Steven T. Griffin

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.

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.

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.

Some Characteristics of the Hollow Cathode Discharge Source from Hollow Cathodes of Different Sizes and Shapes

The empirical evaluation of several types of two-piece hollow cathodes for the analysis of microsamples using both dc and pulsed discharge modes of the hollow cathode discharge is reported. The discharge parameters used are similar to those that have given good precision and sensitivity for microsamples in this laboratory. Both optical microscopy and scanning electron microscopy were used to follow sputtering effects on the cathode geometry. The sputtering patterns in the flat-bottom, two-piece cylindrical hollow cathode revealed directly the cathode dark space on the cathode cavity bottom. The temporal emission profiles from the spherical cathode cavity are similar to those observed in cylindrical cathode cavities. The temporal emission profile changes with the diameter of the cylindrical cathode cavity and is characteristic of the analyte.

Breakdown Voltage Scaling Characteristics of an Argon Hollow Cathode Discharge

Paschen curves are presented for hollow cathodes with diameters varying from 5 mm to 1.5 mm. The curves obey similar functions of (pd), where p is the argon gas pressure and d is the hollow diameter. The curves exhibit similar properties in terms of the location of the breakdown voltage minima. The breakdown voltage increases monotonically for decreasing hollow diameter. The shapes of the various curves and the (pd) minima location are unaffected by aspect ratio.

Advanced terahertz imaging system performance model for concealed weapon identification

The U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) and the U.S. Army Research Laboratory (ARL) have developed a terahertz-band imaging system performance model for detection and identification of concealed weaponry. The details of this MATLAB-based model which accounts for the effects of all critical sensor and display components, and for the effects of atmospheric attenuation, concealment material attenuation, and active illumination, were reported on at the 2005 SPIE Europe Security and Defence Symposium. The focus of this paper is to report on recent advances to the base model which have been designed to more realistically account for the dramatic impact that target and background orientation can have on target observability as related to specular and Lambertian reflections captured by an active-illumination-based imaging system. The advanced terahertz-band imaging system performance model now also accounts for target and background thermal emission, and has been recast into a user-friendly, Windows-executable tool. This advanced THz model has been developed in support of the Defense Advanced Research Project Agencys (DARPA) Terahertz Imaging Focal-Plane Technology (TIFT) program. This paper will describe the advanced THz model and its new radiometric sub-model in detail, and provide modeling and experimental results on target observability as a function of target and background orientation.

Comparison of continuous and discrete frequency-versus-radius frequency-modulated reticles

We present a general expression for the transmission function of the discrete frequency-versus-radius reticle and compare such a reticle with the more common continuous reticle. A discrete form of the frequency-versus-radius reticle has an integer number of chopping cycles on a single radius. The discreteform limits the resolution of the reticle in the radial direction, but this limit is not severe for small-target images. However, since no phase reversal occurs, electronic processing is simplified.

Frustrated total internal reflection based sensor for Cavity Ring Down spectroscopy

Homeland Security applications require spectroscopic remote sensing. Contract work has indicated the potential for sampling into a Cavity Ring Down based spectroscopic sensor. Potential contamination issues have led to the development of a frustrated total internal reflection, surface absorption technique to protect the cavity finesse. Autonomous vehicle power, weight and dimensional limitations require the reduction of cavity baseline.

Laser-induced breakdown plasma-based sensors

Laser Induced Breakdown Spectroscopy (LIBS) is dependent on the interaction between the initiating Laser sequence, the sampled material and the intermediate plasma states. Pulse shaping and timing have been empirically demonstrated to have significant impact on the signal available for active/passive detection and identification. The transient nature of empirical LIBS work makes data collection for optimization an expensive process. Guidance from effective computer simulation represents an alternative. This computational method for CBRNE sensing applications models the Laser, material and plasma interaction for the purpose of performance prediction and enhancement. This paper emphasizes the aspects of light, plasma, and material interaction relevant to portable sensor development for LIBS. The modeling structure emphasizes energy balances and empirical fit descriptions with limited detailed-balance and finite element approaches where required. Dusty plasma from partially decomposed material sample interaction with pulse dynamics is considered. This heuristic is used to reduce run times and computer loads. Computer simulations and some data for validation are presented. A new University of Memphis HPC/super-computer (~15 TFLOPS) is used to enhance simulation. Results coordinated with related effort at Arkansas State University. Implications for ongoing empirical work are presented with special attention paid to the application of compressive sensing for signal processing, feature extraction, and classification.

LIBS spectroscopic classification relative to compressive sensing

Laser Induced Breakdown Spectroscopy (LIBS) utilizes a diversity of standard spectroscopic techniques for classification of materials present in the sample. Pre-excitation processing sometimes limits the analyte to a short list of candidates. Prior art demonstrates that sparsity is present in the data. This is sometimes characterized as identification by components. Traditionally, spectroscopic identification has been accomplished by an expert reader in a manner typical for MRI images in the medicine. In an effort to automate this process, more recent art has emphasized the use of customized variations to standard classification algorithms. In addition, formal mathematical proofs for compressive sensing have been advanced. Recently the University of Memphis has been contracted by the Spectroscopic Materials Identification Center to advance and characterize the sensor research and development related to LIBS. Applications include portable standoff sensing for improvised explosive device detection and related law enforcement and military applications. Reduction of the mass, power consumption and other portability parameters is seen as dependent on classification choices for a LIBS system. This paper presents results for the comparison of standard LIBS classification techniques to those implied by Compressive Sensing mathematics. Optimization results and implications for portable LIBS design are presented.