Lightguide Plate Simplifies LCD Illumination

Richard Gaughan

Liquid crystal displays (LCDs) are used for applications in which low weight, minimal power consumption and uniform intensity are important, such as in cell phones, personal digital assistants and laptop computers. The voltage on each pixel of an LCD controls how much light is let through the red, green and blue elements. Traditional LCD backlighting systems feature a light source, a lightguide plate, a backreflector, a diffuser and a layer of prisms. Now scientists at Tsinghua University in Beijing have developed a lightguide plate that incorporates the functions of these components in a single unit.

Typically, LCD back-illumination is provided by directing light from cold-cathode fluorescent tubes or LEDs into the edge of a lightguide plate, in which it is internally reflected and outcoupled using reflective elements, usually preferentially in the direction of the initial propagation. The reflective elements create bright spots in the illumination pattern, so a diffuser averages the illumination to improve uniformity, and prism sheets modify the illumination angle. With an additional backreflector, a typical single system is composed of four or more optical sheets. To eliminate most of these sheets, Di Feng and his colleagues in the departments of physics and of precision instruments at the university combined several functions in a single element.

The scientists began with a standard poly(methylmethacrylate) lightguide plate, incorporating prisms in the back face of the material. Coating the face of the lightguide plate with reflective material eliminated the need for the backreflecting sheet. Varying the shape, density and size of the integral lightguide plate/microprisms selects the intensity, angular distribution and uniformity. A final microcompressor plate, composed of an array of triangular grooves, fine-tunes the angular light distribution.

A 30 × 40-mm proof-of-principle device designed for use with white light demonstrated an intensity uniformity of better than 86 percent. Initial studies with a 235 × 310-mm lightguide plate also have shown good results, although material limitations may make it difficult to create an element much larger than this.

For small displays, uniformity still is an issue, said Feng, but the researchers are working on this and are ready to cooperate with manufacturers to produce brighter and thinner backlight systems for various devices.