In order to increase the trust and willingness to invest in concentrating solar collectors, an accurate labeling and certification process is of utmost importance. Today’s collector testing standards specify methods for assessing the thermal performance of most types of fluid heating solar collectors. However, they do not provide an applicable testing procedure for obtaining the incidence angle modifier of large-scale line-focusing collectors. Such collectors need to be tested directly in the field, where optical characterization by conventional methods, such as factorization, fails. This study presents a new approach to obtain the incidence angle modifier by fitting ray-tracing curves to the measured optical efficiency data set. The new method has been tested on a fixed mirror solar concentrator with a mobile focus. Prior to this study, 49 experimental data points have been obtained for the optical efficiency after measuring for four testing days according to the ISO 9806 rules. These experimental data points served as a basis for the fitting procedure to validate the ray-tracing model. Stable optimized solutions of the collector’s optical parameters have been determined within a reasonable computation time scale. From a comparison of the optimized solution to a simplified ray-tracing simulation, it was seen that the weighted root-mean-square error was improved by 27.7%. In conclusion, the proposed procedure overcomes practical hurdles and has many advantages over conventional methods.