CPV systems aim to deliver electrical power at lower cost than will be possible with traditional photo-voltaics. To achieve this, maximum performance from the solar cells should be obtained, while minimizing the cost of balance-of-system. Therefore it is important that CPV optical systems are evaluated in terms of their impact on solar cell performance. One attribute of particularly lens based CPV systems is the variance in incidence angles of light at the solar cell surface, especially when a secondary optic element is employed. The electrical performance of TJ CPV solar cells for varying angles of incident illumination is studied in detail. The solar cells suffer a loss of performance of up to 58% for oblique illumination. Calculations and ray tracing simulations show that optical losses are caused by Fresnel reflections off the ARC, and scattered reflections off the front metal grid due to surface roughness. Additionally the merit of using secondary optics in spite of this effect is shown for symmetrical CPV systems. For asymmetrical (i.e. prism based) systems however, the loss of performance may be substantially larger. Therefore grid orientation and design in respect to the optical system should be taken into account and optimized in such systems. (C) 2017 The Authors. Published by Elsevier Ltd.