Commercial Snapshot Spectral Imaging: The Art of the Possible

This report summarizes a study on the state-of-the-art and anticipated advancements in commercial snapshot spectral imaging. Spectral remote sensing as a method of determining material properties had been a tool solely used by governmental organizations, universities, and corporations due to the expense of the technology. However, advancements in snapshot multi- and hyperspectral technology will soon make spectral imaging available to the consumer market. The first wave of products will be dominated by visible to near infrared (VNIR) cameras due to the maturity of Charged Coupled Device (CCD)/Complementary Metal Oxide Semiconductor (CMOS) imaging chip technology used in everything from cell phones cameras to the Hubble Space Telescope.

We performed a comprehensive review of the state-of-the-art in snapshot spectral imaging and identified exemplar devices that represent what we believe are the most promising technologies for achieving a low-cost hyperspectral imager that can either be fit on a cell phone or small form factor stand-along camera similar to a DSLR. It should be noted that there are other small size hyperspectral imagers available on the market, such as the Headwall Nano-Hyperspec1 and Corning microHSI2. However, the Headwall and Corning systems are not snapshot systems; they are line-scanning cameras that require motion from a platform, such as a UAV. We identify four main categories of snapshot spectral imaging:

• Fabry-Perot micro-electromechanical system (MEMS) interferometer.
• Custom color Bayer matrix filters.
• Static Fabry-Perot color tile matrix filters.
• Line-scanner with internal scan mirror.

The first is micro-electromechanical system (MEMS) Fabry-Perot interferometer, which dynamically scans the spectral bandpass by changing the distance between two etalons while capturing individual frames to build up the data cube. The trade-off is that you obtain the full spatial resolution at the cost of increased time necessary to scan across all the wavelengths. The second and third technologies are variants of the same concept of creating customized color filters in front of the focal plane. All consumer-level color cameras have 3-color Bayer matrix with red-green-blue (RGB) filters that approximate human eye color response. One approach is to add more colors to the Bayer matrix. Another is to place static Fabry-Perot filters with specific band passes in front of the focal plane. Both methods capture all the spectral bands at once. One of the trade-offs with this approach is sacrificing spatial resolution for additional spectral bands. Though not strictly “snapshot,” there are also line-scanning devices that employ moving internal parts to perform the scan.

We identified six applications where we feel snapshot spectral imaging could have a significant impact.

• Counter Denial & Deception (e.g., detecting camouflage)
• Search & Rescue
• First Responders
• Medicine
• Law Enforcement/Security
• Crime-scene Forensics

The applications listed above are where we see immediate benefit given the portability of handheld, snapshot implementation of VNIR imaging technology. We expect other applications will come to fruition when the technology becomes more widely available to the general consumer market.