Glasses: Smart glass
Smart materials are those materials specifically designed to have one or more properties that will change in a desired manner in response to an anticipated external stimuli. Smart glass, then, is any type of glass that fits this category, of which there are many.
Thermochromic glasses are glasses which change color (typically tint) in response to changes in heat. Often these glasses are responsive enough to change in direct response to sunlight, letting in more light (i.e. being more transparent) when the sun is not shining as brightly. This can help control the amount of light needed within a structure, as well as the energy consumption of heating or cooling, depending on the climate. Thermochromic windows are typically produced in layers, as shown in the upper left image above.
Electrochromic glasses, then, are glasses which change color (or tint) in response to the amount of voltage applied to the glass. These types of glasses (often used for windows) allow occupants to tint the glass at will, sometimes for the same reasons as mentioned above, but occasionally simply for comfort or privacy. Electrochromic glasses offer more control than thermochromic glasses, but it requires the ability to control the voltage as well. (The amount of electricity used, however, can be far less than the amount that could potentially be saved by allowing for natural lighting.)
Finally, photochromic glasses also have a similar effect, those these glasses react to the presence of light, not heat as with thermochromic glasses. Photochromic glasses are most popular in lenses.
Other types of smart glass include suspended particle and polymer dispersed liquid crystals. The latter is not actually a form of glass, but rather a layer between the glass. As with electrochromic glass, the application of voltage changes the tint, but PDLCs react much faster than electrochromic materials.
Technically speaking, it is often glazings added to glasses that help produce these effects, which is why windows of these types of glasses are constructed in layers. The movement of ions or electrons through the layers can often be the basis for the change in tint. As such, materials which claim to be ‘smart glass’ are typically combinations of glass and coatings, thin films, or other layers between the glass. There are, however, exceptions.
Sources/Further Reading: ( 1 – image 1 ) ( 2 – image 2 ) ( 3 – image 3 ) ( 4 – image 4 ) ( 5 – image 5 ) ( 6 ) ( 7 ) ( 8 ) ( 9 ) ( 10 )