# If the reflected ray intersects with the aperture surface the

If the reflected ray intersects with the aperture surface, the ray enters into the cavity. Then the simultaneous equations of the reflected ray and interior surfaces are solved to determine the location of the intersection point.
In order to figure out the photon distribution on the cavity interior surfaces, the surfaces are divided into Ne elements and an array P[i] is used to count the number of photons on each element, where i   is the label of elements and in the range of 1≤i≤Ne1≤i≤Ne. According to the coordinates of the intersection point, it can be determined which GBLD 345 the photon arrives at. If the photon arrives at i element, P[i] is renewed as:equation(36)P[i]=P[i]+1P[i]=P[i]+1
3.2. Simulation of the solar radiation transfer process within the cavity
3.2.1. View factors and Gebhart factors
The view factor and the radiative heat transfer factor (also referred to as Gebhart factors) are two related concepts and are the foundations of Gebhart method.
The view factor Xi,,j is defined as the ratio of the radiation leaving element i that falls directly on element j to the total radiation leaving element i, where i   is in the range of 1≤i≤Ne1≤i≤Ne, and j   is in the range of 1≤j≤Ne+11≤j≤Ne+1. The cavity aperture is treated as element Ne + 1. The view factor depends only on the geometry of the cavity receiver. It is hard to calculate the view factors for complex cavities by any analytic method. Therefore, the Monte-Carlo method is applied to calculate the view factors of the cavity receiver in the present study.