A Simple Filter Could Make LCDs More Efficient

The new approach wastes far less light, saving energy.

A new type of color filter could significantly increase the energy efficiency of liquid-crystal displays (LCDs), which dominate the market in everything from televisions to cell phones.

The best LCDs today only emit about 8 percent of the light produced by their backlights. This means that they drain batteries in portable electronics and ramp up electricity bills in homes (the California Energy Commission estimates that televisions consume 10 percent of the electricity in homes).

Normally, LCDs use several layers of optical devices to colorize, polarize, and shutter light from a backlight, and inefficiencies emerge at every step. Now researchers at the University of Michigan have made an optical film that promises to boost the overall efficiency of LCDs by more than 400 percent–so that 36 percent of light makes it through. The new optical film was developed by researchers led by L. Jay Guo, professor of electrical engineering and computer sciences at the university. The film colors and polarizes the light that passes through an LCD more efficiently than conventional components can.

The color filter is a three-layer sandwich of an insulating material in between two layers of aluminum; the entire stack is less than 200 nanometers thick and is etched with periodic slits, like a grate. The distance between the slits and their width determines the color they’ll produce when illuminated by a white backlight. This is because the grating patterns are on the same size scale as the wavelength of visible light. In a paper published online last week in the journal Nature Communications, the Michigan researchers show that they can make a rainbow of colors using such a filter.

As well as being more efficient, the filter is also simpler to make than current LCDs, says Guo. It is possible to create red, blue, and green subpixels by patterning gratings of differing widths side by side in a single manufacturing step. Conventional LCDs use pigments to define the red, green, and blue subpixels that filter light from the backlight. Each of these colorants is deposited one at a time and then patterned to make a subpixel. The new gratings transmit more light than colorant-based filters; whereas a traditional green filter transmits about 40 percent of the light; a green, grating-based filter transmits 60 percent. Other colors have similar efficiency advantages.