Here's a futuristic notion: Windows that darken on hot sunny days to block heat and glare, clear up on cool or cloudy days to allow in sunlight and warmth, save lots of energy, eliminate the need for blinds or shades and, most important, allow people indoors to be connected all the time to the natural world.

This may sound like magic, but electrochromic windows are here today. You can see them, above, at the student center at Chabot College in Hayward, CA. They're made by a small Minnesota-based company called SAGE Electrochromics, which is about to get bigger: This week, SAGE announced that it sold 50 percent of itself for $80 million to Saint-Gobain, a global building materials firm based in France.

The partnership is a marriage of new and old -- SAGE is a privately-held high-tech company founded in 1989, while giant Saint-Gobain (EU37.8 billion in sales last year, 190,00 employees) traces its beginnings to the 17th century when it manufactured mirrors for the Chateau de Versailles. Until they worked out their deal, SAGE and Saint-Gobain had been competing to develop windows that would electronically control the sun's energy that flows through them.

I spoke today to John Van Dine, SAGE's founder and CEO, who was thrilled by the deal, which will finance a new plant to increase the firm's manufacturing capacity. A New Jersey native, John was working at a solar photovoltaic firm in the 1980s when he got to thinking that "an electronically tintable glass had as much or more potential to save energy per unit of space as a solar cell had to produce it." He began researching the idea at the Princeton University library and a Newark patent office ("This was before the age of the Internet") and quit his job in 1990 to work full-time at SAGE. Years of research and development (some of its financed by the goverment) followed. In 1998, he moved SAGE to Minnesota because it is, in his words, "the Silicon Valley of the windows industry." Many of the world's big window and glass makers (Andersen, Marvin, Pella) are located nearby.

How do SAGE's windows work? The company's website explains:

The SageGlass coating on the glass is made up of five layers. When voltage [less than 5V DC] is applied to these layers in their "clear" state, they darken as lithium ions and associated electrons transfer from the counter electrode to the electrochromic electrode layer. Reversing the voltage polarity causes the ions and associated electrons to return to their original layer, the counter electrode, and the glass untints. This solid state electrochromic reaction is controlled through a low voltage DC power supply. When the SageGlass coating darkens, the sun's light and heat are absorbed and subsequently reradiated from the glass surface -- much the way low-emissivity glass also keeps out unwanted heat.

I didn't say I understood it. I just said the company website explains it.