There are a growing number of commercially available solar thermal collector types: flat plates, evacuated tubes with and without back reflectors and different tubular spacing or low concentration collectors, using different types of concentrating optics. These different concepts and designs all compete to be more efficient or simply cheaper, easier to operate, etc. at ever higher temperatures, and to extend the use of solar thermal energy in other applications beyond the most common water heating purposes. In view of the proper dimensioning of solar thermal systems and proper comparison of different collector technologies, for a given application, there is a growing need for existing and future simulation tools to be as accurate as possible in the treatment of these different collector types. Collector heat losses are usually considered to be well determined, under variable operating conditions, through the use of the heat loss coefficients provided by efficiency curve parameters. Yet, the traditional approach to the optical efficiency fails to describe accurately the optical effects affecting the amount of radiation which actually reaches the absorber. This paper develops a systematic approach to the proper handling of incident solar radiation, folding that with the information available from tests for determination of collector efficiency curve (steady-state) and the way the optics of different collector types uses incident solar radiation and transforms it into useful heat. (C) 2008 Elsevier Ltd. All rights reserved.