Electrochromic glass works by moving ions in one layer of glass through an inner separator layer and into another layer of glass through the use of electrodes sandwiched in the layers. Once the ions are on the other side of the window, they filter light by creating a tint of a certain color. When electricity is applied again, they migrate back to the other side, making the window clear again.
Electrochromic glass can block visible light or infrared radiation or both, and users can adjust the amount of the tint by adjusting the electric current. Once the desired level of tint is reached, the voltage stops. Earlier versions of electrochromic glass needed a constant source of electricity to stay either opaque or clear, but newer versions only require an application of electricity to switch from one state to the other, making them efficient to operate. It's still possible to see through windows in their tinted state while blocking sunlight and heat, which lowers cooling bills in the summer months. Electrochromic glass is used in museum settings to protect sensitive material from damaging UV rays and to cut down on glare on rearview mirrors in cars, but as of 2014, it is not widely used for building windows.