WINDOW TECHNOLOGIES: Properties Primer
Energy that is not transmitted through the glass or reflected off its surfaces is absorbed. Once glass has absorbed any radiant energy, the energy is transformed into heat, raising the glass temperature.
Typical ¼-inch clear glass absorbs only about 7% of sunlight at a normal angle of incidence (also a 30-degree angle of incidence, as shown in the figure to the right). The absorptance of glass is increased by glass additives that absorb solar energy. If they absorb visible light, the glass appears dark. If they absorb ultraviolet radiation or near-infrared, there will be little or no change in visual appearance. Clear glass absorbs very little visible light, while dark-tinted glass absorbs a considerable amount. The absorbed energy is converted into heat, warming the glass. Thus, when "heat-absorbing" glass is exposed to the sun, it feels much hotter to the touch than clear glass. Tints are generally gray, bronze, or blue-green and were traditionally used to lower the solar heat gain coefficient and to control glare. Since they block some of the sun's energy, they reduce the cooling load placed on the building and its air-conditioning equipment. The effectiveness of heat-absorbing single glazing is significantly reduced if cool, conditioned air flows across the glass. Absorption is not the most efficient way to reduce cooling loads, as discussed later.
All glass and most plastics, however, are generally very absorptive of long-wave infrared energy. This property is best illustrated in the use of clear glass for greenhouses, where it allows the transmission of intense solar energy but blocks the retransmission of the low-temperature heat energy generated inside the greenhouse and radiated back to the glass.