Compact hyperspectral system captures ‘5-D’ images

04 Sep 2018

Temporal, spatial and optical properties of targets give researchers a big picture of materials and living tissues.

Researchers working in Jena, Germany, have developed a compact imaging system that can measure the shape and light-reflection properties of objects with high speed and accuracy. This so-called “5D” hyperspectral imaging system—which captures multiple wavelengths of light (1D…) plus spatial coordinates (+3D…) as a function of time (+1D = 5D) —could benefit a variety of applications including optical-based sorting of products and identifying people in secure areas of airports.

The researchers say that with further miniaturization, the imager could enable smartphone-based inspection in applications such as fruit ripeness testing, or in personal medical monitoring.

"Because our imaging system does not require contact with the object, it can be used to record historically valuable artefacts or artwork," commented research team leader Stefan Heist of Friedrich Schiller University Jena and Fraunhofer Institute for Applied Optics and Precision Engineering (IOF), also in Jena.

“This approach can be used to create a detailed and accurate digital archive, he added, while also allowing study of the object's material composition.” Hyperspectral imagers detect dozens to hundreds of colors, or wavelengths, instead of the three detected by normal cameras. Each pixel of a traditional hyperspectral image contains wavelength-dependent radiation intensity over a specific range linked to two-dimensional coordinates.

The new hyperspectral imaging system, developed in collaboration with Gunther Notni's research group from Germany's Ilmenau University of Technology, advances this imaging approach by acquiring additional dimension information. The work is described in a new paper in Optics Express, in which the 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 in visible through NIR wavelengths.

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

Compact prototype

The researchers created a prototype system with a footprint of just 200 x 425 mm—about the size of a laptop. 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 texturized or structured the researchers have incorporated a specially developed high-speed projector into their system. Using a mechanical projection method, a series of aperiodic light patterns artificially texture the target object’s surface. This allows 3-D reconstruction of the surface. 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."

High-speed hyperspectral

The researchers characterized their prototype by analyzing the spectral behavior of the cameras and the 3D performance of the entire system. They showed that it could capture visible to near-infrared 5D images as fast as 17 frames per second, significantly faster than other similar systems.

To demonstrate the usefulness of the prototype to analyze culturally significant objects, the researchers used it to digitally document a historical relief globe from 1885. They also created near-infrared 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 spectrum of citrus plant leaves as they were absorbing water.