Glass Absorptance

Energy that is not transmitted through the glass or reflected off of its surfaces is absorbed. Once glass has absorbed any radiant energy, the energy is transformed into heat, raising the temperature of the glass.

Typical 1/8-inch (3 mm) clear glass absorbs only about 8 percent of sunlight at a normal angle of incidence. 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 these "heat-absorbing" glasses are in the sun, they feel much hotter to the touch than clear glass. They are generally gray, bronze, or blue-green and are used primarily 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. Absorption is not the most efficient way to reduce cooling loads.

All glass and most plastics, however, are generally very absorptive of far-infrared energy. This property led to the use of clear glass for greenhouses, where it allowed the transmission of intense solar energy but blocked the retransmission of the low-temperature heat energy generated inside the greenhouse and radiated back to the glass.