No longer an obstacle (Image: Matthieu Spohn/Getty)

It’s not quite X-ray vision, but a way has been found to transmit simple images through opaque objects using ordinary light – and physicists have used the method to project an image through glass covered in thick paint.

Some things we consider opaque – “not able to be seen through”, in the New Oxford Dictionary of English definition – are slightly translucent, meaning some light does in fact make it through. However, it is scattered so much as it bounces around inside the materials’ lattice of atoms that physicists thought it was beyond practical use for seeing what is on the other side of the object.

A 2007 experiment that managed to focus light through eggshells and a human tooth demonstrated that might not be so. Now the first simple images have been transmitted through an opaque object and reconstructed on the far side, by physicist Sylvain Gigan and colleagues at École Supérieure de Physique et de Chimie Industrielles in Paris, France.


Snapshot

By reverse engineering the scattering process, the team were able to reconstruct an image from the light that had passed through the opaque paint layer. That scattering is complex, but it’s also predictable: the same light wave will always be scattered in the same way.

The way a particular object scatters light is known as its transmission matrix. “If the [layer of paint] is a maze for light, then you could think of the transmission matrix as the map for it,” says Gigan.

His team worked out the transmission matrix for their painted glass slide by hitting it with a weak laser beam more than 1000 times, changing the shape of the beam each time using a spatial light modulator – the same device used to control the light emerging from a video projector. A digital camera on the other side of the glass detected the different scattering patterns produced each time. By comparing what the camera saw with what had been done to the laser beam, the team measured the paint’s complete transmission matrix.

Invisible image

If a simple image was then projected onto the paint, a person simply looking at the painted glass from behind would see only an even glow. But the team used knowledge of the transmission matrix to decode the faint, noisy trace that reached the digital camera and reconstruct the image.

“Once the matrix is known, reconstructing the image is very quick,” Gigan says. “We can achieve almost video-rate focusing or imaging.”

However, it will be some time before the technique is used to transmit and reconstruct any truly interesting images – the test images were very simple patterns: a 256-pixel rectangular grid with a handful of its squares lit up more brightly. “The quality of the images degrades rapidly when increasing the number of pixels, because the signal-to-noise ratio degrades,” says Gigan, although he says there is “room for improvement” with future study.

Allard Mosk at the University of Twente in Enschede, the Netherlands, who together with his colleague Ivo Vellekoop focused light through eggshells and teeth in 2007, is impressed. “We can see that technically this work is at the beginning of a long and exciting road,” he says. Although at the moment the technique is restricted to simple 256-pixel images, he thinks other groups around the world will now be inspired to send larger and more complex images through opaque objects.

Journal reference: A copy of Gigan and colleagues’ paper is available on the Physics Arxiv