1. Field of Invention
The present invention relates generally to single-layer broadband reflective polarizers having electrically controllable polarization efficiencies and reflection bandwidths, and more particularly to single-layer reflective polarizers which can be electrically switched from broadband operation to narrow band operation over the visible and infrared (IR) bands, as well as single-layer reflective polarizers which can be electrically switched from narrow band operation to broadband operation over the visible and IR bands.
2. Brief Description of the Prior Art
Humans have a natural affinity for pictures. Dramatic improvements in modern display devices are possible with the advent of novel polarizer technologies. In many advanced display techniques, a polarizing device having electrically controllable transmission characteristics such as polarization, transmittance, and spectra is desired in order to actively control the display brightness as well as color balance, for example.
Electrically controllable polarizers can also serve as an enabling technology for other applications such as smart window, wherein the window transmission is electrically switchable from a totally reflective state to a totally transparent state by incorporating two switchable polarizers with opposite polarization states. Such a window can provide lighting control and can additionally provide energy conservation benefits if externally mounted.
Since early attempts at utilizing cholesteric film as optical filters and polymer encapsulated nematic liquid crystals for display devices, there has been great attention focused on trying to bring polymeric liquid crystals and cholesteric liquid crystals together to make devices or light control application.
U.S. Pat. No. 5,691,789 discloses a single-layer reflective super broadband circular polarizer and method of fabricating the same by producing a single layer having cholesteric liquid crystal (CLC) order where the pitch of the liquid crystal order varies in a non linear fashion across the layer.
European Patent Application 0 643 121 A published Mar. 15, 1995 discloses a narrow band, switchable polarizing single layer reflector.
PCT application WO97/2358, published Jul. 3, 1997, discloses a switchable polarizing single-layer reflector having a broader bandwidth.
General references on polymer dispersed liquid crystals may be found: “Polymer Dispersed Liquid Crystal Displays”, by J. W. Doane; a chapter in “Liquid Crystals”, Ed. B. Bahadur, World Scientific Publishing, Singapore; and “CLC/Polymer Dispersion For Haze-Free Light Shutters”, by D. Yang et al. Appl. Phys. Lett. 60, 3102 (1992).
Smart Window Design is described in “Electrochromism And Smart Window Design”, by C. Granqvist, Solid State Ionics 53-56 (1992) and “Large Scale Electochromic Devices For Smart Windows And Absorbers”, by T. Meisel and R. Baraun, SPIE 1728, 200 (1992).
The above identified US patents and other prior art references are hereby incorporated by reference.
While there is there has been a great need in the art for a n improved single-layer electrically controllable broad-band reflective polarizer for use in diverse applications, prior art methods and technology have clearly failed to teach how to how to practice the same in a feasible manner.