by Joseph M. Harary
Imagine turning a knob and controlling the amount of light passing through a window. Several technologies for such switchable glazing are available or under development. This article briefly surveys the different types of switchable glazing, and focuses on "smart windows" where the user can actually control the amount of light passing through the glass, rather than passive switchable glazings such as photochromic and thermochromic technologies, which the user cannot control. There are presently three types of smart window technologies: suspended particle device (SPD), liquid crystal and electrochromic. Of these, SPD technology appears to be the most promising smart window technology in terms of cost and performance.
Our company, Research Frontiers Inc., has developed and licenses SPD technology, which can be used to control and vary the amount of light transmission of "smart" windows for use in home and office building windows, skylights and sunroofs. The basic operating principle of an SPD window is simple: microscopic light-absorbing particles are dispersed in droplets within a film. In the "off" state when no voltage is applied, the particles are suspended randomly throughout the film and absorb light. When voltage is applied to coating on the glass or plastic walls on both sides of the SPD films, the particles align and permit light to pass through the film. SPD smart windows are dark in the "off" when no voltage is applied. The SPD windows dark state can be either light transmissive or nearly opaque, depending upon the particle concentration selected for the SPD film. The SPD window can be activated either manually by the user using a common rheostat, or automatically when coupled with a photocell, thermostat or other similar sensing devises.
The differences among the competing technologies for use in variable light transmission windows should favor SPDs, both in terms of cost and performance. Liquid crystal custom windows (now being sold) offer only two positionson and offand are hazy (especially when viewed at an angle) and expensive. As compared to liquid crystal windows, windows using SPD technology permit the production of a window with no noticeable haze when activated, which would also provide intermediate levels of light transmission and should conserve energy. Also, SPD windows can be used in exterior glass applications for homes, buildings and vehicles. Another type of switchable glazing that uses electrochromic technology operates by using a direct current voltage to cause a chemical reaction in the electrochromic materials, causing the material to darken. According to our research, compared with electrochromic technology, smart windows using SPD technology have shown 1) much faster response time, 2) darker "off" states, 3) lower estimated costs, 4) more reliable performance over a wider temperature range, 5) lower current drain, and 6) no "iris effect," where changes first occur on the outside of the device and work their way in toward the center.
With respect to passive switchable glazing technologies such as photochromic or thermochromic technology, which just reacts to ambient environmental conditions such as ultraviolet (UV) radiation or temperature, SPD technology offers the user control over the amount of light transmission and also much faster switching speeds.
What do SPD smart windows offer the glass industry? Some benefits include:
What do SPD smart windows offer the end user? Some benefits include:
Other applications for SPD technology include contrast enhancement filters for video display terminals, automatically self-dimmable automotive mirrors and sun visors, variable light transmission eyewear (such as sunglasses and ski goggles), and high-contrast flat panel information displays for devices such as computers, televisions, telephones and other electronic instruments.
Joseph M. Harary is vice president of Research Frontiers Inc., a NASDAQ company located in Woodbury, NY, that holds more than 100 U.S. and foreign patents.
© Copyright 1998 Key Communications, Inc. All rights reserved. No reproduction of any type without expressed written permission.