Smart glass

Smart glass or switchable glass (also smart windows or switchable windows in those applications) is a glass or glazing whose light transmission properties are altered when voltage, light, or heat is applied. In general, the glass changes from transparent to translucent and vice versa, changing from letting light pass through to blocking some (or all) wavelengths of light and vice versa.

Smart glass technologies include electrochromic, photochromic, thermochromic, suspended-particle, micro-blind, and polymer-dispersed liquid-crystal devices.[1]

When installed in the envelope of buildings, smart glass creates climate adaptive building shells.

Electrically switchable smart glass

Suspended-particle devices

In suspended-particle devices (SPDs), a thin film laminate of rod-like nano-scale particles is suspended in a liquid and placed between two pieces of glass or plastic, or attached to one layer. When no voltage is applied, the suspended particles are randomly organized, thus blocking and absorbing light. When voltage is applied, the suspended particles align and let light pass. Varying the voltage of the film varies the orientation of the suspended particles, thereby regulating the tint of the glazing and the amount of light transmitted.

SPDs can be manually or automatically "tuned" to precisely control the amount of light, glare and heat passing through.

Electrochromic devices

Electrochromic devices change light transmission properties in response to voltage and thus allow control over the amount of light and heat passing through.[2] In electrochromic windows, the electrochromic material changes its opacity. A burst of electricity is required for changing its opacity, but once the change has been effected, no electricity is needed for maintaining the particular shade which has been reached.[3]

First generation electrochromic technologies tend to have a yellow cast in their clear states and blue hues in their tinted states. Darkening occurs from the edges, moving inward, and is a slow process, ranging from many seconds to several minutes (20–30 minutes) depending on window size. Newer electrochromic technologies eliminate the yellow cast in the clear state and tinting to more neutral shades of gray, tinting evenly rather than from the outside in, and accelerate the tinting speeds to less than three minutes, regardless of the size of the glass. Electrochromic glass provides visibility even in the darkened state and thus preserves visible contact with the outside environment.

Recent advances in electrochromic materials pertaining to transition-metal hydride electrochromics have led to the development of reflective hydrides, which become reflective rather than absorbing, and thus switch states between transparent and mirror-like.

Recent advancements in modified porous nano-crystalline films have enabled the creation of electrochromic display. The single substrate display structure consists of several stacked porous layers printed on top of each other on a substrate modified with a transparent conductor (such as ITO or PEDOT:PSS). Each printed layer has a specific set of functions. A working electrode consists of a positive porous semiconductor such as Titanium Dioxide, with adsorbed chromogens. These chromogens change color by reduction or oxidation. A passivator is used as the negative of the image to improve electrical performance. The insulator layer serves the purpose of increasing the contrast ratio and separating the working electrode electrically from the counter electrode. The counter electrode provides a high capacitance to counterbalances the charge inserted/extracted on the SEG electrode (and maintain overall device charge neutrality). Carbon is an example of charge reservoir film. A conducting carbon layer is typically used as the conductive back contact for the counter electrode. In the last printing step, the porous monolith structure is overprinted with a liquid or polymer-gel electrolyte, dried, and then may be incorporated into various encapsulation or enclosures, depending on the application requirements. Displays are very thin, typically 30 micrometer, or about 1/3 of a human hair. The device can be switched on by applying an electrical potential to the transparent conducting substrate relative to the conductive carbon layer. This causes a reduction of viologen molecules (coloration) to occur inside the working electrode. By reversing the applied potential or providing a discharge path, the device bleaches. A unique feature of the electrochromic monolith is the relatively low voltage (around 1 Volt) needed to color or bleach the viologens. This can be explained by the small over- potentials needed to drive the electrochemical reduction of the surface adsorbed viologens/chromogens.

Polymer-dispersed liquid-crystal devices

In polymer-dispersed liquid-crystal devices (PDLCs), liquid crystals are dissolved or dispersed into a liquid polymer followed by solidification or curing of the polymer. During the change of the polymer from a liquid to solid, the liquid crystals become incompatible with the solid polymer and form droplets throughout the solid polymer. The curing conditions affect the size of the droplets that in turn affect the final operating properties of the "smart window". Typically, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that include a thin layer of a transparent, conductive material followed by curing of the polymer, thereby forming the basic sandwich structure of the smart window. This structure is in effect a capacitor.

Electrodes from a power supply are attached to the transparent electrodes. With no applied voltage, the liquid crystals are randomly arranged in the droplets, resulting in scattering of light as it passes through the smart window assembly. This results in the translucent, "milky white" appearance. When a voltage is applied to the electrodes, the electric field formed between the two transparent electrodes on the glass causes the liquid crystals to align, allowing light to pass through the droplets with very little scattering and resulting in a transparent state. The degree of transparency can be controlled by the applied voltage. This is possible because at lower voltages, only a few of the liquid crystals align completely in the electric field, so only a small portion of the light passes through while most of the light is scattered. As the voltage is increased, fewer liquid crystals remain out of alignment, resulting in less light being scattered. It is also possible to control the amount of light and heat passing through, when tints and special inner layers are used.


Micro-blinds control the amount of light passing through in response to applied voltage. The micro-blinds are composed of rolled thin metal blinds on glass. They are very small and thus practically invisible to the eye. The metal layer is deposited by magnetron sputtering and patterned by laser or lithography process. The glass substrate includes a thin layer of a transparent conducting oxide (TCO) layer. A thin insulator is deposited between the rolled metal layer and the TCO layer for electrical disconnection. With no applied voltage, the micro-blinds are rolled and let light pass through. When there is a potential difference between the rolled metal layer and the transparent conductive layer, the electric field formed between the two electrodes causes the rolled micro-blinds to stretch out and thus block light. The micro-blinds have several advantages including switching speed (milliseconds), UV durability, customized appearance and transmission. The micro-blinds is developed at the National Research Council (Canada).

The expression smart glass can be interpreted in a wider sense to include also glazings that change light transmission properties in response to an environmental signal such as light or temperature.

  • Different types of glazing can show a variety of chromic phenomena, that is, based on photochemical effects the glazing changes its light transmission properties in response to an environmental signal such as light (photochromism), temperature (thermochromism), or voltage (electrochromism).
  • Liquid crystals, when they are in a thermotropic state, can change light transmission properties in response to temperature.
  • Various metals have been investigated. Thin Mg-Ni films have low visible transmittance and are reflective. When they are exposed to H2 gas or reduced by an alkaline electrolyte, they become transparent. This transition is attributed to the formation of magnesium nickel hydride, Mg2NiH4. Films were created by cosputtering from separate targets of Ni and Mg to facilitate variations in composition. Single-target d.c. magnetron sputtering could be used eventually which would be relatively simple compared to deposition of electrochromic oxides, making them more affordable. The Lawrence Berkeley National Laboratory determined that new transition metals were cheaper and less reactive, but contained the same qualities, thus further reducing the cost.
  • Tungsten-doped Vanadium dioxide VO2 coating reflects infrared light when the temperature rises over 29 °C (84 °F), to block out sunlight transmission through windows at high ambient temperatures.

These types of glazings cannot be controlled manually. In contrast, all electrically switched smart windows can be made to automatically adapt their light transmission properties in response to temperature or brightness by integration with a thermometer or photosensor, respectively.

Examples of use

Eureka Tower in Melbourne has a glass cube which projects 3 m (10 ft) out from the building with visitors inside, suspended almost 300 m (984 ft) above the ground. When one enters, the glass is opaque as the cube moves out over the edge of the building. Once fully extended over the edge, the glass becomes clear.[4]

The Boeing 787 Dreamliner features electrochromic windows which replaced the pull down window shades on existing aircraft.[5]

NASA is looking into using electrochromics to manage the thermal environment experienced by the newly developed Orion and Altair space vehicles.

Smart glass has been used in some small-production cars including the Ferrari 575 M Superamerica.[6]

ICE 3 high speed trains use electrochromatic glass panels between the passenger compartment and the driver's cabin.

The elevators in the Washington Monument use smart glass in order for passengers to view the commemorative stones inside the monument.

The city's restroom in Amsterdam's Museumplein square features smart glass for ease of determining the occupancy status of an empty stall when the door is shut, and then for privacy when occupied.

Bombardier Transportation has intelligent on-blur windows in the Bombardier Innovia APM 100 operating on Singapore's Bukit Panjang LRT Line, to prevent passengers from peering into apartments while the train is moving[7] and is planning to offer windows using smart glass technology in its Flexity 2 light rail vehicles.[8]

  • The 1982 film Blade Runner contains an early depiction of smart glass in a scene in which a room is darkened with a smart-glass-like shade so Rick Deckard, played by Harrison Ford, can administer a polygraph-style test to determine whether Rachael, portrayed by Sean Young, is an organic robot known as a replicant.
  • The 1993 film Philadelphia features a scene in which a large conference room in the middle of the law firm has walls of glass on three sides. Jason Robards says, "Bill, will you get the windows?", and a switch is thrown, and all the windows immediately become translucent, so that no one can see them firing Tom Hanks' character.
  • In the 1999 game Dino Crisis, there is a "bulletproof glass made of liquid crystal. You can't see through it because it is currently set to "smoke" mode" as the main protagonist Regina describes a panel of glass in the final area of the game.
  • Smart glass is seen in the 2002 motion picture The Sum of All Fears, in which Jack Ryan, played by Ben Affleck, is ushered into a secret room in the Pentagon, the windows of which whiten over as the door is shut.
  • Smart glass can be seen in the third season of the television series 24, where Jack Bauer changed the visibility to frosted glass to conceal the view as he was injecting heroin.
  • Smart glass is mentioned in season three, episode five of CSI: Miami, entitled "Legal", in which a young lady working undercover to expose underage drinking is murdered in a room shielded by what Ryan Wolfe refers to as "intelligent glass", where closing the door completes an electrical circuit, making the glass frost over and become opaque. The episode first aired in 2004.
  • Smart glass is seen in the television series Lie to Me with the interrogation/interview room at the Lightman Group offices consisting of what amounts to a room-sized box within a larger room, with smart glass walls. The walls appear to be white and opaque most of the time, but can be rendered clear to reveal those observing a subject from the outside.
  • Smart glass was featured in 2005 video game Tom Clancy's Splinter Cell: Chaos Theory in a fifth mission, "Displace International", enabling the main character to quickly switch between on and off modes with his OCP pistol attachment.
  • Smart glass was shown in the movie Iron Man (2008), after the reporter Christine Everhart wakes up after a one-night stand with Tony Stark.
  • Smart glass is seen in use in White Collar season 1 episode 8 "Hard Sell" when Neal comes to tell Daniel Reed that Avery plans to betray him. Daniel flips a switch and his office window becomes frosted over, preventing Avery from peeking inside while they talk.
  • Smart glass was featured in the 2012 James Bond movie Skyfall, revealing Raoul Silva to M after he is captured.
  • Smart glass was used in the bathroom in The Real World: Austin.
  • Dimmable smart glass was featured in the 2014 film, Captain America: The Winter Soldier, in the S.H.I.E.L.D. office in Washington, D.C.
  • Smart glass was featured in the 2014 animated feature Big Hero 6, used by Tadashi Hamada for his office.
  • In the fifth season of Angel, smart glass lines the interior wall of Angel's office, and can be frosted over at the flick of a switch under Angel's desk. (The fictional vampire-safe "necro-tempered glass" lines the outer walls of the building.)
  • Electrochromic glass can be seen in wide use at 2016 video game Deus Ex: Mankind Divided. EC glass is frequently used for blocking/unblocking vision between rooms and surrounding environment.

See also


  1. Baetens, R.; Jelle, B.P.; Gustavsen, A. (2010). "Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: A state-of-the-art review". Solar Energy Materials and Solar Cells. 94 (2): 87–105. doi:10.1016/j.solmat.2009.08.021. hdl:11250/2473860.
  2. Xu, Ting; Walter, Erich C.; Agrawal, Amit; Bohn, Christopher; Velmurugan, Jeyavel; Zhu, Wenqi; Lezec, J.; Talin, A.Alec (27 January 2016). "High-contrast and fast electrochromic switching enabled by plasmonics". Nature Communications. 7: 10479. doi:10.1038/ncomms10479. PMC 4737852. PMID 26814453.
  3. Mortimer, Roger J. "Switching Colors with Electricity". American Scientist. Retrieved 2 August 2018.
  4. "Experience the terrifying Edge at Eureka Tower". Herald Sun. 29 April 2007.
  5. "How The Magical WIndows in Boeing's 787 Dreamliner Work". Gizmodo. 10 August 2011. Retrieved 2 August 2018.
  6. McGrath, Jenny (7 August 2015). "Cooling down the house: A new smart glass could block both heat and light". Digital Trends. Retrieved 3 August 2018.
  7. "Bombardier INNOVIA APM100 (C801), Singapore". SG Trains. 2015-07-23. Archived from the original on 2015-07-23. The Bombardier INNOVIA APM100 (C801) trains are Singapore's first variant of LRT cars, which operates on the 14 station Bukit Panjang LRT Line operated by SMRT Light Rail Ltd. They were first developed by Adtranz as the CX-100, which was later acquired by Bombardier Transportation and renamed in 2001.
  8. "Bombardier to Feature Vision Systems' Nuance With SPD-SmartGlass From Research Frontiers at InnoTrans 2014 in Berlin, Germany". CNN Money. 2014-09-18. Archived from the original on 2014-09-19. This electronically dimmable window technology provides unsurpassed thermal insulation: SPD-SmartGlass substantially rejects solar heat from entering through windows. When compared to conventional automotive glass, Mercedes-Benz reported that the use of SPD-SmartGlass significantly reduced the temperature inside the vehicle by up to 18 °F/10 °C. This increases passenger comfort and reduces air conditioning loads, thereby saving fuel and reducing CO2 emissions.

Further reading

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