As we set our sights on the new millennium, the possibility of living in an entirely automated home is becoming more and more of a reality. The convenience, energy benefits and added security of automatically controlled lights, doors and appliances are beginning to have a greater presence on the market.

One facet of home automation people rarely consider, however, is the ability to control the amount of light that passes through their windows. Such "smart" window technology – in which you can regulate the amount of light transmission with a computer, photocell or even by simply turning a knob – now exists.

There are a number of benefits gained by living in a home that has "smart" windows which can be automatically or manually darkened: harmful ultraviolet rays are blocked; energy bills are decreased through the reduction of heating, lighting and air-conditioning costs; fading of furniture, artwork and other valuables is nearly eliminated; users are given control over their privacy and environment; and the need for costly blinds and shades is eliminated. In short, you have increased comfort and control over your environment.

Several technologies – called "switchable glazings" – are available or under development to accomplish these ends. This article, which briefly surveys the different types of switchable glazings, focuses on active switchable glazings (i.e.: "smart windows") which allow the user to 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.

SPD technology has been developed and is being licensed to the glass and window film industry by Woodbury, New York-based Research Frontiers Incorporated (RFI). This technology can be used to control and vary the amount of light transmission of "smart" windows for use in home and office buildings, skylights and sunroofs. technology has been developed and is being licensed to the glass and window film industry by Woodbury, New York-based Research Frontiers Incorporated (RFI). This technology can be used to control and vary the amount of light transmission of "smart" windows for use in home and office buildings, skylights and sunroofs.

The basic operating principle of an SPD window is simple: microscopic light-absorbing particles are dispersed in droplets within a film. This film is then placed between two panes of glass (or plastic). In the "off" state when no voltage is applied, the particles are suspended randomly throughout the film and absorb light, making the window dark. In this state, the SPD window can transmit varied levels of light or be nearly opaque, depending upon the selected concentration of particles for the SPD film.

When moderate voltage is applied to the glass or plastic wall coatings on both sides of the SPD film, the particles align and permit light to pass through. By varying the amount of voltage, the user can adjust the level of light transmission. The window film can be activated either manually by the user with a common rheostat, or automatically when coupled with a photocell, thermostat or other similar sensing device.

Liquid crystal technology is presently marketed by several companies throughout the world. Only two positions – on or off – are offered in liquid crystal custom windows currently being sold. When compared to SPD windows, they have a higher haze when activated (especially when viewed at an angle) and are more expensive. Unlike SPD windows, liquid crystal windows do not permit intermediate levels of light transmission. They also do not save as much energy or block solar energy because they only scatter light as it passes through.

Electrochromic technology operates by using a direct current voltage to create a chemical reaction in the electrochromic materials, causing the window to darken. As compared to 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," in which changes first occur on the outside of the device and work their way in towards the center. operates by using a direct current voltage to create a chemical reaction in the electrochromic materials, causing the window to darken. As compared to 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," in which changes first occur on the outside of the device and work their way in towards the center.

As compared with passive switchable glazing technologies such as photochromic or thermochromic -- which merely react to environmental conditions such as ultraviolet radiation or temperature -- SPD technology offers the user control over the amount of light transmission and much faster switching speeds.

SPD technology creates for the user the key ingredient of practical window shading technology -- control over his/her environment. Such cost-effective control allows people to experience the benefits of "smart" windows. SPD "smart" windows will soon be accessible to the average consumer, making the home of the future a reality today.

Mr. Harary is Vice President of Research Frontiers Incorporated, a Nasdaq company located in Woodbury, New York. Research Frontiers currently holds over 100 U.S. and foreign patents and patent applications, covering light control technology. Further information about SPD smart windows, and about Research Frontiers and the various companies that have licensed SPD technology for a variety of applications can be found by visiting Research Frontiers' web site at www.refr-spd.com   or by calling the company at (888) SPD-REFR.