Recovering Color Images from Scattered Light

A new method takes light from colored numerals that has been scattered by a mostly opaque surface and uses its speckle patterns and a coded aperture to reconstruct the image in five different frequencies before combining them into a final image. (Source: M. Gehm, Duke U.)

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. “Others have been able to reconstruct color images from scattered light, but those methods had to sacrifice spatial reso­lution or required prior characteri­zation of the scatterer in advance, which frequently isn’t possible,” said Michael Gehm, associate professor of electrical and computer engi­neering at Duke. “But our approach avoids all those issues.”

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 became 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 new memory effect imaging approach developed by the authors Xiaohan Li, a PhD student in Gehm’s lab, Joel Greenberg, associate research professor of electrical and computer engi­neering, and Gehm breaks through this limi­tation.

The trick 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. And the amount it shifts is directly related to the color of light passing through. “This shift is small compared to the overall size of what’s being imaged, and because our detector is not sensitive to color, it creates a messy combination,” said Li. “But the shift is enough to give our algorithm a toehold to tease the indi­vidual speckle patterns apart from each color, and from that we can figure out what the object looks like for each color.”

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.

“There are a lot of applications where people really want to know how much energy there is in specific spectral bands emitted from objects located behind opaque occlu­sions,” said Greenberg. “We’ve shown that this approach can accomplish this goal across the visible spectrum. Knowing the aperture pattern and how much it shifts as a function of wavelength provides the key we need to dis­entangle the messy sum into separate channels.” (Source: Duke U.)

Reference: X. Li et al.: Single-shot multispectral imaging through a thin scatterer, Optica 6, 864 (2019); DOI: 10.1364/OPTICA.6.000864

Link: Signal and Information Processing, Electronic and Computer Engineering Dept, Duke University, Durham, USA

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