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.