During a calculation technique, contributions to reflected light from multiple discrete cells in a model of a multilayer stack in a reflective photo-mask may be determined based on angles of incidence of light in a light pattern to the multilayer stack, a polarization of the light in the light pattern, and a varying intensity of the light in the light pattern through the multilayer stack. Then, phase values of the contributions to the reflected light from the multiple discrete cells are adjusted, thereby specifying optical path differences between the multiple discrete cells in the multilayer stack that are associated with the defect. Moreover, the contributions to the reflected light from multiple discrete cells are combined to determine the reflected light from the multilayer stack. Next, k-space representations of the contributions to the reflected light from the multiple discrete cells are selectively shifted based on the angles of incidence.