The coated diffuser films used to manage the light in liquid crystal displays can give manufacturers a real headache. Usually made from polyester film coated with micron-sized optical beads, these films can lose bits of coating and develop optical defects as they make their way through the LCD manufacturing process, driving down yields substantially. GE Advanced Materials has come up with a new line of polycarbonate diffuser films that can eliminate these coating-related failures.
Called Illuminex and based on Lexan optical-quality polycarbonate, these monolithic diffuser films have no coating at all. "There's nothing to flake off," says Todd Hoff, GE's industry manager for displays. Instead of the coating, Illuminex films get their optical properties from two proprietary technologies developed by GE scientists. The first uses additives to incorporate diffusion properties into the resin itself. The second involves a surface modification to the films as part of the melt-calendaring process. According to Hoff, this texturing process creates a "random surface profile" that helps distribute light evenly, one of the primary functions of diffuser films (see graphic).
In terms of optical performance—which LCD makers measure in terms of hiding power, luminescence, and viewing angle—the Illuminex films "meet or exceed the performance of coated products," Hoff says. And the films may offer another kind of performance edge as well. With an HDT of about 135C, versus 80-85C for many polyester products, the new Illuminex films promise to do a better job resisting the permanent waviness that can develop when displays go into high-heat and -humidity environments.
But these films really start to shine in the yield department. Many display makers adopted GE's new technology in the past year and have seen 10 percent yield improvements, Hoff reports. "That all comes down to the elimination of coating failures from handling," he says. There are the sources of coating failure too, such as CTE mismatch between the coating and film substrate. Overall yield losses from diffuser films can top 20 percent in some operations.
GE has developed several grades »of Illuminex films for use as bottom and top
diffusers in a wide variety of consumer electronics and automotive displays.
Current products come in thicknesses of 127 and 203 microns for products ranging
in size from PDAs up to large television sets. Right now, there's no Illuminex
product for the smallest displays, such as those used in most cell phones, but
Hoff does not entirely rule them out for the future.
Light spreaders on left: Diffuser films
help manage the light in liquid crystal displays. Here's how they fit in
with other display components. The three graphics on the right: Illuminex
films do away with the coatings that give LCD diffuser films their optical
properties. Instead, GE scientists came up with propertary technologies to
modify the polymer used in the film as well as the surface of the film.