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


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Miniature Snapshot Multispectral Imager

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This article first appeared in the April, 2012 issue of Defense Tech Briefs Magazine.

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