. All rights reserved.