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Compact hyperspectral system captures “5D” images

31 August 2018 | News
Photo of the hyperspectral imager

Researchers from Friedrich Schiller University Jena, Ilmenau University of Technology and Fraunhofer Institute for Applied Optics and Precision Engineering, Germany, have developed a compact imaging system that can measure the three-dimensional shape as well as the spectral properties of objects with high speed and accuracy.

The new hyperspectral imaging system can acquire additional dimension information. In Optics Express (10.1364/OE.26.023366), researchers describe how each pixel acquired by their new 5D hyperspectral imager contains the time, x, y and z spatial coordinates, and information based on light reflectance ranging from the visible to the near infrared.

“State-of-the-art systems that aim to determine both the shape of the objects and their spectral properties are based on multiple sensors, offer low accuracy or require long measurement times,” said Stefan Heist. “In contrast, our approach combines excellent spatial and spectral resolution, great depth accuracy and high frame rates in a single compact system.”

The researchers created a prototype system with a footprint of 200 × 425 mm. It uses two hyperspectral snapshot cameras to form 3D images and obtain depth information. By identifying particular points on the object’s surface that are present in both camera views, a complete set of data points in space for that object can be created. However, this approach only works if the object has enough texture or structure to unambiguously identify points.

To capture both spectral information and the surface shape of objects that may not be highly textured or structured the researchers incorporated a specially developed high-speed projector into their system. Using a mechanical projection method, a series of aperiodic light patterns are used to artificially texture the object surface. This allows robust and accurate 3D reconstruction of the surface. The spectral information obtained by the different channels of the hyperspectral cameras are then mapped onto these points.

“Our earlier development of a system projecting aperiodic patterns by a rotating wheel made it possible to project pattern sequences at potentially very high frame rates and outside the visible spectral range,” said Heist. “New hyperspectral snapshot cameras were also an important component because they allow spatially and spectrally resolved information to be captured in a single image, without any scanning.”

To demonstrate the usefulness of the prototype to analyse culturally significant objects, the researchers used it to digitally document a historical relief globe from 1885. They also created NIR 5D models of a person’s hand and showed that the system could be used as a simple way to detect veins. The imager could also be used for agricultural applications, which the researchers showed by using it to capture the 5D change in reflection of citrus plant leaves as they were absorbing water.

The researchers plan to optimize their prototype by using hyperspectral cameras with a higher signal-to-noise ratio or that exhibit less crosstalk between the different spectral channels. Ideally, the system would be tailored to specific applications. For example, cameras with high imaging rates could be used to analyse dynamically changing object properties, while using sensors with high resolution in the infrared wavelength might be useful for detecting chemical leaks.

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