In this paper, we focus on the variation of the transmittance of the receiver glass envelope as a function of the incidence angle and we measure its impact on the annual optical efficiency of a LFR plant using ray-tracing techniques. For this purpose, we draw up a detailed model of the LFR collector installed on the roof of the School of Engineering of the University of Seville, Spain. We also calculate the optical efficiency with and without a secondary reflector and with constant or variable transmittance receiver glass envelope properties. We run simulations using a clear-sky annual 1-min synthetic data set as input and calculate an average annual optical efficiency using efficiency matrices and Incidence Angle Modifiers (IAM) obtained from ray-tracing simulations. We find that the effect of the variation of the receiver glass envelope optical properties, as a function of the incidence angle, reduces the annual optical efficiency by 2.5% when the LFR plant has a basic secondary reflector and by 0.7% when there is no secondary reflector, according to the results obtained when using constant optical properties. We also evaluate the performance of the system with an optimised secondary reflector design. (C) 2020 Elsevier Ltd. All rights reserved.Y