Imagine darkening a window with the turn of a knob. Such technology exists, but has been costly to install. The National Renewable Energy Laboratory (NREL) in Golden, Colo., is trying to erase this barrier by combining two technologies: electrochromic materials that darken in response to electrical input, and photovoltaic (PV) devices that produce electric current from light. A transparent PV coating on the window provides the juice to activate the window's change from clear to nearly opaque, eliminating the expense of wiring the windows into the building's power. The PV coating would double as a light sensor, so windows could darken as the sun shone brighter. The PV and electrochromic films could both be deposited on a polymer sheet that is then glued to an existing windowpane, says NREL's David Benson.