An Even Smaller Pocket Projector

Researchers in Germany have developed the world's thinnest "pico" video projector. The prototype device contains an array of carefully shaped microlenses, each with its own miniature liquid-crystal display (LCD). The device is just six millimeters thick, but it produces images that are 10 times brighter than would normally be possible with such a small device.

Handheld pico projectors can be used to display movies, maps, or presentations on nearby surfaces. But the projections can be difficult to view in direct sunlight because the light source isn't very powerful. The new lens system is small enough to be incorporated into a slim smart phone.
Increasing the brightness of a projection normally means increasing the area of the light source used, says Marcel Sieler, a researcher at the Fraunhofer Institute for Applied Optics and Precision Engineering in Germany. Sieler was involved with developing the prototype. But to increase the area in this way requires a thicker lens to focus the larger image. "As the area of the light source increases, so does the volume of the lens," says Sieler. The result is a much bigger projector.
Sieler and colleagues created a novel type of lens that focuses light from a relatively large light source while remaining thin. The prototype video projector consists of 45 microlenses colored red, green, or blue. Each lens has an LCD with 200 by 200 pixels behind it. The light passing through each LCD is focused through a lens, and together each image is superimposed on top of each other to produce the final image. The design was inspired by a type of microlens array known as a "fly's eye condenser," which is normally used to mix light from different sources.
The resolution of the projector is close to that of a WVGA projector, which has 800 by 480 pixels. The new projector has a brightness of 11 lumens, says Sieler, compared to 10 to 15 lumens for existing pico projectors. Sieler says that if the prototype were the same size as an existing pico projector, it would produce about 90 lumens. The next challenge is to make the LCD pixels smaller, from 8.5 microns each to less than three microns, says Sieler.
Microlens arrays are not new, notes Tim Holt, executive director of the Institute of Photonics at the University of Strathclyde. But Holt says that molding each lens in a way that focuses light at a single convergent point is novel.
The LCD behind each lens is sandwiched inside a transparent material, allowing the entire lens to be more compact. To make this kind of integration possible, the researchers needed a new transparent material. Glass wasn't suitable because its melting point is so high that the LCD components would be destroyed during the molding process, says Michael Popall, head of the Fraunhofer Institute for Silicate Research, who helped develop the new pico projector. Transparent polymers tend to have the opposite problem: their melting point is so low they would melt and deform under the projector's light source.
Popall and colleagues developed what he calls a hybrid organic-inorganic material. "This is an optical material that is totally transparent in the visual range, and which you can process like a polymer," he says.
In February, the new prototype will be demonstrated at the Nano Tech 2011 conference in Tokyo.

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