The Research Search For The Least Beneficial Overcast Sky And Progress In Defining Its Luminance Gradation Function

Abstract Recently illuminance levels under ISO/CIE homogeneous standard sky types were characterised in their relative terms after ISO/CIE (2004, 2003) standardised as normalised by the luminance in the zenith. Sky luminance and horizontal illuminance based on the gradation and scattering indicatrix functions, including the extreme overcast cases frequently encountered in nature, were recently determined in absolute physical units of luminance in kilocandles per meter square and of illuminance in kilolux. The historical search to find energy and visibility critical sky luminance distributions shows a progression of steps in studying the worst or critical overcast situations. That progression has enabled the determination and evaluation of interior illuminance for comparison of the merits of dual daylighting and artificial lighting under established criteria for comfortable visibility.