ABSTRACT
The primary function of a window is to allow daylight into the building. Before the invention of glass, windows were open apertures in the external walls of buildings and allowed wind and rain to enter through them. In consequence, the apertures were small and not particularly efficient at allowing daylight to enter the building. Modern windows comprise a sheet (or sheets) of transparent or opaque glass enclosed in a framework of timber, steel, aluminium or PVCu (plastic). This enables windows to be much larger and their daylighting efficiency is much improved.
