Programmable matched filter and Hadamard transform hyperspectral imagers based on micromirror arrays

Abstract

Hyperspectral imaging (HSI), in which each pixel contains a high-resolution spectrum, is a powerful teclmique that can remotely detect, identify, and quantify a multitude of materials and chemicals. The advent of addressable micro-mirror arrays (MMAs) makes possible a new class of programmable hyperspectral imagers that can perfonn key spectral processing functions directly in the optical hardware, thus alleviating some ofHSI ' s high computational overhead, as well as offering improved signal-to-noise in certain important regimes (e.g. when using uncooled infrared detectors). We have built and demonstrated a prototype UV-Visible micro-mirror hyperspectral imager that is capable not only of matched filter imaging, but also of full hyperspectral imagery via the Hadamard transfonn teclmique. With this instrument, one can upload a chemical-specific spectral matched filter directly to the MMA, producing an image showing the location of that chemical without further processing. Target chemicals are changeable nearly instantaneously simply by uploading new matched-filter patterns to the MMA. Alternatively, the MMA can implement Hadamard mask functions, yielding a full-spectrum hyperspectral image upon inverting the transfonn. In either case, the instrument can produce the 2D spatial image either by an internal scan, using the MMA itself, or with a traditional external push-broom scan. The various modes of operation are selectable simply by varying the software driving the MMA. The design and perfonnance of the prototype will be discussed, along with experimental results confmning the signal-to-noise improvement produced by the Hadamard technique in the noisy-detector regime . Requirements for a possible future extension to the long-wave infrared will also be examined.

Avoiding the side-effect of high-performance spectrometers with coded apertures

“…In our spectrometer, we choose the S matrix as the encoding matrix W. If all the elements in each row of W − are binned, the sum is 2∕n 1, where n is the order of W [11]. The decoded error in each channel is proportional to the intensity of the smear noise and shares the same intensity.…”

Adaptive hyperspectral imaging with a MEMS-based full-frame programmable spectral filter

“…Each Hadamard function contains roughly half of the spectral bands in the set, dramatically increasing the amount of light that falls on the detector, and it can be shown that for an N-band detector-noise-limited system, one can realize a factor of √N improvement in signal to noise compared to single-band collection. 16 Finally, as we'll discuss below, intensity modulation at the DMD enables the direct measurement of common matched-filter detection functions, which skips the data cube collection entirely and performs the multispectral data analysis directly in the optical hardware.…”

“…15 More recent designs have expanded this capability to 1-D pixel arrays for pushbroom imaging applications. 16,17 In each case, the concept is straightforward: light emerging from each source is collimated, dispersed by a grating or prism, and re-imaged on a 1-D array of micro-mirrors (2-D for imaging), forming a spectrally dispersed image of the source. Each mirror (column) in the array controls a different narrow wavelength band, and can be independently addressed or tilted to either of two positions.…”

Full-frame programmable spectral filters based on micro-mirror arrays

“…The original Los Alamos micro-mirror array multiplexing hyperspectral imager. 1 The concept of operation is illustrated on the left: Light from the scene is dispersed by a diffraction grating and imaged onto the micro-mirror array, which selects certain wavelengths and rejects others to implement a chemical-specific filter or Hadamard encoding scheme. The selected wavelengths are recombined by a second grating and imaged onto the detector array.…”

“…[1][2][3][4] But all of these designs share a similar shortcoming: they remain, in essence, push-broom imagers, still requiring a time-consuming scan either of a 1-dimensional single-line field of view, or, in some cases, of the spectral pattern programmed onto the DMD, in order to build up a 2-dimensional spatial image. This paper describes the design and performance of a new type of multi-band programmable filter that truly behaves like a filter, allowing full-frame 2D spatial images to be obtained in a single shot without scanning, with the same spectral band selection applied across the entire frame.…”