We propose a model to describe the behavior of the absorption profile as a function of the thickness changes of the photoresist layers (Shipley 1350-J), used to make high-quality holographic elements. We also show that the surface topology of the substrate is an important parameter that modulates the layer's thickness on the substrate. We obtain a depth modulation Δd in the exposure time (assuming a uniform irradiance), which is brought about by changes in the thickness of the photoresist layers at different points on the substrate; the thickness parameter is a function of the absorption coefficient of the photoresist.