In this paper are defined analytic relationships allowing estimating the concentration ratio losses at the focus of any type of thermodynamic solar facilities affected with opto-mechanical errors. The approximate expressions are based on a simple Lorentz function, and are usable for introducing various families of defects, such as mirror shape accuracy, alignment and tracking errors, and astigmatism. These relations are not intended for replacing more sophisticated computing codes (Monte-Carlo or convolution models), but are suitable for first-guess performance prediction and preliminary system engineering and design. (C) 2014 Elsevier Ltd. All rights reserved.