Robert W. Brandstetter

Photopolymer elements for an optical correlator system

An optical correlator system that employs photopolymer materials fabricated to form holographic optical elements and multiple matched filter memories is described. The photopolymer media offer some distinct advantages over other materials; in particular, these are in situ operations, high-efficiency one-step dry chemistry, and ease of operation. By comparison, silver-halide media in general require off-line multistep wet chemistry processing which is slow and inconvenient; the resultant holographic element exhibits low efficiency. To be sure, the use of photopolymers for holographic elements has its shortcomings, being several orders of magnitude less sensitive, with spatial frequency limits at the low end and somewhat limited wavelength operation. In this paper a comparison is made of two Van der Lugt architectures, one being an all-photopolymer element system and the second a benchmark correlator system employing silver-halide materials. Representative correlation objects were prepared and processed for each medium. This was followed by a series of test measurements for peak levels, signal-to-clutter, and tracking response. For a wider range of applications, filter impulse responses are examined and compared for the classic Van der Lugt filter and binary-phase-only-filter-type elements. It is shown that when care is taken in processing, the photopolymers can offer an extremely convenient and effective medium for in situ recording of both amplitude and phase filter elements, while providing significantly more versatility than the silver halides and, generally, with higher efficiency and system throughput.

Optical correlator simulation and experimental evaluation for three-dimensional parts

The performance of an optical correlator is analyzed as a vision system for a manufacturing robot. The correlator architecture is mathematically modeled and a simulation of the system is undertaken using representative airplane parts. Of particular interest in this study is the effect of three-dimensional part data on a two-dimensional correlation process. The model accounts for the apparent changes in part dimensions as a function of z-axis components, position in a parts tray, and the angular field of view of the vision system camera. The impact of these changes on the level of the correlation function and background discrimination is then assumed. Complementing the analytical model simulation, an actual optical correlator system is arranged to operate with parts placed at various locations in the physical tray area in order to obtain their corresponding levels. It was found that, by the introduction of multiple filters, the representative 3-D parts can be detected and tracked within the area of the tray, even when apparent part dimensions changed over the field of view of the imaging camera. The results of this research indicate that the correlator vision system concept shows promise for future 3-D manufacturing robotics applications.

Experimental comparison of optical binary phase-only filter and high-pass matched filter correlation

An experimental comparison of two optical correlation systems in their ability to do target detection, identification, and discrimination is described. A common set of four binary input images was used for the experiments, which included single and multiple target scenes. The set included symmetric and asymmetric objects. The two optical correlation systems compared were (a) a high pass matched filter (HPMF) VanderLugt system and (b) a binary phase-only filter (BPOF) system. In the HPMF system, the input image and filter are film-based, with the filter somewhat Gaussian apodized to achieve high passing. The BPOF system has one magneto-optical spatial light modulator as the input device, and another as the filter device. The experimental measurements compared were (1) target auto- and crosscorrelation, (2) auto- and crosscorrelation for multiple target scenes, (3) spatial extent of the correlation peaks, and (4) sidelobe levels in multiple target scenes. In spite of the fundamental differences in the correlators compared (i.e., film vs. real time image/filter), the use of binary imagery and high pass filters in both cases gave comparable results in target detection, identification, and discrimination. Both the similarities and the differences are described and summarized.

Optical correlator vision system for a manufacturing robot assembly cell

An optical correlator system is to be interfaced with an existing robot assembly cell to provide the necessary manufacturing machine vision. In operation various parts are scanned by the vision system which provides identification and location of each pre-selected part. This information is forwarded to the robot controller which translates these data to perform mechanically articulated part retrieval placement and fastening to the assembly being manufactured. A tray of parts would be analyzed with individual parts located for use in an ongoing manufacturing process. As new parts arrive optical memories are fabricated off line and subsequently conveyed to the vision system on demand. To enhance parts identification and location under variable ambient conditions the video from the scanned scene is preprocessed to reduce the effects of lighting and contrast variations. Using laboratory and shop data a variety of actual aircraft parts have been used to fabricate matched filters for use in the vision correlator. Tests conducted with the correlator show that individual parts can be identified located and differentiated from groups of parts with similar appearance. As a next step the correlator as a vision system will be configured for interface with a representative robot cell for premanufacturing tests.

Processing imagery for a spatial light modulator

A real-time optical correlator employs a spatial light modulator (SLM) to record ongoing, changing scenes. Current SLMs are quite useful in this role although some suffer from limited dynamic range and, therefore cannot respond fully to such variations as light changes on the target. In this paper, a method is described for preprocessing an image before it is impressed upon an SLM. The processed image, in effect, alters the transfer characteristics and serves to make the image relatively invariant with changes in scene and environmental target conditions. The restructured image will appear invariant to the SLM - or at least quite constant - and, therefore, invariant to the optical matched filter residing in the memory. The correlator operates as though the target and scene conditions were fixed, confined to more acceptable, narrowed conditions.