We have developed a model to calculate the refractive index modulation in photorefractive chiral smectic A and C materials. The index pattern is calculated considering the simultaneous presence of a sinusoidal spacecharge field and an applied field. The material parameters considered by the model are the principal indices of refraction of the mesophase, the relative amplitude of the applied and space-charge field, the electroclinic coefficient (for S-A*), and the spontaneous tilt angle (for Sc*). The model also takes into account the sample orientation and light polarization. Results show large variations of the index modulation with the various paremeters considered. Experimental data obtained from polymer-dispersed chiral smectic A samples are in excellent agreement with the predictions of the model.