Glare from windows: current views of the problem

This paper discusses daylight glare as a source of visual discomfort in terms of glare from the unobstructed sky and not glare related to direct or reflected sunlight. There is a difference between the glare experienced from a window and the glare experienced from a large source of artificial light of the same subtended area as the sky, due to the pyschological differences in the visual content of the field of view. There is also a difference in the discomfort glare produced by one small source or a number of small sources and a source with the same glare index subtending a large solid angle, due to the effect of the larger source on the adaptation level of the visual mechanism. The paper reviews the laboratory studies using artificial light sources of large area which were carried out in England and the USA and also the studies using daylight seen through real windows which were carried out in France and in England independently. The link between the practical studies and the laboratory studies is indicated and the derivation of quantitative requirements is shown. The analysis of the theoretical results indicates the effect of the various parameters involved. The conclusion is that discomfort glare from a single window (except for a rather small one) is practically independent of size and distance from the observer but is critically dependent on the sky luminance. Practical guidance is given on the design of windows and their surroundings to reduce the problems of glare from the sky, which is otherwise certain to be uncomfortable for a very large part of the year. The requirements for the balance of interior and exterior luminances are discussed, and a method of calculation and its practical application is put forward.