The optical efficiency of concentrating solar thermal and photovoltaic systems with cylindrical geometries is asymmetrical about the optical axis. Biaxial models, based on projected incidence angles, are often used to estimate the annual performance of asymmetric concentrators. However, the use of projected angles tends to underestimate optical losses in the cover glass. In this work, a biaxial model for the incidence angle dependence of the optical efficiency, which uses the transverse projected incidence angle for determining the influence of the reflector and the real incidence angle to determine the influence of the glazing is proposed. The model gives an absolute value of the optical efficiency and it is valid for concentrating systems with translational symmetry, as well as for flat plate collectors and planar photovoltaic modules. The model is validated for a system with an east-west aligned parabolic reflector without a cover glass and it is shown that the dependence on the optical efficiency of the reflector on the longitudinal angle of incidence is negligible. The model is compared with the commonly used biaxial model and it is found that the difference is a couple of percentage points when the difference between the longitudinal projected incidence angle and the real incidence angle is large and the angle of incidence on the glass is high. (c) 2005 Elsevier Ltd. All rights reserved.