The transmittance, reflectance, and absorptance of a single cover, allowing for both reflection and absorption losses, can be determined by ray-tracing techniques. For the perpendicular component of polarization, the transmittance, reflectance and absorptance of the cover are [4]:

(2.2.1)

(2.2.2)

(2.2.3)

Similar results are found for the parallel component of polarization. For incident unpolarized radiation, the optical properties are found by the average of the two components.

2.3. TRANMITTANCE-ABSORPTANCE PRODUCT (та)

Some of the radiation passing through the cover system is reflected back to the cover system while the remainder is absorbed at the plate. In turn, the reflected radiation from the plate will be partially reflected at the cover system and back to the plate as illustrated in Figure 2.2.1. In this figure, т is the transmittance of the cover system at the desired angle, a is the angular absorptance of the absorber plate, and Pd refers to the reflectance of the cover system for diffuse radiation incident from the bottom side. It is assumed that the reflection from the absorber plate is diffuse and unpolarized. The multiple reflection of diffuse radiation continues so that the fraction of the incident energy ultimately absorbed becomes [4]:

(та) |eff = та І[(1-а) pd]n =______ ш___

n=0 1-(1- a) Pd