WINDOW TECHNOLOGIES: Properties Primer

Solar energy transmission through three types of glass under standard ASHRAE conditions.

Reflectance

Just as some light reflects off of the surface of water, some light will always be reflected at every glass surface. A specular reflection from a smooth glass surface is a mirror-like reflection similar to the image of yourself you see reflected in a store window. The natural reflectivity of glass is dependent on the type of glazing material, the quality of the glass surface, the presence of coatings, and the angle of incidence of the light. Today, most glass manufactured in the United States is float glass, which reflects 4% of visible light at each glass-air interface or 8% total for a single pane of clear, uncoated glass. The sharper the angle at which the light strikes, however, the more the light is reflected rather than transmitted or absorbed (see figure to the right). Even clear glass reflects 50% or more of the sunlight striking it at incident angles greater than about 80°. (The incident angle is formed with respect to a line perpendicular to the glass surface.)

The reflectivity of various glass types becomes especially apparent during low light conditions. The surface on the brighter side acts like a mirror because the amount of light passing through the window from the darker side is less than the amount of light being reflected from the lighter side. This effect can be noticed from the outside during the day and from the inside during the night. For special applications when these surface reflections are undesirable (i.e., viewing merchandise through a store window on a bright day), special coatings can virtually eliminate this reflective effect.

Most common coatings reflect in all regions of the spectrum. However, in the past twenty years, researchers have learned a great deal about the design of coatings that can be applied to glass and plastic to preferentially reflect only selected wavelengths of radiant energy. Varying the reflectance of far-infrared and near-infrared energy has formed the basis for high-performance low-E coatings.

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