An optical model is presented to determine the constraints imparted by self-absorption on the luminescence emitted from down-conversion (DC) materials. An analytical formula was derived demonstrating that, for a given DC material, an optimal thickness exists that maximizes the intensity of the emitted radiation, while minimizing the impact of self-absorption. This defines a new limit for the optical efficiency of a DC material that is governed by its geometry. Subsequently, the model was validated through experimental analysis of a specific down-converting borate glass co-doped with Ce3+-Yb3+, whose optimal thickness has been determined to be 0.83 mm. The model clarifies the origin of the disparity between the theoretical and the experimental efficiencies reported for some materials. The results from this work assist with the design and implementation of DC layers for photovoltaic devices, as well as providing a framework for optimization of DC materials to other fields of optics and photonics. (C) 2013 Elsevier B.V. All rights reserved.