Michael T. Eismann

Iterative design of a holographic beamformer.

A design method is presented for computing the phase functions of an energy efficient system using two holographic elements for converting a Gaussian beam into a uniform beam with rectangular support in the far field of the source. The method is based on a modification of the Gerchberg-Saxton algorithm which includes an x-y separability constraint on the phase of one of the holographic elements. A beamforming system was fabricated using this method, and experimental results were obtained which support the design approach.

Thermal multispectral detection of military vehicles in vegetated and desert backgrounds

Recent data collections using an infrared hyperspectral measurement system have provided a significant measurement database of military vehicles in vegetated and desert backgrounds. This paper summarizes the results of a study performed to assess the detection performance potential of multispectral sensors using this database. Specific issues addressed include approaches to optimal band selection; robustness of band combinations with target, background, and environment diversity; and sensor noise requirements. All of these issues are vital to assessing the feasibility and utility of infrared multispectral sensors in operational scenarios.

Infrared multispectral target/background field measurements

Extensive measurements of targets and backgrounds were made in the field by an infrared Fourier transform spectrometer. These measurements were made to provide statistically valid estimates of target to background spectral contrast and background spatial and spectral statistics to support the use of multispectral sensing techniques for detecting military targets in clutter. The details of the spectrometer, the targets, the backgrounds, and the measurements are given.

Target detection in desert backgrounds: infrared hyperspectral measurements and analysis

Infrared multispectral sensors are being investigated as a means for day and night target detection. Infrared multispectral sensors would exploit high spectral band-to-band correlation to reject high background clutter. An infrared Fourier transform spectrometer-based field measurement system was used to collect spectral signature data of targets and desert scrub and sand backgrounds from a 100 foot tower at White Sands Missile Range. The measurements include target-to-background spectral contrast, subpixel targets, background spectral correlation, and background spatial power spectra. The measurements have been analyzed to determine multispectral signal-to-clutter ratios versus target, background, diurnal, and weather variations, background correlation versus temperature clutter variations, and spectral correlation versus spatial scale. These measurements contribute to the expanding target and background infrared hyperspectral signature database. The results of the analysis demonstrate the utility and robustness of infrared multispectral techniques for target detection.

Holographic polar formatting and realtime optical processing of synthetic aperture radar data

A two holographic optical element (HOE) system for polar formatting of spotlight mode synthetic aperture radar (SAR) data was designed, fabricated, and successfully tested. With the addition of a spatial light modulator, a third phase-compensating HOE, and a Fourier transform lens, the real-time polar formatting of SAR data and SAR image formation was experimentally demonstrated.

Analysis of infrared hyperspectral measurements by the joint multispectral program

A series of infrared hyperspectral field measurements was made at Wright Patterson Air Force Base and the U.S. Army White Sands Missile Range by the Joint Multispectral Program (JMSP) between November 1993 and June 1994. In these experiments, a highly sensitive Fourier transform spectrometer (FTS) was used to collect data from test panels, military and civilian vehicles, and various types of natural backgrounds. Post-collection data analyses are being conducted by the JMSP to assess the potential of thermal multispectral processing for detecting and classifying low-contrast ground targets in natural clutter environments. One target material of special interest is CARC paint, which is currently applied to U.S. Army vehicles in various colors to create woodland and desert camouflage patterns. CARC-painted test panels were observed in a wide variety of backgrounds and weather conditions during all of the JMSP experiments. It is shown here that certain fine-scale spectral features of this paint can support reliable two-color discrimination of CARC-coated test panels in different natural backgrounds, even under low contrast and high clutter conditions. The paper also examines environmental variations in two key parameters that determine spectral detectability; specifically, the observed target-background spectral contrast signature (which provides the required coloring), and the background spectral correlation (which provides for multiband clutter suppression).

Design and fabrication of a wide-field holographic lens for a laser communication receiver

The design and fabrication of a wide field-of-view (FOV) holographic lens for use in a laser communication receiver are described. The holographic lens was designed to image incoming laser radiation at 850 nm onto a solid state detector array over a FOV of ±22.5 x ±16 deg (±26.7 deg in the diagonal direction). Experimental results obtained with the holographic wide FOV laser receiver are presented. The location of the focused spot can be determined by interpolation to subpixel accuracy, providing the laser receiver with an angular resolution better than 1.5 mrad.

Holographic Beamformer Designed By An Iterative Technique

A fundamentally energy efficient, two element holographic beamformer for converting a Gaussian beam into a rectangular uniform beam in the far-field was designed, fabricated, and tested. The design method is based on a modification of the Gerchberg-Saxton iterative Fourier transform algorithm to include an x-y separability constraint on the phase of one of the holographic elements. This modification gave a design with improved beamforming performance. The two required holographic elements were fabricated using a combination of optical and computer-generation techniques. The experimental results verify the effectiveness of the design approach.

Design And Applications Of Multiple Hologram Systems

An important application of holographic optical elements (HOE) is coordinate transformations (or mappings) to put data into a format that can be operated on by an optical processor. In optical coordinate transformation, a specific one-to-one mapping of pixel locations in the input plane to pixel locations in the output plane is produced. For some desired transformations, the data redistribution cannot be realized with a single continuous-phase hologram, and in these cases a multiple hologram system must be considered. The processing of spotlight synthetic aperture radar (SSAR) data is used as a sample application to illustrate the concept. The design and fabrication of the holograms for SSAR data processing is discussed and experimental results are given.

Holographic lens for wide-FOV laser receiver

Light weight and compactness are critical requirements in the design of many optical systems and the use of holographic optics often provides an ideal solution to the size and weight problem. In this paper, the design and fabrication of a wide field-of-view (FOV) holographic lens for use in a laser receiver are described. The holographic lens is designed to image incominlaser radiation at 850nm onto a solid state detector array over a field of view of x (±26.7 in the diagonal direction). Over the limiting aperture, the effective f/number of the holographic lens for each incident angle is f/2.5. Over the entire lens aperture, however, the f/number of the holographic lens is f/0.7 in order to accommodate the 53.4 FOV. Experimental results obtained with the holographic wide FOV laser receiver are presented. The location of the focussed spot can be determined to within one pixel of the detector array, providing the laser receiver with an angular resolution better than 1 mrad.