The optical properties of the effective layer in the part that the direct radiation indirectly passed through the blind by reflections between the slats are determined by first considering the blind as the curved slat without thickness for the sake of simplicity.
The transmittance of the effective layer for the curved slat blind in this part can be determined from the cell composed from two adjacent slat surfaces into a closed enclosure as suggested by EnergyPlus (2005).
The slat surfaces of the blind are assumed to be perfect diffuse surfaces.
This closed enclosure is composed of 6 surfaces as shown in Fig. 5.
The slat is subdivided into 2 surfaces; the surface that receives the solar direct radiation (de) and the surface that is blocked from the solar direct radiation (ef).
The 6 surface closed enclosure in this model is different from the closed enclosure used in ISO15099 (ISO 15099, 2003) and EN 13363 (EN 13363-2, 2005).
A 14 surface closed enclosure is used in ISO15099.
Each slat is subdivided into five equal parts.
A 6 surface closed enclosure is used in EN13363.
But the definition of the surfaces on the slat is different from the developed model and ISO15099.
Instead of dividing the slat into separate surfaces, EN13363 defines 2 surfaces on each slat surface.
The first surface is assigned to the whole slat surface.
The second surface is assigned to the part of the slat surface that receives the solar direct radiation.
The second surface is superimposed on the first surface.
Therefore the view factors that used to calculate the optical properties are different from the developed model.
Both models (ISO15033 and EN13363) are only applicable for a flat slat blind.