A light coloured blind reflects some of the solar radiation and absorbs the rest. This causes the blind to heat until it is losing heat at the same rate as it receives it from the sun. If the blind is in the room, most of the energy it absorbs is added to the room's cooling load. If it is between the panes of a double window, however, some of the absorbed energy is transferred to the outside air and the room's cooling load is reduced accordingly. It is desirable therefore, to use light coloured blinds and, if possible, to place them between the panes, of a double window.
Heat absorbing glass is also widely used to reduce solar heat gain.
Glasses are available that absorb over 70 per cent of the incident radiation so that transmission to the inside of a building is about 20 per cent when the angle of incidence is small and even less when it is large. Absorbing glass is not as good as these figures indicate, however, because the energy that is absorbed by the glass is dissipated to the surroundings on both sides of the window. The proportion of absorbed energy that is transferred to the inside depends on the relative magnitudes of the heat transfer coefficients at the inside and outside surfaces. If cool air is introduced into the room through a grill along the window sill more than half the absorbed energy is transferred to the room side. Thus, the use of heat absorbing glass may cause a higher maximum cooling load than occurs with ordinary glass because part of the absorbed energy is transferred to the room air very soon after it bas been absorbed by the glass.