A novel passive millimeter wave imaging (PMMW) system that utilizes a special arrangement of N equals 36 focal plane elements is presented. N MMW radiometric receivers generate a N X N PMMW image. We discuss the system fabrication and testing.
TRW has developed a passive millimeter wave demonstration camera using its unique millimeter wave monolithic integrated circuit (MMIC) technology. It operates in a 10 GHz band around 89 GHz, has a field of view of 10 degree(s) by 15 degree(s), and can process and display data in real-time at video rates. Its focal plane consists of 1040 MMIC direct detection receivers.
Radio vision systems on the basis of radio optical devices combined with sensor measurements were created. Mathematical modeling of the super-resolution regimes for the systems were carried out for different receiving schemes and varied levels of applied distortion. The main purpose of this report is the comparison of resolution from an 8-mm wave and 3-mm wave radiometric range system.
W-band radiometer have been shown to be effective in detecting metallic and non-metallic weapons concealed under heavy garments in both indoor and outdoor situations. We now have a system providing near real time-time display and weapon detection in a man portable demonstration system. The system consists of 34 W-Band detectors in a 12 inch Cassegrain system with a mechanical scanner. The system also includes a long wave infrared camera, which aids the operator in selecting targets.
A close-range high-resolution radiometric imaging system is described that demonstrates a capability to see through clothing and provide high resolution images of concealed weapons. The radiometer receiver is lens-fed and operates in W-band. It uses a system of linear actuators to rapidly scan the receiver at a range of approximately 1 meter from the subject. The images are of a high quality and are believed to be the best that have been published to date in the open literature. They represent a major step forward in the application of passive millimeter-wave technology to commercial applications such as security screening for concealed weapons detection. The paper describes the design of the experimental imaging system and includes images that demonstrate a state-of-the-art weapons detection technology.
The threat in modern life necessitates the use of security systems in many areas. Systems, whether manual search or automated, need to be able to detect concealed munitions beneath clothing and in baggage. Systems which scan people, unlike baggage, must be safe to avoid damaging those who must be repeatedly scanned. Passive millimeter wave (mmw) systems have the ability to scan people through clothing to detect concealed objects without irradiating the individual. The performance of such systems is dependent on operating frequency, which is a trade-off between resolution, clothing transmission, and material visibility. Transmission and reflection spectra of clothing, skin, and other materials which may be worn under clothing, over the frequency range 60 to 500 GHz, are presented with their implications for operating frequency. The practicalities of imaging are discussed, the differences between the indoor and outdoor situation highlighted, and the limitations of the indoor case described. Imagery of persons with concealed objects, obtained using DERA's MITRE 94 GHz mmw imager, are presented.
As a result of its relatively short wavelength coupled with relatively high penetration of many materials, millimeter- wave imaging provides a powerful tool for the detection of concealed articles. By using a passive approach such as that implemented here, it is possible to image (detect) concealed weapons and articles or look through certain types of walls, all without generating any form of radiation that might raise health concerns. In this paper we show the results of two years of upgrades to our imager that have resulted in true 30 Hz imaging and 5 Kelvin instantaneous thermal sensitivity. High resolution ground based video and image data taken at distances from 12 to over 66 feet together with airborne flight test data will be presented as representing state-of-the-art in passive millimeter-wave imaging.
A novel millimeter-wave imaging technique has been developed for personnel surveillance applications, including the detection of concealed weapons, explosives, drugs, and other contraband material. Millimeter-waves are high-frequency radio waves in the frequency band of 30 - 300 GHz, and pose no health threat to humans at moderate power levels. These waves readily penetrate common clothing materials, and are reflected by the human body and by concealed items. The combined illumination cylindrical imaging concept consists of a vertical, high-resolution, millimeter-wave array of antennas which is scanned in a cylindrical manner about the person under surveillance. Using a computer, the data from this scan is mathematically reconstructed into a series of focused 3D images of the person. After reconstruction, the images are combined into a single high-resolution 3D image of the person under surveillance. This combined image is then rendered using 3D computer graphics techniques. The combined cylindrical illumination is critical as it allows the display of information from all angles. This is necessary because millimeter-waves do not penetrate the body. Ultimately, the images displayed to the operate will be icon-based to protect the privacy of the person being screened. Novel aspects of this technique include the cylindrical scanning concept and the image reconstruction algorithm, which was developed specifically for this imaging system. An engineering prototype based on this cylindrical imaging technique has been fabricated and tested. This work has been sponsored by the Federal Aviation Administration.
This paper discusses the current state-of-the-art in millimeter-wave radiometric imagers being developed for law enforcement use within the United States. The most prevalent application at present is the detection of concealed weapons (guns, knives, etc.) with a secondary application involving the detection of drugs and explosives. Three main topics will be addressed: the phenomenology of concealed weapon detection at millimeter wavelengths, the promise and short comings of first generation radiometric imagers, and the characteristics of second generation systems currently under development.
This paper describes the design and development of a novel reflective lens for use at millimeter and microwave frequencies. This lens uses twist-polarization techniques and effectively simulates a refractive lens of index 3 in a cheap, light-weight structure consisting of two wire grid polarizers and a Faraday rotator. This paper also discusses how a high-sped rotary scanning mechanism can be used to produce a linear scan pattern again using twist-polarization techniques. This linear scanner employs a wire grid polarizer in the form of a roof prism, a Faraday rotator or quarter wave plate and a rotating disc inclined with respect to its axis of rotation. Through multiple reflections the original conical scan is cancelled in one direction while it is doubled in the other direction to form a linear scan pattern.
Antenna-coupled microbolometers have demonstrated an adequate sensitivity for mm-wave detection. The detector operating principle is based on a lithographic antenna, which is terminated by a matched resistive bolometer. The incident RF power is dissipated in the bolometer element, and the resulting temperature rise is measured with low noise electronics. In this paper, we summarize the recent development in microbolometer technology for millimeter-wave imaging. The aim is to develop a 20 element linear array of detectors with an optical NEP < 10 (DOT) 10-12/(root)Hz at room temperature. In order to reach good performance, excellent thermal isolation of the bolometer element is required. This can be achieved either by silicon micromachining techniques or by the use of low thermal conductivity substrate materials. For good optical performance, a careful design of the coupling structures is required. The lithographic antenna candidates best suited for array applications is discussed. Also the requirements for integrated quasioptical elements are presented.
The main limitation to the sensitivity of a radiometer or imager is its equivalent noise temperatures, Te. Placing a low noise amplifier (LNA) at a radiometer's front end can dramatically reduce Te. LNA performance has steadily improved over recent years, and here we report on a W-band LNA with the lowest Te measured at room temperature. Furthermore, we present statistical RF data showing high yield and consistency for future high volume production that is needed for commercial radiometric imaging array applications such as security screening, aircraft landing, and other systems.
The movements of clouds and moist air are responsible for temporal changes in sky radiation temperatures. Detailed here are measurements of the sky radiation temperatures made during periods of cloud movement and light rain. The measurements were made over the frequency bands 26 to 40 GHz (in the ka-band) and 90 to 98 GHz (in the w-band) using a direct detection and heterodyne radiometer. Temporal derivatives and the spectra of the temperature changes are examined. The measurements indicate that the most rapid temperature changes are due to cloud movements. The standard deviations of these temperature changes were measured at 0.030 K/s for the ka-band and 0.072 K/s for the w-band. The spectral analysis indicated that these changes took place at frequencies less than 10 mHz. The higher frequency fluctuations in the sky radiation temperatures were less than 20 mK and less than 30 mK at ka-band and w-band respectively. This level of fluctuation represents an upper limit set by the difficulty of achieving DC coupling over the long measurement periods of approximately 1000 s.
This paper investigates the use of the rotational molecular resonance lines of carbon monoxide for the detection of this gas. Active and passive techniques are investigated, concluding that a passive technique is feasible, but that an active technique is not. The passive technique would use a three channel spectral radiometer tuned on and around the resonance line. Concentrations of 10 parts per million could be detected over a path length of 100 m. Such a system could be incorporated into a passive millimeter wave imager to detect and measure remotely concentrations of carbon monoxide. Using active techniques, it is shown that modulated radiation temperature changes are too small to be measured using radiometry.
A Passive MMW sensor capability of 0.5 Krms, at W-Band, achieved with an integration time of less than 0.1 ms is now considered state-of-the-art. Though some engineering details remain concerning Passive Millimeter Wave (PMMW) configurations best suited for imaging applications, motion picture frame rates for most applications appear to be a near future reality. The efficacy of applying these new capabilities to all-weather sensing may in many cases be determined by the characteristics of the all-weather scenario. The Munitions Directorate of the Air Force Research Laboratory is currently investigating phenomenology associated with the PMMW scenario that will influence the achievable capabilities of PMMW sensors, when used under adverse weather conditions. Model predictions and verification measurements are described along with preliminary results.
Passive millimeter-wave imagery has tremendous potential for imaging in adverse conditions. However, poor resolution and long acquisition times pose serious limitations to this potential. Therefore, an important issue is the optimization of the sampling pattern. Ordinarily, a focal plane sensor array has sensors placed in a rectangular grid pattern at sub-Nyquist density, and the array must be dithered to sample the image plane at the Nyquist density in each dimension. However, the Nyquist density oversamples the image due to the usually circular support of the diffraction-limited image spectrum. We develop an efficient algorithm for optimizing the dithering pattern so that the image can be reconstructed as reliably as possible from a periodic nonuniform set of samples, which can be obtained from a dithered rectangular-grid array. Taking into account the circular frequency support of the image, we sequentially eliminate the least informative array recursively until the minimal number of arrays remain. The resulting algorithm can be used as a tool in exploring the optimal image acquisition strategy.
In this presentation we show how antialiasing can be achieved through image processing. Bandlimited images have their bandwidths extended using non-linear image restoration techniques. In these images the spatial frequencies within the passband are known but not the correct irradiance at nay point in the scene. In antialiasing however converse is true. When the bandwidth is to be increased by a factor of two the irradiance at alternate picture points is known but not the correct spectrum. Nevertheless bandwidth extension and removal of aliasing is possible using these non-linear techniques. They have been applied to real imagery using a recently developed 2D Lorentzian algorithm which has then bee used to further enhance the images.
To quantify the benefits of applying image restoration algorithms to passive millimeter imagery a synthetic scene was generated and restored and the image quality assessed. The synthetic scene consisted of a three bar fan pattern and a large rectangular black area. This pattern was blurred with a Gaussian point spread function, and two-percent noise added to the simulated imagery. The blurred noisy image was then passed through a series of image restoration algorithms.
Methods of mathematical modeling of multiple ray radio vision systems of image formation with antenna pattern distortion compensation in millimeter wavelength range were developed and improved at the Faculty of Physics of the Moscow State University. At present, intensive experimental investigations of multiple-ray radio vision systems on the basis of radio-optical devices with the sensor measurements are being carried out. The main purpose of the report is demonstration of the local-linear method capability of super-resolution on the model pictures with a small signal/noise ratio.
The designers of modern devices of vision prefer a scanning step of receiving system antenna with a smaller size aperture than the main lobe pattern of the Point Spread Function. This report is devoted to the development and application of a local-linear method of additional resolution enhancement for such a receiving system in radio vision and optics.
Real time passive millimeter-wave imaging systems have a wide variety of uses from aircraft navigation and landing in fog to detection of concealed weapons. A useful imaging system for flight platforms requires a large number of pixels and a high frame rate combined with a small antenna volume and a lost cost. We present a millimeter-wave imaging system which uses 32 MMIC low noise amplifiers to display a 60 X 75 pixel image at a 30 Hz frame rate. The system's pupil-plane phased array architecture allows for a relatively thin large aperture antenna. A remotely located processor utilizes microwave guiding circuit boards to perform phase and frequency discrimination on the radiation received by the antenna array.