Smartphone cameras have improved dramatically since the first camera-equipped cell phone was introduced in 1999, but they are still subject to tiny errors in the alignment of different wavelengths of light in the final image. That’s not a concern for your average Instagram selfie, but it’s far from ideal when it comes to scientific image analysis, for example.
Nature seems to have provided a solution in the eyes of the mantis shrimp, which inspired researchers at North Carolina State University (NCSU) to develop a new type of optical sensor. The sensor is small enough for smartphone applications, but it’s also capable of breaking down visible light wavelengths into narrower bands than current smartphone cameras can manage, as well as capturing polarized light, according to a recent paper published in the journal Science Advances.
Human eyes have three photoreceptors for detecting red, green, and blue light. Dogs have just two photoreceptors (green and blue), while birds have four, including one for detecting ultraviolet (UV) light. Octopuses, meanwhile, can detect polarized light. But mantis shrimp (aka stomatopods) have the most complex eyes of all: they can have between 12 and 16 individual photoreceptors and can thus detect visible, UV, and polarized light.
Mantis shrimp have three “pseudo-pupils stacked on top of each other. There are tens of thousands of clusters of photoreceptor cells called ommatidia, similar to the eyes of flies. Six rows of ommatidia in the middle of the eye, known as the midband, are each able to detect either specific wavelengths of light or polarized light. The first four rows are devoted to the former, including UV light, while the last two rows are lined with tiny hairs that confer the ability to detect the latter.
Each compound eye can move independently and thus also boasts independent depth perception, since about 70 percent of each eye focuses on the same point in space. Because of this, the eyes function a bit like scanning a photograph; mantis shrimp are constantly moving their eyes to scan their environment, spotting a band of color, moving a row of ommatidia, and repeating the scan.
Those properties inspired the NCSU researchers to base the design for their new organic light sensor on the structure of mantis shrimp eyes. Dubbed the Stomatopod Inspired Multispectral and POLarization sensitive (SIMPOL) sensor, it features elements (six polarization-sensitive organic photovoltaics and four polymer retarder films), vertically stacked along a single optical axis, just like the rows of ommatidia in the mantis shrimp—all on a single pixel. So it can detect hyperspectral and polarization light simultaneously.
“Our work demonstrates that it is possible to create small, efficient sensors that can simultaneously capture hyperspectral and polarimetric images,” said NCSU co-author Brendan O’Connor. “I think this opens the door to a new breed of organic electronic sensing technologies.”
The researchers built a proof-of-concept prototype of the SIMPOL sensor and tested its capabilities in the lab. They found that, while the standard smartphone CCD cameras use three spectral imaging sources for red, green, and blue light, the SIMPOL can handle four spectral channels and three polarization channels, all at the same time. Modeling simulations suggest that the team could improve its design further to build detectors capable of sensing as many as 15 spectral channels simultaneously.
“Lots of artificial intelligence (AI) programs can make use of data-rich hyperspectral and polarimetric images, but the equipment necessary for capturing those images is currently somewhat bulky,” said co-author Michael Kudenov, also of NCSU. “Our work here makes smaller, more user-friendly devices possible. And that would allow us to better bring those AI capabilities to bear in fields from astronomy to biomedicine.”