“Smart glass” and “switchable glazing” are generic terms that refer to all types of passive and active systems. Photochromic and thermochromic glazing, which are light- and heat-sensitive, respectively, are considered passive, because they do not require electricity. Those transitional sunglasses are smart, but passive. Active-control or electrochromic systems offer more options, but must be hard-wired to a power source.

Active technologies are the focus of most of the current research. Electrochromic windows are generally considered to be the most suitable chromogenic technology for energy control in buildings. They reduce or block light transmission and alter transparency in response to environmental signals, such as glare, sunlight, or temperature. The change from transparent to tinted is achieved when a small electrical current is applied to the window. The window returns to transparent when the voltage is turned off. Light transmittance during operation varies from 5 to 80 percent. Once the change in tint is initiated, the electrochromic glazing does not need constant current to maintain the tinting. In addition, the film can be tuned to block certain wavelengths, such as solar (heat) energy.