Windows that can transmit a controllable fraction of incident light intensity would be beneficial in a variety of applications. Thus, windows of buildings or vehicles, windows between rooms, and personal items such as glasses or goggles are often used in situations in which it would be advantageous if their optical transparency could be adjusted, for example by electrical means.
Great efforts have been expended to improve the ways and means of selectively controlling the transmission of light through window structures. A common approach to light control involves using an opaque window shade to reduce the transmission of electromagnetic radiation. Such shades may either be purely mechanical (the most common type) or may be controlled by a motor. Another approach to variable control of light transmission can be achieved by mechanically rotating a pair of polarizing films where the relative angle between polarizing axes of the polarizing films are changed.
Another approach to light control involves the use of polymer films or doping glass with metal ions to absorb or reject certain wavelength range of the electromagnetic radiation. Light transmission through windows using such technologies is fixed once the window structures are constructed.
In recent times, there has been great interest in using variable light transmission glass or glazing to achieve electromagnetic radiation control. A summary of variable tinting window developments can be found in, for example, “Chromogenic Switchable Glazing, Towards Development of the Smart Window” by Carl Lempert, published in the June 1995 Proceedings of the Window Innovation Conference, Toronto, Canada; and “Optical Switching Technology for Glazings” by Carl Lempert, published in Thin Solid Films, Volume 236, 1993, pages 6-13.
Several different types of chromogenic switchable glazing structures have been discovered using suspended particle devices, electrochromic effects, and certain types of liquid crystal. In general, the structures absorb the incident light or diffuse the incident light.
A glazing structure disclosed in U.S. Pat. No. 5,940,150 proposes an adjustable trans-reflective window that is primarily based on two layers of cholesteric liquid crystals and their ability to reject or to transmit one circular polarized light depending on the electric stimulus applied while capable of transmitting the other. However, a specific cholesteric liquid crystal typically works in only a narrow bandwidth of light and a stack of liquid crystal materials are needed to broaden the working wavelength range of the device. Thus, significant material as well as device development effort is needed in order that the glazing structure be practical. The patent also teaches a glazing structure sandwiching a controllable retardation film between two linear reflective polarizers. Such a glazing structure will reflect theoretically 50% to 100%, and transmit 50% to 0%, of naturally polarized light depending on the external stimulus applied. The theoretical contrast ratio between the maximum reflection state and the minimum reflection state of the glazing structure employing linear reflective polarizers disclosed in that patent is therefore only 2:1, making it less desirable as an electrically controllable mirror.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those skilled in the art upon a reading of the specification and a study of the drawings.