Calculation of Externally Reflected Component (ERC):

Charts
If there is an obstruction opposite the window, then the lower limit for the sky component will be a line drawn from point P to the top of this obstruction with reference to Figures 6.1 and 6.2. The sector between this line and the sill PB line will give the ERC, Follow the same procedure as for the sky component. Multiply the result by the reflectance of the obstruction (if known) or by 0.2 if the CIE-sky protractors were used. Using the uniform sky protractors, take half of the reflectance value or 0.1 as the multiplying factor.

(The ERC is initially calculated in the same manner as the sky component. Sight lines are drawn to the top edge of obstructions visible from the reference point. This gives an equivalent sky component, which is converted to the externally reflected component by allowing for the reduced luminance of the obstructing surface. For uniform sky protractors the initial ERC is divided by 10. For CIE sky protractors the initial ERC is divided by 5.

Calculation of Internally Reflected Component (IRC):

The internally reflected component of daylight inside a room depends on the reflectances of the room surfaces and on the size of the windows and obstructions. The process of multiple reflections is complex but formulas and tables are available to calculated the IRC.

                              
Table 2 The Minimum IRC of Daylight Factor.

The average IRC can be determined quite precisely from the BRS inter-reflection formula. The simplified form of the is :

Average IRC =0.85W ( Cr_fw + 5r_cw )/ A (1-r)

where 0.85 = transmittance of window glazing assumed
           W     = window area, m²
           A       = total area (ceiling + floor + walls including windows)
           r       = average reflectance of area A
          r_fw     = average reflectance of floor and the three walls below the plane at the
                        level mid-height of the window (excluding the window wall)
           r_cw   = average reflectance of ceiling and the upper (remaining) part of the above
                        three walls
           C        = a coefficient depending on external obstructions