Conductive film lets light shine through

St. Paul, MN--At first glance, it looks like an ordinary frosted window--simple, colorless, and technically unremarkable. It's the type of glass often seen at a cashier's window or in a doctor's office--the kind that obstructs the view without blocking light.

Apply a trickle of electrical current to this "technically unremarkable" product, however, and it suddenly transforms itself. In the blink of an eye, the frosted glass turns transparent, enabling passersby to see through it. Engineers envision scores of applications for the material, including office partitions, bus windows, automotive sunroofs, and visors for training firemen and pilots, to name just a few.

The key to the new technology is a thin film product that fits inside the laminated glass of an ordinary window. Known as 3M Privacy Film, it contains three major components: two polyester films, each about 3 mils thick; and a 17-micron matrix coating layer containing liquid crystals, like those used in watches and computer displays. A thin conductive layer of indium tin oxide is applied to the interior side of each polyester film, allowing application of current.

In the film's unpowered state, its rod-like liquid crystal materials lie randomly within the droplets of the matrix coating. When a small amount of current (20 mA/square foot) is applied in the form of an electrical field, however, the liquid crystals act like magnetic particles within a magnetic field. "The matrix has holes like a slice of Swiss cheese," notes Stephen A. Miller, technical manager of the Specified Products Construction Products Department at 3M. "The liquid crystals are in the holes and randomly arranged like footballs in a bag. When an electric field is applied, the crystals align." Result: passersby can see through the film when the crystal droplets switch.

Users place 3M Privacy Film between two panes of glass. Wires from the film lead through a power conditioner and a switch, then on to a 120V electrical source. Applying an electrical field to the film requires a flick of the switch. When the switch is turned on, 76% of incident light passes through the film-and-glass sandwich. With it off, light scatters: about 48% passes through, but only 1% of specular light passes. As a result, individuals who look through the glass see light, but are able to make out few images.

Engineers at 3M point to two main challenges in the development of the film: creation of a defect-free coating technique and inventing an electrical connection for the film. Their proprietary coating technology enables them to manufacture reliable film in volume. The thin film connector acts as a single-edge "bus bar," enabling installation of the film and glass in a window with minor adjustments to a standard sash.

Used in home and office construction, the film provides natural light and privacy, while reducing glare. It also allows instant switching of the window from frosted to clear, and saves on the cost of window treatment installation and maintenance.

Firefighters have employed the technology on visors that simulate smoke conditions. By raising and lowering the voltage applied to the film, users can increase or decrease the effect. Similarly, it has been considered as a way to simulate smoke in a cockpit during pilot training.

Currently, 3M is searching for additional applications for the technology, and encourages design engineers to contact the company with ideas.

Additional details...Contact Jim Mannix, 3M Center, Building 225-4S-08, St. Paul, MN 55144, (612) 733-2222.