Snapshot Systems And Hyperspectral Imaging
Hyperspectral imaging, which combines spectral and spatial information, has enabled advances in diverse fields such as precision farming, machine vision, and biomedical imaging. A hyperspectral camera produces a 3D data cube composed of 2D spatial (x, y) and 1D spectral (λ) data. It is typically characterized by the spectral range, the number of spectral channels, and the spatial size of the entire image. The data cubes are acquired either by scanning techniques, such as push-broom sensors, or, in a term proposed by Hagen et al. (2012), snapshot technology for non-scanning spectral cameras, meaning the complete data cube is gathered with one sensor readout.
We coined the phrase “hyperspectral snapshot cameras” to describe the unique technology of our cameras. Recently, this term has become an umbrella definition for all non-scanning imaging spectroscopy techniques. However, there are different technologies available to choose from. To clarify the distinction, we now refer to our cameras as spectral video cameras, i.e., True Snapshot HSI. In this article, True Snapshot HSI is compared to Snapshot-imitating HSI and Scanning HSI.
True Snapshot HSI
Light Field HSI
The data cube is read out entirely, with high spatial and spectral resolution. In 2019, Cubert introduced a fundamentally new snapshot technology that enables high spatial and spectral resolution simultaneously. The ULTRIS X20 was the first hyperspectral camera based on light field technology, where both the intensity and direction of incident light rays are used to produce spectral images. The light field approach creates a vast number of images, each equipped with its unique optical filter.
Advantages
Prism-based Multipoint HSI
The data cube is read out entirely, but the spatial resolution is low. Using prism-based sensor technology, Cubert designed its first hyperspectral snapshot camera, the FireflEYE 185. For the first time ever, a hyperspectral camera was able to use the entire incoming light, while not being reduced to one line (push-broom scanning) or one single wavelength pass (tunable filters). With up to 70% of the incident light reaching the sensor, the 185 had a great signal-to-noise ratio and achieved integration times of less than 1 ms.
Advantages
Disadvantages
True Video Spectroscopy
While the data cubes from all snapshot and scanning cameras are similar, comparable, and in many cases substitutable, the image production itself makes a big difference. One of the biggest differences is light efficiency. A true snapshot imaging spectrometer, like the multipoint or light field approach, collects the entire 3D data cube (all spectral pixels) in a single integration period. The available incoming light is completely utilized and not reduced to one line (push-broom scanning) or one single wavelength pass (tunable filters) at one given time unit. This better light efficiency provides excellent signal qualities and higher signal-to-noise ratios in the shortest time. Time is the second advantage because a snapshot imaging spectrometer captures the entire image in a few milliseconds or less, making it a game-changer in real-time imaging and video spectroscopy.
Cubert’s video imaging technology combines high-resolution and small size with high-speed spectral imaging, forming the basic requirements for true video spectroscopy. In fact, snapshots capture static moments, while real-life situations often encompass an array of changes, such as motions, actions, and processes. Machine vision and inspections are more easily and accurately applied to industrial processes using camera technology that can keep pace with such processes. Image-based quality assessment or event mapping for moving objects can support higher accuracies, e.g., in the food industry, pharmacy, or color characterization. Finally, our cameras can feed automated inspection systems, crucial decision algorithms, process control, and robot guidance in time-critical industrial processes.
Snapshot-imitating HSI
Filter-on-chip HSI
The data cube is read out entirely, but the spectral resolution is limited. The filter-on-chip technology provides high spatial resolution with high image speed while facing limited spectral resolution and reduced signal-to-noise ratios (SNR). The integrated filters typically are composed of several cells, each of which filters a given wavelength band, and are deposited directly onto CMOS wafers. Typically, filter-on-chip cameras are classified as hyperspectral cameras, although the setup only allows a distinct number of spectral bands (no continuous coverage) and therefore is multispectral.
Advantages
Disadvantages
Tunable Filter
A 2D image in one band is collected at the same time. To generate a hyperspectral data cube, a filter wheel scans through the spectrum, taking images in various bands.
Advantages
Disadvantages
High Adaptability
Until now, high-resolution hyperspectral cameras have involved high acquisition costs and complex, time-consuming usability. This made it difficult for the industry to integrate the technology efficiently into machine vision systems. Acquiring hyperspectral images that are accurately recorded and provide reliable image information simultaneously has always been a challenge, which is why cameras have mainly been used in R&D environments for top-level research and the development of new applications.
The light field approach Cubert has developed is a game-changer. With its ease of use, speed, and the high potential for downsizing the technology to cost regions of common digital cameras, the ULTRIS becomes accessible to decision-makers in industry. In the long run, we even see high potential for mobile applications. Hyperspectral video systems combined with strong retrieval algorithms can provide helpful information in almost real-time, finally bringing this complex technology to everyday use.
Scanning HSI
Push-Broom HSI
A 2D dataset consisting of x and λ is collected at once. The spectral data cube (x, y, λ) is generated by scanning the object, either by moving the object underneath the camera (e.g., on a conveyor belt) or by moving the camera (e.g., on an aircraft).
Advantages
Disadvantages
Spatiospectral Scanning HSI
Spatiospectral scanning utilizes a linear variable filter, positioned in front of a 2D imaging array. A full-frame image is generated at once, but each line (y) is captured in a different spectral band (λ). To generate the data cube, scanning needs to be done in a particular way and requires pixel coregistration, making it more complex than push-broom imaging.
Advantages
Disadvantages
Literature
Hagen, N.A., Gao, L.S., Tkaczyk, T.S. und Kester, R.T., 2012. Snapshot advantage: a review of the light collection improvement for parallel high-dimensional measurement systems. Optical Engineering, 51(11), S. 111702.
About the Author
Dr. René Heine is the Co-Founder and CEO of Cubert GmbH, a leader in real-time spectral imaging. Since founding the company in 2012, René has been instrumental in shaping Cubert’s technological direction and growth. He holds a Doctor of Physics degree from the University of Ulm, where he graduated magna cum laude, and completed his diploma thesis at Harvard Medical School. René’s deep expertise in physics and his vision for cutting-edge imaging technologies drive Cubert’s innovations and advancements in hyperspectral solutions.
