Optical EngineeringInput couplers for thin light-guides and light-emitting diodes
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As the demand toward a thin liquid crystal display accelerates, a backlight system must become thinner. Compact and efficient coupling of an LED to a thin light-guide is one of the key technologies. First, we consider simple face-down and prism coupling, and carry out experiments using a chip-type LED with a 2.4-mm-diam emission area and a 3.0-mm-thick light-guide. Face-down coupling guides only 12% of the LED output into the light-guide. With a right-angle prism, the efficiency is improved to 23%. We try to improve the coupling efficiency by altering the propagation direction of the light with an optical film. However, our experiments and simulations with off-the-shelf prism sheets and diffuser films show that the coupling efficiency decreases. Next, we carry out ray-tracing simulations for various input coupler designs. A chip-type LED with a 0.42-mm-diam emission area and a 0.1-mm-thick light-guide are assumed. Calculated efficiency greater than 50% is obtained by a square prism mounted obliquely on the light-guide, while the footprint of the coupler is about 1.3 mm. An input coupler based on an oblique cylinder improves the calculated efficiency to 73.5% at the expense of its footprint of 4.82 mm.