Nils J. Fonneland

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.

A near IR Fabry-Perot interferometer for wide field, Low resolution hyperspectral imaging on the Next Generation Space Telescope

We discuss work in progress on a near-infrared tunable bandpass filter for the Goddard baseline wide field camera concept of the Next Generation Space Telescope Integrated Science Instrument Module. This filter, the Demonstration Unit for Low Order Cryogenic Etalon (DULCE), is designed to demonstrate a high efficiency scanning Fabry-Perot etalon operating in interference orders 1 - 4 at 30 K with a high stability DSP based servo control system. DULCE is currently the only available tunable filter for lower order cryogenic operation in the near infrared. In this application, scanning etalons will illuminate the focal plane arrays with a single order of interference to enable wide field lower resolution hyperspectral imaging over a wide range of redshifts. We discuss why tunable filters are an important instrument component in future space-based observatories.

Multiview: a novel multispectral IR imaging camera

The Surveillance Sciences Directorate of the Northrop Grumman Advanced Systems and Technology organization is developing a novel Multispectral IR camera known as Multiview. This prototype system is capable of simultaneously acquiring 4-color SWIR/MWIR 2D imagery that is both spatially and temporally registered utilizing a single 2562 HgCdTe snapshot IR FPA capable of frame rates in excess of 240 Hz. The patented design offers an extremely compact package that contains the entire optomechanical assembly (lenses, interchangeable filters, and cold shield) in less than a 3 in3 volume. The unique imagery collected with this camera has the potential to significantly improve the effectiveness of clutter suppression algorithms, multi-color target detection and target-background discrimination for a wide variety of mission scenarios. This paper describes the key aspects of the Multiview prototype camera design and operation. Multiviews ability to dynamically manage flux imbalances between the four subbands is discussed. Radiometric performance predictions are presented along with laboratory validation of many of these performance metrics. Several examples of field collected imagery is shown including examples of transient rocket plume data measured at 240 Hz sample rate. The importance and utility of spatio-temporal multi-band imagery is also discussed.

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.

Development and testing of a MWIR Fabry-Perot interferometer for hyperspectral imaging

This paper describes the theory of design, operation, and testing of a tunable MWIR Fabry-Perot interferometer operating in low orders. This device is called the agile bandpass tunable filter (ABTF) due to the fact that the spectral bandwidth can be changed by a large factor by changing the order. In first order the system can be tuned over the entire 3.5-5 micrometers spectral region with only a single order sorting filter. We provide a short introduction to tunable filters an then briefly discuss the requirements that low order operation places on the Fabry-Perot dielectric mirrors. Operation in low orders forces one to abandon the classical Fabry-Perot approximation that the mirrors are negligibly thin compared to the plate separation. Rather, one must now account for the phase properties of the dielectric stack mirrors as they produce phase effects comparable to the plate separation. We next address the issue of control of the Fabry-Perot. This is accomplished through a closed-loop system using capacitive sensor on the Fabry-Perot flats to measure the separation of the plates. Additionally we describe how the ABTF is characterized using a FTIR to measure the bandpass shape and position, and we show some examples of measurements made with the ABTF used as a hyperspectral imaging system with a 256 X 256 HgCdTe camera. We conclude with a discussion of potential applications and future work.

Agile band-pass tunable filter in gas measurement

The agile bandpass tunable filter (ABTF) is a new instrument with variable resolution and center wavelength that allows MWIR and LWIR infrared cameras to recognize spectral images that are characteristic of specific materials, including gases. Benefits of this new technology include the ability to: (1) Task infrared camera-based sensors to search for specific materials, and (2) Transform infrared camera data into signatures easily understood by untrained users. The application of a working prototype hyperspectral ABTF is described. The prototype is capable of rapidly selecting bandpasses from among several spectral resolutions, and is easily tunable over the MWIR spectral band. The application of the ABTF to signature imaging of gases is shown.

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.