Engineers at Duke University have developed a method for extracting a color image from a single exposure of light scattered through a mostly opaque material. The technique has applications in a wide range of fields from healthcare to astronomy.
When light is scattered as it passes through a translucent material, the emerging pattern of “speckle” looks as random as static on a television screen with no signal. But it isn’t random. Because the light coming from one point of an object travels a path very similar to that of the light coming from an adjacent point, the speckle pattern from each looks very much the same, just shifted slightly.
With enough images, astronomers used to use this “memory effect” phenomenon to create clearer images of the heavens through a turbulent atmosphere, as long as the objects being imaged were sufficiently compact. While the technique fell out of favor with the development of adaptive optics, which do the same job by using adjustable mirrors to compensate for the scattering, it has recently become popular once again. Because modern cameras can record hundreds of millions of pixels at a time, only a single exposure is needed to make the statistics work.
While this approach can reconstruct a scattered image, it has limitations in the realm of color. The speckle patterns created by different wavelengths are typically impossible to disentangle from one another.
The key to solving this problem is to use a coded aperture followed by a prism. A coded aperture is basically a filter that allows light to pass through some areas but not others in a specific pattern. After the speckle is “stamped” by the coded aperture, it passes through a prism that causes different frequencies of light to spread out from each other. This causes the pattern from the coded aperture to shift slightly in relation to the image being captured by the detector. The amount it shifts is directly related to the color of light passing through.
The researchers show that, by focusing on five spectral channels corresponding to violet, green and three shades of red, the technique can reconstruct a letter “H” full of nuanced pinks, yellows and blues. Outside of this difficult proof-of-principle, the researchers believe their approach could find applications in fields such as astronomy and healthcare.
In astronomy, the color content of the light coming from astronomical phenomena contains valuable information about its chemical composition, and speckle is often created as light is distorted by the atmosphere. Similarly, in healthcare, color can tell researchers something about the molecular composition of what’s being imaged, or it can be used to identify biomolecules that have been tagged with fluorescent markers.