WINDOW TECHNOLOGIES: Advanced
Liquid Crystal Device Windows
A variant of the liquid crystal display technology used in wristwatches is now serving as privacy glazing for new windows. A very thin layer of liquid crystals is sandwiched between two transparent electrical conductors on thin plastic films and the entire emulsion or package (called a PDLC or polymer dispersed liquid crystal device) is laminated between two layers of glass. When the power is off, the liquid crystals are in a random and unaligned state. They scatter light and the glass appears as a translucent layer, which obscures direct view and provides privacy. The material transmits most of the incident sunlight in a diffuse mode, thus its solar heat gain coefficient remains high.
When power is applied, the electric field in the device aligns the liquid crystals and the glazing becomes transparent in a fraction of a second, permitting view in both directions. Most such devices have only two states, clear and diffusing, and the power (about 0.5 W/sf, operating between 24 and 100 volts AC) must be continuously applied for the glazing to remain in the clear state. The visible transmittance range is typically 50–80% and the SHGC is 0.55–0.69, although dyes can be added to darken the device in the off state. Some manufacturers offer products in a variety of colors and for curved and flat-shaped glass. Glazings can be fabricated up to 3-by-7.5-foot sheets. Ultraviolet (UV)-stable formulations now permit exterior applications but UV stability and cost remain as issues.