Wi-Fi router used to make hologram

Enlarge (credit: Friedemann Reinhard/Philipp Holl)

One of the most visually striking technologies to be enabled by the laser is the hologram. But holography doesn't have to use lasers. Indeed, depending on what you want to image, laser light might not be a good choice-lasers are notoriously bad at seeing through walls, for example. Wi-Fi signals, in contrast, go right through most walls, and there's now a nice demonstration of holography using a Wi-Fi router.

3D images in 2D

The basic idea behind holography is that when light is scattered from an object, it encodes the 3D structure of the object. We typically lose some of that information in the recording process. It's not just that we only capture a fraction of the scattered light, but we only record how bright the light is.

When we record the brightness of a light beam, we lose the phase of the light field. To put it at its most simple, although we may know the direction from which the light came and how bright it is, we do not know how far the light has travelled since it scattered from an object. While it's a little more subtle than that, this sort of information is what you actually need if you want a 3D image. The distance travelled is recovered by measuring the phase and amplitude of the light field.

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