You've no doubt heard of smart bombs and smart houses.

Last year, the National Fenestration Ratings Council (NFRC), a Maryland-based organization that tests the efficiency of window products from an energy-performance standpoint, issued a rating for “smart windows.”

Smart windows are windows with so-called switchable glass.

They work like this: A chemical film is applied to the glazing. That film can then be activated by electrical current, in effect resulting in a kind of electronic glass.

Say the sun's direct rays are pouring through your living room windows. You hit a switch, and the windows grow dark, filtering out not only the visible light but the far more destructive ultraviolet light that comes with it. Basically, what you have is a push-button light filter built into the window glass.

MASS-MARKET PRODUCT? Lou Podbelski, vice president of sales and marketing for Sage Electrochromics in Faribault, Minn., the only manufacturer now making switchable glass, says the process the company uses to create the glass is similar to the processes that resulted in low-E — low-emissivity — glass, which is now common in both residential and commercial windows.

Window manufacturers embracing electronic glass include skylight maker Velux, as well as upper-end wood window producer Marvin.

The essential difference between what Podbelski calls “switchable glass” and the tinted glass currently in widespread use is, he says, that with switchable glass you have control: “You can flip a switch and have a nice, comfortable room.” When you want light, it's there. When you don't, it isn't.

According to Podbelski, the benefits of switchable glass will be evident primarily in climates characterized by high sun exposure. In such regions, he claims, energy savings from using switchable glass will be substantial.

COSTS TO COME DOWN Right now, the cost of windows fitted with switchable glass would be somewhere between two and three times the cost of standard windows.

Podbelski, however, says Sage Electrochromics anticipates that as demand for the window technology increases and output rises correspondingly, the price will come down between 60% and 70% during the next five years.

NFRC executive director Jim Benney says that switchable glazing has been around for as long as 15 years, but that “it was slow and very expensive.” Now, he says, the manufacturer has “made it more durable, efficient, and cost-effective. But,” he points out, “it's still a high-end product.”

Even so, he adds, the NFRC expects that the demand for dynamic glazing technology will “grow substantially in the coming years.”