When the blind is in the position that there is some of the incident direct radiation directly passing through the blind without touching the slats, the whole surface of the slat shall receive the rest of the incident direct radiation.
In this case the optical properties due to the part of direct radiation which is interreflected between two adjacent slats can still be determined in the same procedure as described above except changing the analysis from the 6 surface closed enclosure to the 4 surface closed enclosure (neglecting surfaces 5 and 6, surfaces 3 and 4 are extended to the whole width of the curved slat).
The optical properties in Eqs. (37) and (38) can be calculated based on a portion of a unit incident direct radiation flux that excluded the portion of the radiation that is passing through the blind without touching the slats.
The front absorptance of the effective layer for direct radiation in this condition can be expressed as