User:Chris Howard/Smart glass

Smart glass or switchable glass, also called smart windows or switchable windows in its application to windows or skylights, refers to electrically switchable glass or glazing which changes light transmission properties when voltage is applied.

Certain types of smart glass can allow to control the amount of light and heat passing through: with the turn of a button, it changes from transparent to opaque, partially blocking light while maintaining a clear view of what lies behind the window. Another type of smart glass can provide privacy at the switch of a button.

Smart glass technologies are electrochromic devices, suspended particle devices, and liquid crystal devices.

The use of smart glass can save costs for heating, air-conditioning and lighting and avoid the cost of installing and maintaining motorized light screens or blinds or curtains. When opaque, liquid crystal or electrochromic smart glass blocks most UV, thereby reducing fabric fading; for SPD-type smart glass, this is achieved when used in conjunction with low-e low emissivity coatings.

Critical aspects of smart glass include installation costs, the use of electricity, durability, as well as functional features such as the speed of control, possibilities for dimming, and the degree of transparency of the glass.

Electrochromic devices
Electrochromic devices change light transmission properties in response to voltage and thus allow to control the amount of light and heat passing through. In electrochromic windows, the electrochromic material changes its opacity: it changes between a colored, translucent state (usually blue) and a transparent state. A burst of electricity is required for changing its opacity, but once the change has been effectuated, no electricity is needed for maintaining the particular shade which has been reached. Darkening occurs from the edges, moving inward, and is a slow process, ranging from many seconds to several minutes depending on window size. Electrochromic glass provides visibility even in the darkened state and thus preserves visible contact with the outside environment. It has been used in small-scale applications such as rearview mirrors.

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.

Suspended particle devices
In suspended particle devices (SPDs), a thin film laminate of rod-like particles suspended in a fluid is placed between two glass or plastic layers, or attached to one layer. When no voltage is applied, the suspended particles are arranged in random orientations and tend to absorb light, so that the glass panel looks dark (or opaque), of blue or, in more recent developments, grey or black colour. When voltage is applied, the suspended particles align and let light pass. SPDs can be dimmed, and allow instant control of the amount of light and heat passing through. A small but constant flow of electricity is required for keeping the SPD smart window in its transparent stage.

Polymer dispersed liquid crystal devices
In polymer dispersed liquid crystal devices (PDLCs), liquid crystal droplets are arranged in a sheet between two layers of glass. In the "off" state, they are randomly oriented and, when switched on, they align according to the electic field. The liquid crystals scatter light, without blocking it, thus the glass looks white even when in its transparent state. There is no possibility to control the amount of light and heat passing through, and the device operates in on/off states only. This technology has been used in interior settings for privacy control (for example conference rooms, intensive-care areas, bathroom/shower doors) and as a temporary projection screen. It has been marketed under the name of "switchable privacy glass".

Related areas of technology
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.
 * Vanadium dioxide VO2 coating reflects infrared light when the temperature rises over 29 degrees Celsius, to block out sunlight transmission through windows at high ambient temperatures.

These types of glazing 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.

The topic of smart windows in a wider sense includes also self-cleaning glass and the automatic opening or closing of windows for ventilation purposes, for example according to a timer or in response to a rain sensor.

Corporate websites

 * SmartGlass International Ltd. (electrochromic glass, liquid crystal glass) Corporate website
 * Research Frontiers Inc. (SPD-SmartGlass) Corporate website
 * Polytronix Inc. (PDLC liquid crystal glass) Corporate website
 * SAGE Electrochromics Inc. (SageGlass electrochromic glass) Corporate website

Category:Glass Category:Liquid crystals Category:Solar design Category:Smart materials Category:Thin-film optics Category:Windows