Air is reasonably transparent, but over long distances, things such as humidity, turbulence, and the like can distort images. This is because the density of air isn’t the same everywhere, and it changes in time and space. The situation becomes even more complicated when trying to look through fog, biological tissues, or other inhomogeneous materials. Similarly, reflection off most (non-polished) surfaces doesn’t produce coherent images, no matter how shiny the surface looks.
Despite the loss of information, researchers have developed a number of techniques to reconstruct the appearance of the original object.
Ori Katz, Eran Small, and Yaron Silberberg have now shown they can produce a fully three-dimensional image even after light has gone through thin, inhomogeneous layers. Known as turbid materials, these layers contain microscopic particles or density fluctuations that scatter light, preventing focusing. To accomplish this, they used wavefront shaping, whereby they pass the scattered light through a special modulator. This modulator produces constructive interference between light from two different wavefronts, allowing a coherent image to be produced. As a bonus, the image can be produced in real time, as opposed to related methods that require computer reconstruction.
When light passes through a turbid medium, the photons scatter off the inhomogeneities. If the source is incoherent, like an ordinary incandescent or fluorescent bulb, this results in a blurry image—if any image can be formed at all. If the light is coherent, such as a laser, scattering results is a speckled pattern. In either case, a clear view of the original object may not be possible. This spells doom for medical imaging, astronomy, and other applications. (The authors also suggested it gets in the way of peering through shower curtains. We at Ars condone such voyeuristic pursuits for consenting scientific partners only).
The researchers illuminated a printed letter “A”—the object—using an ordinary tungsten halogen lamp (a light bulb), which produces undirected incoherent white light. They used a thin polycarbonate film as the turbid medium. While a static medium like that doesn’t change in time as a fog or other fluids do, the authors showed it was enough to keep the image of the “A” from forming.