Long a lab curiosity, "smart windows"-energy savers that darken in response to an electric current-appear headed for U.S. commercial production in the fall of 2001. Sage Electrochromics of Faribault, Minn., has teamed with 3M and the Center for Ceramic Research at Rutgers University to develop a process in which five thin layers of ceramic, totaling only one fiftieth the thickness of a human hair, are baked onto glass panes. Application of electricity to the coating causes the window to tint; the higher the voltage, the darker it gets. Turning the dimmer knob all the way causes the window to block up to 95 percent of light. Similar glass has been used in automotive rearview mirrors, but there have been obstacles to high-volume production of large panes suitable for homes and offices-obstacles Sage says it has now overcome. The glass was developed with $3.5 million from the National Institute of Standards and Technology's Advanced Technology Program.