A team of researchers from Jaume I University (UJI) and the University of València (both in Spain) has developed a single-pixel optical system based on compressive sensing that can overcome the fundamental limitations imposed by light scattering and, hence, enable deeper penetration through tissue.
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The research team used an off-the-shelf digital micromirror array from a commercial video projector to create a set of microstructured light patterns that are sequentially superimposed onto a sample. They then measure the transmitted energy with a photodetector that can sense the presence or absence of light, but has no spatial resolution. Then, they apply a signal processing technique called compressive sensing, which is used to compress large data files as they are measured. This allows them to reconstruct the image.
|The researchers placed an image of the Cheshire Cat in front of a scattering layer that completely hid it, then used the image they were able to retrieve from photodetected values recorded by the photodiode to demonstrate that disorder-assisted imaging can indeed recover an object's fine details—even using a sensor without spatial resolution. (Image courtesy of Jesús Lancis/Jaume I University)|
The research team's approach also uses a single-pixel sensor to capture the images. While most people think that more pixels result in better image quality, there are some cases where this isn't true, says Jesús Lancis, the paper’s co-author and a researcher in the Photonics Research Group at UJI. In low-light imaging, for instance, it's better to integrate all available light into a single sensor. If the light is split into millions of pixels, each sensor receives a tiny fraction of light, creating noise and destroying the image.
"Something similar happens when you try to transmit images through scattering media," Lancis says. "When we use a conventional digital camera to get an image, we only see the familiar noisy pattern known as 'speckle.' In compressive imaging, since we aren't using pixelated sensors, it should be less sensitive to light scrambling and enable transmission of images through scattering."
|Researchers placed an image of the Cheshire Cat behind a layer of material that hides the object. A standard multi-pixel camera cannot see the cat because the material in front scatters the light (a). However, by using a single-pixel optical system based on compressive sensing, the researchers could overcome the imaging limitations traditionally imposed by scattering to recover fine details about the hidden object (b). (Image courtesy of Optics Express)|
The team's technique could operate through dynamic scattering. "Most scattering media of interest, like biological tissue, are dynamic in the sense that the scatter centers continuously change their positions with time—meaning that the speckle patterns are 'in motion.' This is ideal for some applications because monitoring the changes of the speckle can reveal information about the sample, but the drawback is that it's a major nuisance to transmit or get images," Lancis points out. "Our technique, however, requires no calibration of the medium, and its fluctuations during the sensing stage don't limit imaging ability."
Next, the research team plans to break the barriers of light penetration depth inside a scattering medium with the state-of-the-art megapixel-programmable spatial light modulators used in consumer electronics, Lancis says. To do this, they'll need to demonstrate that their technique works even when the sample is embedded inside the tissue.
Full details of the work appear in the open-access journal Optics Express; for more information, please visit http://dx.doi.org/10.1364/OE.22.016945.
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