This research work has been carried out in effort to establish the evaluation method for the discomfort glare from windows. The existing Glare constant formulas are determined from the glare source located on the line of sight and the exponents in the formulas have fixed values through the entire visual field. In order to apply the existing Glare constant formulas to windows, it is necessary to confirm that the exponents have suitable values at the positions elsewhere on the line of sight. The experiments were conducted to obtain the precise exponents at various positions. Sensitivity to glare was tested on the Glare Testing Instrument consisting of a semi-spherical screen and the optical fiber lamp. Twenty-five observers aged between 20 and 47 had participated in the experiments. The results show that the exponents are variable according to the source position, source size, and background luminance. The modified Glare Constant Gw formula is proposed considering the variation of the exponents. The charts for determining the exponents of source size and background luminance are also proposed. The modified Gw, formula has the practical advantage in which the Glare Constant can be obtained directly at the position of the source.
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