The optical properties for direct radiation can be further separated into two components. The first component is the
transmittance due to the direct radiation which is directly passing through the blind without touching the slat surfaces.
The second component is the transmittance due to the part of direct radiation which is interreflected between two adjacent slats.
The transmittance of the effective layer for the curved slat blind without thickness in the part that some of the direct radiation is directly passing through the blind without touching the slat surfaces, which is sometime called direct-to-direct transmittance, can be defined as