We present a single disperser spectral imager that exploits recent theoretical work in the area of compressed sensing to achieve snapshot spectral imaging. An experimental prototype is used to capture the spatiospectral information of a scene that consists of two balls illuminated by different light sources. An iterative algorithm is used to reconstruct the data cube. The average spectral resolution is 3.6 nm per spectral channel. The accuracy of the instrument is demonstrated by comparison of the spectra acquired with the proposed system with the spectra acquired by a nonimaging reference spectrometer.

/pdf/single-disperser-design-for-coded-aperture-snapshot-spectral-4q8778zsaj.pdf

Single disperser design for coded aperture snapshot spectral imaging

This paper describes a single-shot spectral imaging approach based on the concept of compressive sensing. The primary features of the system design are two dispersive elements, arranged in opposition and surrounding a binary-valued aperture code. In contrast to thin-film approaches to spectral filtering, this structure results in easily-controllable, spatially-varying, spectral filter functions with narrow features. Measurement of the input scene through these filters is equivalent to projective measurement in the spectral domain, and hence can be treated with the compressive sensing frameworks recently developed by a number of groups. We present a reconstruction framework and demonstrate its application to experimental data.

http://www2.engr.arizona.edu/~gehm/pubs/OptExp.15.14013.pdf

Single-shot compressive spectral imaging with a dual-disperser architecture

We have previously reported on coded aperture snapshot spectral imagers (CASSI) that can capture a full frame spectral image in a snapshot. Here we describe the use of CASSI for spectral imaging of a dynamic scene at video rate. We describe significant advances in the design of the optical system, system calibration procedures and reconstruction method. The new optical system uses a double Amici prism to achieve an in-line, direct view configuration, resulting in a substantial improvement in image quality. We describe NeAREst, an algorithm for estimating the instantaneous three-dimensional spatio-spectral data cube from CASSI’s two-dimensional array of encoded and compressed measurements. We utilize CASSI’s snapshot ability to demonstrate a spectral image video of multi-colored candles with live flames captured at 30 frames per second.

Video rate spectral imaging using a coded aperture snapshot spectral imager

Applied Iterative Methods is a self-contained treatise suitable as both a reference and a graduate-level textbook in the area of iterative algorithms. It is the first book to combine subjects such as optimization, convex analysis, and approximation theory and organize them around a detailed and mathematically sound treatment of iterative algorithms. Such algorithms are used in solving problems in a diverse area of applications, most notably in medical imaging such as emission and transmission tomography and magnetic-resonance imaging, as well as in intensity-modulated radiation therapy. Other applications, which lie outside of medicine, are remote sensing and hyperspectral imaging. This book details a great number of different iterative algorithms that are universally applicable.

/pdf/applied-iterative-methods-kor6ztq27v.pdf

Applied Iterative Methods

This paper presents a prism-mask system for capturing multispectral videos. The system is composed of a triangular prism, a monochrome camera, and an occlusion mask. Incoming light beams from the scene are sampled by the occlusion mask, dispersed into their constituent spectra by the triangular prism, and then captured by the monochrome camera. Our system is capable of capturing frames with high spectral resolution at video rates. It also allows for different trade-offs between spectral and spatial resolution by adjusting the focal length of the camera. We demonstrate multispectral video acquisition with various spectral resolutions and spatial resolutions, as well as different frame rates. The effectiveness of our system is further evaluated with several applications, including human skin detection, physical material recognition, video segmentation, RGB video generation, and illumination identification.

A Prism-Mask System for Multispectral Video Acquisition

The plateau equalization algorithm for display of IR images is defined to include histogram equalization and histogram projection as special cases. A maximum gain parameter is defined and surveyed for a large number of images, determining the useful range of plateau values. Analysis of hardware experience with histogram projection explains why it produces intensity jitter in very low contrast scenes. Detailed flow charts for hardware implementation of plateau equalization are provided.

Plateau equalization algorithm for real-time display of high-quality infrared imagery

A spectral imager constructs a three-dimensional (two spatial and one spectral) image from a series of two-dimensional images. We discuss a technique for spectral imaging that multiplexes the spatial and spectral information on a staring focal plane and then demultiplexes the resulting imagery to obtain the spectral image. The spectral image consists of 100×100 spatial pixels and 25 spectral bands. The current implementation operates over the 3–5-μm band, but can easily be applied to other spectral regions. This approach to spectral imaging has high optical throughput and is robust to focal plane array nonuniformities. A hardware description, the mathematical development, and experimental results are presented.

High-throughput hyperspectral infrared camera

A conjecture that the number of Costas arrays is a monotonic increasing function of array size is disproved by extension of the known values to n=17. A probabilistic estimation formula is developed which predicts the peak at n=16 and tracks the known values to (typically) 5-6%. >

On the number of Costas arrays as a function of array size

Twelve-bit digitized images taken with PtSi Schottky barrier detector arrays have been processed on Sun work stations. Two techniques for 8-bit global display are compared: the standard method of histogram equalization and a newly devised technique of histogram projection. The latter assigns equal dynamic range to each occupied level, while the former
does so according to the density of the occupied levels. The projection technique generally gives distinctly superior results based on an extensive
set of indoor, outdoor, day, and night imagery. For cases in which the two algorithms have complementary advantages, the techniques can be combined in effect by a weighting of their distribution functions, which often gives the desirable features of each. The new projection algorithm also can be used as a powerful and robust local contrast enhancement technique.
An alternative method of contrast enhancement, a global algorithm based on modular (sawtooth) displays, affords a comparable degree of enhancement at less computational cost.

Display of wide dynamic range infrared images from PtSi Schottky barrier cameras

Our algorithm development for point target surveillance is closely meshed to our laboratory IR cameras. The two-stage approach falls into the category of `track before detect' and incorporates dynamic programming optimization techniques. The first stage generates merit scores for each pixel and suppresses clutter by spatial/temporal subtractions from N registered frames of data. The higher the value of the merit score, the more likely that a target is present. In addition to the merit score, the best track associated with each score is stored; together they comprise the merit function. In the second stage, merit functions are associated and dynamic programming techniques are used to create combined merit functions. Nineteen and thirteen frames of data are used to accumulate merit functions. Results using a total of 38 and 39 frames of data are presented for a set of simulated targets embedded in white noise. The result is a high probability of detection and low false alarm rate down to a signal to noise ratio of about 2.0. Preliminary results for some real targets (extracted from real scenes and then re- embedded in white noise) show a graceful degradation from the results obtained on simulated targets.

Virgil E. Vickers

Papers

Plateau equalization algorithm for real-time display of high-quality infrared imagery

High-throughput hyperspectral infrared camera

On the number of Costas arrays as a function of array size

Display of wide dynamic range infrared images from PtSi Schottky barrier cameras

Signal-processing algorithms for point-target detection in consecutive-frame staring imagery