ISIS: an information-efficient spectral imaging system

Abstract

A specialized hyperspectral imager has been developed that preprocesses the spectra from an image before the light reaches the detectors. This "optical computer" does not allow the flexibility of digital post-processing. However, the processing is done in real time and the system can examine = 2 x lo6 scene pixels/sec. Therefore, outdoors it could search for pollutants, vegetation types, minerals, or man-made objects. On a highspeed production line it could identify defects in sheet products like plastic wrap or film, or on painted or plastic parts.ISIS is a line scan imager. A spectrally dispersed slit image is projected on a Spatial -Light Modulator. The SLM is programmed to take the inner product of the spectral intensity vector and a spectral basis vector. The SLM directs the positive and negative parts of the inner product to different linear detector arrays so the signal difference equals the inner product. We envision a system with one telescope and =4 SLMs. INTRODUCTIONDifferent classes of materials can be separated from one-another because they have different reflectance spectra in the 0.5-pm to 2.4-pm spectral region. Figure 1 a is an image of a scene containing soil, trees, and a tank. Figure IC, can be calculated for these spectra using spectral linear discriminant analysis2. These vectors, reminiscent of eigenvectors, would emphasize the differences between the spectra of the different material classes.One can search for specific materials in an area by scanning it with a hyperspectral imager, processing the data, and comparing the results with known results. The data processing consists of taking the spectra from each image pixel and taking the inner product (dot product) of the pixel's spectra and each of the spectral basis vectors. The constants computed from the inner products can then be compared with known results. As an example, consider searching a scene ( Fig. 1 a) using two basis vectors. For each pixel of the scene, the two inner products have been taken and the results have been plotted, producing the scatter plot shown in Figure Id. Note that the background and the material sought tend to cluster into three different areas, one of which is "interesting".Most hyperspectral instruments use a line-scan-imager architecture. These instruments move an image of the scene across a slit. After passing through the slit, the spectra are DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or ...

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