In the last few years, head-up displays (HUDs), which project information onto the driver's view of the road, have started appearing in a few high-end cars. But a more compact kind of projection device, small enough to fit inside a rearview mirror, could see this kind of display more widely deployed.

A head-up display overlays information on a normal view of the road. For example, symbols can be used to show the car's current speed or the distance to the vehicle ahead without the driver having to look away from the road.

The new projection device, developed by Light Blue Optics, based in Cambridge, UK, uses a technique called holographic projection that allows it to be far smaller than current in-car HUD systems. "We can make an HUD so small you can put it into a rearview mirror or wing mirror," says Edward Buckley, Light Blue Optics's head of business development.

Details of Light Blue Optics's prototype were presented today at the Society for Information Display's Vehicles and Photons 2009 symposium, in Dearborn, MI. The prototype projects an image through a two-way wing mirror so that it appears to be about 2.5 meters away, superimposed over the reflected road scene. The picture appears to originate from a point in space in front of the mirror, only from a narrow perspective.

Existing HUDs require relatively large liquid-crystal arrays and optics to generate an image, says Buckley. "In a BMW 5 Series, the size is about five litres," he says. "We can make it about one-tenth of the size. This means you can start to put these virtual image displays where you couldn't previously."

Holographic projection uses constructive and destructive interference of light to make up the picture, allowing the device to be much smaller. "[Size is] the number-one detriment in getting HUDs into vehicles," says Mark Larry, an expert on in-car displays at Ford who co-chaired the symposium.

Holographic projectors use liquid crystal on silicon (LCOS) to modulate beams of red, green, and blue laser light to create a complete image. Holographic projection does not actually involve creating a hologram, but rather uses principles of holography to create a projected image through optical interference. Buckley says the technology could work equally well on a forward-facing display such as a windshield.