The optical efficiency of an evacuated tube array varies
across the day depending on the configuration of the tube
array, i.e. tube dimensions, tube spacing and the distance
between the tubes and the reflector plane. The transverse
incidence angle modifier of the water-in-glass collector under
test is presented in Fig. 4 Budihardjo et al. (2003). The
variation of the optical efficiency with longitudinal incidence
angle is modelled as (Duffie and Beckman, 1991):