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.