Traditional polymer-dispersed liquid-crystal-type (PDLC) glazing has been commercialized for privacy-window and automobile sunroof applications. A drawback of this technology is that the intrinsic refractive-index mismatch of polymers and liquid crystals (LCs) causes serious haze in the transparent configuration, particularly at large viewing angles. As an alternative, we have developed novel multifunctional displays that use a polymer-stabilized cholesteric-texture (PSCT) coating. PSCT permits three optical states, a white frosted appearance for privacy protection, a transparent phase for see-through purposes, and an information-display functionality, all employing a single panel. The mix of materials in PCST is better matched than that of PDLC. At the same time, the amount of polymer is significantly reduced, with the result that the transparent state of PSCT glazing is virtually haze-free. A second advantage of PSCT is bistable states.1,2 Finally, when properly engineered, PSCT windows coupled with suitable electric driving3 also permit information display. This is impossible to achieve with PDLC-based devices.
Figure 1. Frosted state of a PSCT panel at zero voltage.
Like PDLC glazing, PSCT panels are covered with two transparent substrates (either glass or a clear polymer sheet), whose inner surface is laminated with a transparent conductive coating, such as indium tin oxide (ITO). A thin layer of monomer and cholesteric LC-material mixture is sandwiched between the two substrates. UV exposure leads to the formation of a polymer network. Similar to PDLC, at zero voltage the PSCT—in a so-called focal-conic texture—strongly scatters light, and it appears whitish opaque. Under a sufficient voltage, the PSCT is switched to the homeotropic (vertical-to-surface) optically transparent state.
The particular properties of the PSCT device enable the light-scattering focal-conic state to be maintained when the voltage, V, is increased from zero (V0) to a certain value, Vb. Further increasing the voltage, to Vh, induces the material to switch to its transparent appearance. Reducing the potential difference from Vh to Vb causes the transparency to persist until V is further decreased. Both the focal-conic and the homeotropic (transparent) states are therefore stabilized at Vb. The appearance of the PSCT device at Vb depends on the initial state of the appliance. This is known as bistability. With a proper electric driving mechanism, a pixilated PSCT panel can display information. Both frosted and clear pixels are maintained at a voltage of Vb if a static character is shown. The image is refreshed and rewritten as the pixels are voltage-scanned line by line. If the scanning rate and response time are fast enough, PSCT panels can be used for video display.
We have fabricated a demonstration PSCT panel with a resolution of 64×64 pixels. Figure 1 shows the panel in its frosted, opaque state. When all the pixels are switched to the transparent state, the panel becomes optically clear (see Figure 2). If the individual pixels are forced into either their frosted or clear appearance, the resulting contrast allows display of characters (see Figure 3).
Figure 2. Transparent state of a PSCT panel at bistable voltage.
Figure 3. Character-display configuration of a PSCT panel.
This type of glazing can be applied, for instance, to advertising in store windows. It simultaneously allows both character display and see-through functionality (see Figure 4), in addition to privacy protection. More general applications include conference-room and executive-office windows and doors, automobile windows, and counter message boards for banks and ticket offices. Flexible, ITO-coated, clear-plastic film substrates can be laminated onto existing window glass, thus making it safety glass.
Following on from the construction of our successful prototype PSCT panel, we aim at fabricating large panels with higher resolution as our next step. We also require better ITO coating patterns, to narrow the size of the inactive mesh lines between pixels.
Figure 4. Character display and see-through glazing.
Le Li, Ben Tang, Shenggang Wang, Jie Yang, Jiangbin Zhao
Kent Optronics, Inc.
Hopewell Junction, NY
Le Li is an expert in traditional and nonlinear optics, with emphasis on liquid-crystal and polymer structures, optical components and devices. He is the CEO of Kent Optronics Inc., which he joined in April 2000.