Smart windows are made of materials that can be easily switched between a transparent state and a state that is opaque, translucent, reflective, or retro-reflective. The switching is done by applying an electric voltage to the material, or by performing some other simple, often mechanical, operation. They can be used to regulate the flow of light and radiant heat into or out of a building or other space.
 Hi-tech designs
There are various hi-tech types of smart window. Some use liquid crystals, sandwiched between films of Polaroid. An electric field orients the liquid crystals so they interact with the plane of polarization. Depending on the voltage, the light may, or may not, be able to pass through both sheets of Polaroid. (This is essentially the same technology as is used in Liquid Crystal Displays for digital devices, video screens, and so on.) Other designs work like venetian blinds, sometimes on a very small scale so the slats of the blinds can not easily be seen with the naked eye.
 Low-tech designs
An interesting design has recently been produced by the Norwegian inventor Kim Øyhus. He calls it the SunValve. He has published the invention without patenting it, so it can be freely used by anyone. Essentially, his window consists of two sheets of retro-reflective plastic material fixed back-to-back with a narrow space between them. When the space is filled with air, the window is retro-reflective, so it sends unwanted light or heat back toward its source. The retro-reflection depends on the occurrence of total internal reflection at the interface between the plastic and the air in the space. To make the window transparent, a liquid with a refractive index equal, or nearly equal, to that of the plastic is introduced into the space between the sheets. This stops the reflection, so light then passes easily through the entire assembly. This window is simple and cheap, and would be suitable for use in the developing world.
He has produced a YouTube video in which he demonstrates and explains the SunValve. It can be accessed by clicking on the following URL.