A phase change material is applied as a very thin film to a transparent substrate such as glass, which material when switched from the amorphous to the crystalline state and back again can affect the reflectivity/transmittance of the combined substrate-coating system. When used with glass panels in the fabrication of relatively large area window glass, the change in spectrally selective transmittance can be used to modulate the amount of sunlight passing through the glass, and thus reduce the amount of cooling required for an interior space in the summertime, and the amount of heating required of that same interior space in the wintertime, while also optimizing the use of visible daylight. Exemplary of a suitable phase change material for glass coating is GeSb or BiSn. Heating of the phase change material to initiate a change in phase can be provided by the application of electric energy, such as supplied from a pulsed power supply, or radiant energy, such as from a laser.
STATEMENT OF GOVERNMENTAL SUPPORT
 The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The government has certain rights in this invention.