1. Putting Two Images On One Surface

Microsoft’s Surface touch-screen system is a PC that thinks it’s a coffee table. The screen is actually a rear-projection system in the body of the ‘table’ and the touch works by infra-red. And although it was designed to detect the heat of your fingers touching, dragging and rotating objects on the screen, Surface’s infra-red camera can detect things above the surface as well, like faces. That got the researchers at Microsoft Research in Cambridge, England thinking about how to the user experience and the user interface out into the real world.

The SecondLight prototype looks like a standard Surface computer, but when you put a piece of tracing paper or translucent plastic onto the image SecondLight can project a second image onto that paper, making it look as if it’s a lens stripping away a layer of the display to show you what’s underneath. Microsoft Research’s Steve Hodges shows this off with an image of the night sky, where the sheet of paper revealed a star map with the names of constellations; with a satellite image of Cambridge where the second image was a map with street names and with a photograph of a cow that revealed an encyclopedia entry about the cow.

Use a sheet of plastic or a scroll of paper to reveal the second image on SecondLight.

SecondLight uses a switchable LCD display that’s similar to the privacy glass you sometimes see in designer offices (and nightclub restrooms in Las Vegas). Usually the liquid crystal molecules in polymer stabilized cholesteric textured liquid crystal (PSCT-LC) are oriented in random directions and they scatter light in all directions. That gives you an opaque film, like a standard Surface display, that you can project an image on to.

But when you apply a voltage to PSCT-LC the liquid crystal molecules untwist and line up so that the material becomes transparent and SecondLight can project an image through it, onto another opaque surface – like the tracing paper sitting on top of it. Privacy glass doesn’t switch back and forth between transparent and opaque that quickly; PSCT-LC splits the liquid crystals into small areas that can switch in less than 0.5ms.

To get SecondLight to show both images, it needs to switch the PSCT-LC surface back and forth between transparent and diffuse 60 times a second (because at 60Hz you don’t usually see flicker in an image). It also needs to project two images and switch between those 60 times a second, using twin Hitachi CPX1 60Hz projectors and two DisplayTech optical shutters; these use ferroelectric liquid crystal material that switches between being transparent and being solid black.

Inside SecondLight: twin projectors, shutters and cameras.

SecondLight reveals street names using just a sheet of plastic