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Miniature Snapshot Multispectral Imager

Compact multispectral imaging sensors are used in defense, medical, and environmental monitoring.

Multispectral imagers use an optical device that can separate the colors to obtain the spectral content in the scene. Such an optical device could be a grating or prism, a filter wheel, a diffractive optic lens, a Fabry-Perot (F-P) etalon, or a tunable filter. All of these optical devices are used with a focal plane array (FPA) and suitable optics for a given spectral region. All such imagers collect images in a time-sequential manner. Recently, some multispectral imagers have been developed that collect images of the scene at all the spectral bands simultaneously.

The Snapshot Multispectral Imager uses a commercial camera. A 4°—4 filter array is installed in front of the FPA. In front of the filter array, a micro-optic assembly with telecentric microlenses, baffles, and light stop array are placed.
One such imager has been developed using microelectromechanical system (MEMS) technology to fabricate a filter bank covering a spectral region with each filter operating at a specific peak wavelength with a design spectral width. Such a filter array is then combined with a microlens assembly based on telecentric design to image the same scene at each spectral band without much overlap with neighboring bands using a honeycomb baffle design at a subsection of the FPA.

This snapshot miniature multispectral imaging system operates in the shortwave infrared (SWIR) region. The two main components used in the design of the snapshot multispectral imager are the multichannel F-P filter array and the microlens assembly. The filter array uses 16 F-P etalons, each tuned to a different peak wavelength. The filter bank consists of a 4°--4 two-dimensional (2D) array of fixed wavelength filters, each with a narrow bandpass. Instead of using two high-reflectivity metal mirrors with a cavity in between to fabricate each of the 16 F-P filters, a multilayer dielectric stack mirror design was chosen to obtain optimal filter performance. Each of the mirrors on either side of the cavity in the FP etalon was fabricated using 11 alternating layers of two different dielectric materials: titanium dioxide (TiO2) and silicon dioxide (SiO2) with different refractive index values.

In the mirror design, the layer thickness for each material was chosen equal to one-quarter of the selected center wavelength of the filter to obtain the desired spectral full width at half maximum (FWHM) with high filter finesse. An SIO2 cavity between the two dielectric mirrors was chosen with a corresponding thickness for each of the peak wavelengths. Each of the 16 F-P filters had the same top and bottom multilayer dielectric mirror, but the cavity thickness for each filter was different, equal to around one-half of the corresponding center wavelength of the filter. The F-P filters were fabricated by deposition of evaporated thin material films. Different cavity layer thicknesses for the 16 spectral filters were achieved by using only four 2-shadow masks for deposition of the evaporated SiO2 films.

The other main component that is required to obtain simultaneous multispectral images of a scene is a microlens assembly that can image the scene into 16 different spectral channels, coming through each filter on a subsection of the FPA of the SWIR camera independently, i.e., the out-of-band leakage from the surrounding filters has to be made as small as possible. This was achieved by using two arrays of 4°—4 microlenses with field stops and baffles. Commercial off-theshelf lenses were used with some machining to match the size of the individual filter. The f-number of microlenses, baffle thickness, and field stop distances were chosen after radiometric considerations to optimize the light transmission to the FPA. The filter bank operates from 1487 to 1769 nm with a spectral bandpass ~10 nm. The design of the filter array can be customized for any spectral region from the ultraviolet (UV) to the longwave infrared (LWIR). Also, filter and custom optics sizes can be matched to optimize performance using a variety of cameras with different size focal planes.

The fabricated filter was installed in front of a commercial camera with an indium gallium arsenide (InGaAs) FPA, and the camera imaging lens was replaced by the microlens assembly. The microlens assembly was carefully tested with the commercial camera for its 4°—4 independent image formation capability. A number of filter arrays was fabricated and characterized for their spectral characteristics. The snapshot multispectral imager with 16 independent channels was calibrated to evaluate its imaging performance. A number of indoor and outdoor scenes were imaged and the images were analyzed. It was clear from these images that each spectral channel operated independently without noticeable out-of-band leakage.

This work was done by Neelam Gupta of the Army Research Laboratory, Philip R. Ashe of SpectralSight, and Songsheng Tan of Infotonics Technology Center. ARL-0115

Topics:
Assembling Exteriors Fabrication Filters Imaging and visualization Insulation Lenses Manufacturing processes Metals Microelectromechanical devices Mirrors Mirrors Optics Optimization Physical Sciences Sensors and actuators Titanium
This Brief includes a Technical Support Package (TSP).
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Miniature Snapshot Multispectral Imager

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    Defense Tech Briefs Magazine

    This article first appeared in the April, 2012 issue of Defense Tech Briefs Magazine (Vol. 6 No. 2).

    Read more articles from this issue here.

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    Overview

    The document titled "Development of a Miniature Snapshot Multispectral Imager" presents a comprehensive overview of a novel imaging system designed to operate in the short wavelength infrared (SWIR) region. Authored by Neelam Gupta, Philip R. Ashe, and Songsheng Tan, the report details the development of a compact, lightweight, and low-power multispectral imager that utilizes a 4×4 Fabry-Perot filter array. This filter array operates within a spectral range of 1487 to 1769 nm, with a bandpass of approximately 10 nm, making it suitable for various applications, including the detection of chemicals used in improvised explosive devices (IEDs).

    The imager's design incorporates a microelectromechanical system (MEMS) shadow mask technique to fabricate the Fabry-Perot etalon, which features multilayer dielectric mirrors. The filter array is integrated into a commercial handheld indium gallium arsenide (InGaAs) camera, where the standard imaging lens is replaced with a custom-designed 4×4 microlens array. This configuration allows for telecentric imaging performance across 16 sub-imaging channels, enhancing the system's versatility.

    The report outlines the design and characterization processes of the filter array and microlens optical package, providing insights into the optical performance, tolerance analysis, and radiometric analysis. It also discusses the fabrication process of the MEMS filter array and presents experimental results that demonstrate the imager's capabilities in capturing multispectral images of various indoor and outdoor scenes.

    The flexibility of the microlens optics and filter design is highlighted, indicating that the system can be tailored for different wavelength regions, from ultraviolet (UV) to longwave infrared (LWIR), and that the spectral bandpass can be customized to meet specific requirements. This adaptability makes the imager a valuable tool for a range of applications beyond IED detection, potentially extending to environmental monitoring, agricultural assessment, and other fields requiring multispectral imaging.

    In summary, the document provides a detailed account of the development, design, and potential applications of a miniature snapshot multispectral imager, emphasizing its innovative features and the significance of its capabilities in various domains.

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