Sunlight shining through the window could one day project your favourite picture onto the wall, thanks to a new system that generates complex images by carving intricate patterns into the surface of a transparent sheet.
Shine a light through a wine glass and you’ll see refracted slivers of brightness overlaying the glass’s shadow. It is complex versions of these bright patterns, called caustics, that are now being exploited to reproduce a photographic image.
Tim Weyrich, a computer graphics researcher at University College London, worked with researchers at Disney Research in Zurich, Switzerland, and Princeton University to manufacture Plexiglas slabs that generate an array of fuzzy elliptical patches which together form a predefined image. Each 10-centimetre-square slab contains over 1000 tiny curved areas that act as lenses to project these bright patches.
Weyrich and his team determine the exact pattern of patches required by looking at the energy distribution of a greyscale image: brighter regions have more energy, while darker ones have less. They then work out the collection of curved patches needed to reproduce this energy distribution.
If that sounds tricky, manufacturing the required surface is even harder. Each curved patch has to be painstakingly carved out by a computer-controlled mill, and producing a single slab can take up to three days. Weyrich hopes this can eventually be speeded up.
Another limitation with the current method is that lighter regions show more detail because they are made up of smaller patches in which the light is more concentrated, compared with the larger patches of more diffuse light making up the dimmer regions. This effect could be avoided by using differently sized carvings in the glass - large ones to focus large amounts of light for highlight areas and smaller ones for the shadows - but that would make designing the surface still more complicated.
Weyrich and colleagues have also applied the techniques to reflected light by manufacturing metallic surfaces that generate highlights in the shape of a desired image. These could be used as a security feature similar to the holograms now imprinted on credit cards, because they would be hard to forge or copy.
Weyrich’s system is unlikely to replace simple projectors for producing an image, says Gustavo Patow, a computer graphics researcher at the University of Girona, Spain. But it could find other uses. Car manufacturers could exploit the technique to shape headlight beams into an exact pattern on the road, avoiding the risk of driver glare, Patow suggests. “Being able to control it so finely is amazing.”