RF imaging, architectures, and applications: introduction to the feature issue.

This Applied Optics feature issue spotlights developments in the emerging areas of millimeter, submillimeter, and terahertz imaging.

Super-resolution for flash ladar imagery.

Flash ladar systems are compact devices with high frame rates that hold promise for robotics applications, but these devices suffer from poor spatial resolution. This work develops a wavelet preprocessing stage to enhance registration of multiple frames and applies super-resolution to improve the resolution of flash ladar range imagery. The triangle orientation discrimination methodology was used for a subjective evaluation of the effectiveness of super-resolution for flash ladar. Results show statistically significant increases in the probability of target discrimination at all target ranges, as well as a reduction in subject response times for super-resolved imagery.

Modeling passive millimeter wave imaging sensor performance for discriminating small watercraft.

Passive millimeter wave (pmmW) imagers are quickly becoming practical sensor candidates for military and nonmilitary tasks. Our focus was to adapt the Night Vision [U.S. Army Research Development and Engineering Command, Communications and Electronics Research Development and Engineering Center, Night Vision and Electronics Sensors Directorate (NVESD)] passive thermal infrared imager performance models and apply them to pmmW imaging systems for prediction of field performance for the task of small watercraft and boat identification. The Night Vision Lab's infrared sensor model has been evolving since the 1950s, with the most current model being NVThermIP [Night Vision Thermal and Image Processing (NVThermIP) Model Users Manual, Rev. 9 (U.S. Army RDECON, CERDEC, NVESD, 2006)]. It has wide recognition as an engineering tool for sensor evaluation. This effort included collecting pmmW signatures for a representative set of targets, conducting an observer perception experiment, and deriving the task difficulty criteria that can be used in NVThermIP for identification of boats. The task difficulty criteria are used by designers and managers to create systems capable of meeting specific performance criteria in the field.

Performance of infrared systems in swimmer detection for maritime security.

The detection of swimmer activity in harbor areas around piers and ships is an important aspect of Anti-Terrorism/Force Protection (AT/FP) sensing efforts in the U.S. Navy and Coast Guard. A series of data collections and perception experiments were conducted to validate the use of thermal target acquisition models against swimmer targets. The results were analyzed to derive the discrimination criteria necessary for sensor design for maritime force protection.

Predicting the probability of facial identification using a specific object model.

Many security and surveillance tasks involve either finding an object in a cluttered scene or discriminating between like objects. For example, an observer might look for a person of known height and weight in a crowd, or he might want to positively identify a specific face. The paper "Modeling target acquisition tasks associated with security and surveillance" [Appl. Opt. 46, 4209 (2007)] describes a specific-object model used to predict the probability of accomplishing this type of task. We describe four facial identification experiments and apply the specific-object model to predict the results. Facial identification is accurately predicted by the specific-object model.

Characteristics of infrared imaging systems that benefit from superresolution reconstruction.

There have been numerous applications of superresolution reconstruction algorithms to improve the range performance of infrared imagers. These studies show there can be a dramatic improvement in range performance when superresolution algorithms are applied to undersampled imager outputs. These occur when the imager is moving relative to the target, which creates different spatial samplings of the field of view for each frame. The degree of performance benefit is dependent on the relative sizes of the detector/spacing and the optical blur spot in focal plane space. The minimum blur spot size achievable on the focal plane is dependent on the system F/number. Hence, we provide a range of these sensor characteristics, for which there is a benefit from superresolution reconstruction algorithms. Additionally, we quantify the potential performance improvements associated with these algorithms. We also provide three infrared sensor examples to show the range of improvements associated with provided guidelines.

Midwave infrared and visible sensor performance modeling: small craft identification discrimination criteria for maritime security.

The new emphasis on Anti-Terrorism and Force Protection (AT/FP), for both shore and sea platform protection, has resulted in a need for infrared imager design and evaluation tools that demonstrate field performance against U.S. Navy AT/FP requirements. In the design of infrared imaging systems for target acquisition, a discrimination criterion is required for successful sensor realization. It characterizes the difficulty of the task being performed by the observer and varies for different target sets. This criterion is used in both assessment of existing infrared sensor and in the design of new conceptual sensors. We collected 12 small craft signatures (military and civilian) in the visible band during the day and the long-wave and midwave infrared spectra in both the day and the night environments. These signatures were processed to determine the targets' characteristic dimension and contrast. They were also processed to band limit the signature's spatial information content (simulating longer range), and a perception experiment was performed to determine the task difficulty (N50 and V50). The results are presented and can be used for Navy and Coast Guard imaging infrared sensor design and evaluation.

Target-acquisition performance in undersampled infrared imagers: static imagery to motion video.

In this research we show that the target-acquisition performance of an undersampled imager improves with sensor or target motion. We provide an experiment designed to evaluate the improvement in observer performance as a function of target motion rate in the video. We created the target motion by mounting a thermal imager on a precision two-axis gimbal and varying the sensor motion rate from 0.25 to 1 instantaneous field of view per frame. A midwave thermal imager was used to permit short integration times and remove the effects of motion blur. It is shown that the human visual system performs a superresolution reconstruction that mitigates some aliasing and provides a higher (than static imagery) effective resolution. This process appears to be relatively independent of motion velocity. The results suggest that the benefits of superresolution reconstruction techniques as applied to imaging systems with motion may be limited.

Atmospheric modulation transfer function in the infrared.

In high-resolution ultranarrow field-of-view thermal imagers, image quality over relatively long path lengths is typically limited by atmospheric degradation, especially atmospheric blur. We report our results and analyses of infrared images from two sites, Fort A. P. Hill and Aberdeen Proving Ground. The images are influenced by the various atmospheric phenomena: scattering, absorption, and turbulence. A series of experiments with high-resolution equipment in both the 3-5- and 8-13-microm regions at the two locations indicate that, as in the visible, image quality is limited much more by atmosphere than by the instrumentation for ranges even of the order of only a few kilometers. For paths close to the ground, turbulence is more dominant, whereas for paths involving higher average elevation, aerosol modulation transfer function (MTF) is dominant. As wavelength increases, turbulence MTF also increases, thus permitting aerosol MTF to become more dominant. A critical role in aerosol MTF in the thermal infrared is attributed to absorption, which noticeably decreases atmospheric transmission much more than in the visible, thereby reducing high-spatial-frequency aerosol MTF. These measurements indicate that atmospheric MTF should be a basic component in imaging system design and analysis even in the infrared, especially as higher-resolution hardware becomes available.