Sky component grids for glazed vertical windows

Daylight contribution at a point due to sky visible through different areas of a window is a function of the sky luminance, the projected solid angle of these areas at the reference point on the working plane and the transmittance of glass. The transmittance of glass varies considerably with the angle of incidence, which depends upon the relative positions of different parts of a window opening with respect to the reference point. A sky component grid, representing the sky component values from different elemental areas of openings in a window wall, forms one of the simplest ready reckoners in daylighting design. This paper presents the methodology of generating sky component grid for any sky condition. The relevant data on mean altitude of grid elements, the mean angle of incidence for different grid elements, and differential sky factors are included for facilitating the computation of grid values. The sky component grid for Indian clear design sky based on exact equations of sky component for a horizontal plane, with a due allowance for transmittance of 3 mm thick clear glass, is provided for comparing the accuracy of the methodology based on differential sky factors. The values computed from the data and the methodology presented in this paper agree closely with the exact values. The deviation of computed values from exact values is less than ±10%.