It has been suggested earlier that the incidence angle modifier K-tau alpha for low concentrating collectors with tubular absorbers could be factorised according to K-tau alpha(theta(t), theta(l)) proportional to K-tau alpha(theta(t), 0)K-tau alpha(0, theta(l)), where theta(t) and theta(l) are the projected incidence angles in the transversal and longitudinal projection planes, respectively. Ray-tracing calculations on low-concentrating CPC collectors with flat absorbers parallel to the cover show that a K-tau alpha factorisation overestimates the annual delivered energy from the collector by about 4-5%, when compared to calculations using the full incidence angle modifier. Data from outdoor testing has been used for characterization of incidence angle behaviour for a truncated CPC with a concentration of C=1.56. Multiple linear regression analysis was used. This analysis technique makes feasible the determination of angular dependent incident angle modifiers and is an efficient tool to use for all collectors which cannot be characterised by standard equations of the incidence angle dependence.