Carl E. Halford

Design and analysis of apposition compound eye optical sensors

Methods are presented for designing and estimating the performance of artificial apposition compound eye optical sensors. Apposition compound eyes have been investigated for a wide range of applications from robotics to smart weapons. While artificial apposition compound eyes have been constructed and demonstrated, optical design issues and performance prediction for these systems have never been adequately addressed in the literature. Apposition compound eyes are a useful paradigm for applications where wide field of view is critical but high spatial resolution is not required. Natural arthropod compound eyes and their biological models are first scrutinized to give insight into designing and modeling artificial apposition compound eye optical sensors. Such sensors are shown to have comparable sensitivity to traditional single-aperture sensors for extended sources. A method for enhancing resolution over the extended-source case is demonstrated for single point sources. For the first time, the frequency response of an artificial apposition compound eye is addressed while taking into account the differences between artificial and natural ommatidia. Both a numercal integration technique for determining the contrast transfer function and a Fourier-transform modulation-transfer-function method are presented and validated with experimental results.

Active and passive imaging in the THz spectral region : phenomenology, dynamic range, modes, and illumination

The useful compromise between resolution and penetration power of the submillimeter or terahertz (THz) spectral region has long made it attractive for a variety of imaging applications. However, many of the demonstrations of imaging in this spectral region have used strategically oriented targets, especially favorable concealment materials, proximate imaging geometries, etc. This paper reports the results of studies aimed at better understanding the phenomenology of targets, the impact of this phenomenology on various active and passive imaging strategies, and most importantly, the development of imaging strategies that do not require the aforementioned special circumstances. Particular attention is paid to the relationship between active and passive images, especially with respect to how they interact with the illumination- and detector-mode structures of various imaging scenarios. It is concluded that the very large dynamic range that can be obtained with active single-mode systems (including focal-plane arrays) can be used in system designs to overcome the deleterious effects that result from the dominance of specular reflections in single-mode active systems as well as to strategically orient targets to obtain recognition. This will aid in the development of a much more robust and generally useful imaging technology in this spectral region.

Modeling the target acquisition performance of active imaging systems

Recent development of active imaging system technology in the defense and security community have driven the need for a theoretical understanding of its operation and performance in military applications such as target acquisition. In this paper, the modeling of active imaging systems, developed at the U.S. Army RDECOM CERDEC Night Vision & Electronic Sensors Directorate, is presented with particular emphasis on the impact of coherent effects such as speckle and atmospheric scintillation. Experimental results from human perception tests are in good agreement with the model results, validating the modeling of coherent effects as additional noise sources. Example trade studies on the design of a conceptual active imaging system to mitigate deleterious coherent effects are shown.

Active and Passive Millimeter and Sub-Millimeter-Wave Imaging

We have developed several millimeter/submillimeter/terahertz systems to study active and passive imaging and associated phenomenology. For measuring the transmission and scattering properties of materials, we have developed a dual rotary stage scattering system with active illumination and a Fourier Transform spectrometer. For imaging studies, we have developed a system based on a 12-inch diameter raster-scanned mirror. By interchange of active sources and both heterodyne and bolometric detectors, this system can be used in a variety of active and passive configurations. The laboratory measurements are used as inputs for, and model calibration and validation of, a terahertz imaging system performance model used to evaluate different imaging modalities for concealed weapon identification. In this paper, we will present examples of transmission and scattering measurements for common clothing as well as active imaging results that used a 640 GHz source and receiver.

Parameters of spinning AM reticles

A new method of obtaining amplitude modulation (AM) for determining target location with spinning reticles is presented. The method is based on the use of graded transmission capabilities. The AM spinning reticles previously presented were functions of three parameters: amplitude vs angle, amplitude vs radius, and phase. This paper presents these parameters along with their capabilities and limitations and shows that multiple parameters can be integrated into a single reticle. It is also shown that AM parameters can be combined with FM parameters in a single reticle. Also, a general equation is developed that relates the AM parameters to a reticle transmission equation.

Imaging with frequency-modulated reticles

A novel imaging technique in which frequency-modulated retides encode different pixel locations by light modulation is presented. In this technique a reticle modulates different pixel locations at different frequencies, and photodetectors collect the resulting signals. Filters decode these signals to recreate the image on a display. The technique allows multiplexing many pixels onto a fewer number of detectors by utilizing the bandwidth of the detectors more effectively. Since frequency modulation creates an additional dimension for the detector, a single detector can function as a linear array, a linear array can function as a staring array, or the additional dimension can be used to convey spectral or other information. At wavelengths requiring expensive focal plane components, costs can be greatly reduced.

Atmospheric turbulence modulation transfer function for infrared target acquisition modeling

A new direction for the US Army Night Vision and Electronic Sensors Directorate is the development of ultra-narrow field of view (UNFOV) infrared target acquisition (TA) systems. Frequently, the per- formance of these systems is limited by atmospheric turbulence in the imaging path. It is desirable to include the effects of atmospheric turbu- lence blur in infrared TA models. The current TA models are currently linear shift invariant (LSI) systems with component modulation transfer functions (MTFs). The use of additional MTFs, to account for atmo- spheric turbulence, requires that the turbulence blur have LSI properties. The primary unresolved issue with the treatment of turbulence blur as an MTF is the LSI characteristics of the blur. Significant variation in spatial blur and temporal blur prohibit the use of a single MTF in an LSI target acquisition model. Researchers at Ben-Gurion University (BGU) use a TA model that includes an LSI blur, which is a temporal average of the turbulence blur. The research described here evaluates the BGU-type treatment of atmospheric MTF and determines it reasonable for inclusion in the US Armys TA model. In addition to the spatial characteristics, the temporal variation of the turbulence blur is also described.

Concealed weapon identification using terahertz imaging sensors

Terahertz imaging sensors are being considered for providing a concealed weapon identification capability for military and security applications. In this paper the difficulty of this task is assessed in a systematic way. Using imaging systems operating at 640 GHz, high resolution imagery of possible concealed weapons has been collected. Information in this imagery is removed in a controlled and systematic way and then used in a human observer perception experiment. From the perception data, a calibration factor describing the overall difficulty of this task was derived. This calibration factor is used with a general model of human observer performance developed at the US Army Night Vision and Electronic Sensors Directorate to predict the task performance of observers using terahertz imaging sensors. Example performance calculations for a representative imaging sensor are shown.

Sparse Detector Imaging Sensor with Two-Class Silhouette Classification.

This paper presents the design and test of a simple active near-infrared sparse detector imaging sensor. The prototype of the sensor is novel in that it can capture remarkable silhouettes or profiles of a wide-variety of moving objects, including humans, animals, and vehicles using a sparse detector array comprised of only sixteen sensing elements deployed in a vertical configuration. The prototype sensor was built to collect silhouettes for a variety of objects and to evaluate several algorithms for classifying the data obtained from the sensor into two classes: human versus non-human. Initial tests show that the classification of individually sensed objects into two classes can be achieved with accuracy greater than ninety-nine percent (99%) with a subset of the sixteen detectors using a representative dataset consisting of 512 signatures. The prototype also includes a Webservice interface such that the sensor can be tasked in a network-centric environment. The sensor appears to be a low-cost alternative to traditional, high-resolution focal plane array imaging sensors for some applications. After a power optimization study, appropriate packaging, and testing with more extensive datasets, the sensor may be a good candidate for deployment in vast geographic regions for a myriad of intelligent electronic fence and persistent surveillance applications, including perimeter security scenarios.

Marriage of FM reticles to focal plane arrays

A technique for segmenting focal planes to complement spinning frequency modulation (FM) reticles in tracking applications is presented. The technique involves the relaxation oftwo parameter constraints in the FM reticle equation. The advantages are a reduction in susceptibility to countermeasures and increased resolution. The disadvantages are increased electronics loading and a reduction in the maximum trackable target size.