By Charles Wardell. Switchable windows, which use electricity to change from clear to shaded, have been on the horizon for years. Marvin introduced one in the early '90s that used liquid crystal technology, but then quietly withdrew it. Andersen showed several concept windows based on electrochromic technology at the International Builders' Show two years ago but has yet to announce a product.

Now, a practical solution has become available. Since mid-July, the Louisville, Ky.-based ThermoView ( has been selling replacement windows using suspended particle device (SPD) technology.

The SPD glass consists of inner and outer panes surrounding a film coated with microscopic crystals. The crystals naturally absorb light, making the glass dark; when a voltage is applied, the particles realign to let light pass through. Adjusting the voltage varies how light the glass gets. The manufacturer says that the glass changes color in a couple of seconds, as opposed to the 10 minutes or more required by current electrochromic technologies.

ThermoView is selling at retail outlets in California but will ship direct nationwide. The SPD technology is owned by Woodbury, N.Y.-based Research Frontiers ( It has also licensed its technology to Fort Lauderdale, Fla.-based SPD Systems ( for custom skylights. Research Frontiers president Joe Harari is in discussions with other window makers as well, though he declines to name names.