Terahertz Bi-Directional Reflection Function Measurements for Homeland Security

http://nist.gov/pml/div682/grp00/terahertz-bi-directional.cfm
My Note: I can't determine the date this published, but it's new to me, and I wanted to share it.

Summary:

Far-infrared (or THz, 25 to 300 micron wavelength) femtosecond laser methods are employed to generate high power broadband pulses for far-field imaging applications. We are developing real-time pulsed imaging of objects as well as exploring THz hyperspectral imaging as a means to discover concealed threat materials relevant to Homeland Security. Measurement of the bi-directional reflection function (BRDF) at THz frequencies is being conducted to better understand pulse propagation and distortion effects after reflection from common concealment materials.

Description:

We employ novel ultrafast laser terahertz techniques to generate high power picosecond pulses for reflection imaging applications. A large area (2 cm x 2 cm) biased GaAs photoconductive switch produces THz pulses with sufficient bandwidth (0.3 to 2 THz) for far-field reflection hyperspectral imaging. Reflected pulses are collected by a large area reflection telescope and the THz image is relayed onto a (1x1) cm² ZnTe electro-optic image up-convering crystal. Weak 800 nm gating pulses from the Ti:Sapphire laser system (seeTerahertz Apparatus/Facilities page) are used to extract the image imposed on the detection crystal and relayed onto a cooled CCD detector for imaging.  

In parallel to our THz spectroscopy efforts, we are also developing novel methods for real-time pulsed THz imaging of far-field objects and to perform hyperspectral imaging of inhomogeneous materials, threat agents and other species relevant to Homeland Security needs. This wavelength range is particularly attractive because THz radiation readily transmits through most plastics, cloth, papers and other non-metallic, low-density matierals. Detailed analysis of high power source and THz pulse propagation is required as well as specialized collection optics to optimize image resolution. Spectral characterization of materials in an image scene at several meter reflection distances is being explored.    

BRDF measurements with (top) and without (below) the index card sample taped to the sample mount front surface. The without sample case shows strong evidence of THz pulse reflection from the sample mount. These measurements were taken without the input iris that is normally set at diameter of 1.5 cm (to restrict the beam size) in front of the sample. Evidence for stray optical reflections is also found from responses before zero delay time.

Angular pulsed Terahertz reflection amplitude map for a sample of corduroy cloth with 3.2 mm rib spacing. (a) Power dependent response showing temporal oscillations resulting from individual rib scatter radiating from the cloth front and rear surfaces; (b) Superimposed Fourier Transform amplitude and power reflection map of the raw scattering data exhibiting the spatial equivalent of diffraction and fit to the data exhibiting several orders of diffraction.

In earlier imaging experiments (circa 2003-2008), it was found that near-field imaging (<1 meter) of reflective objects retained expected resolution at THz frequencies. However, objects placed at ca. 3 meters from the generator produced distorted images, probably arising from pulse interference or depolarization effects. To explore these possibilities, DHS is currently funding a collaborative effort to directly measure and model GHz through THz BRDF of a variety of concealment materials. Since the BRDF has not previously been measured in this wavelength region, our new results will be compared to conventional BRDF models to test their applicability and accuracy for predicting image quality for hyperspectral and related applications. 

Bi-directional Reflection Function apparatus CAD design (top) and its implementation attached to pulsed THz imaging apparatus (bottom). The stacked rotation stages permit manual selection of the incident angle of the pulsed THz output beam and angularly synchronized collection mirrors allow propagation of horizontal or vertically polarized radiation to be collected and processed by the imager.

These investigations use a state-of-the-art, KHz rep-rate amplified 45 femtosecond pulsed laser for broadband (0.2-2.5) THz GaAs antenna generation and detection using a ZnTe electro-optic crystal. See the Terahertz Apparatus/Facilities page.

Selected Publications:  

• M. B. Campbell and E. J. Heilweil, “Non-invasive detection of weapons of mass destruction using THz radiation,” in Proceedings of SPIE Vol. 5070 Terahertz for Military and Security Applications, edited by R. Jennifer Hwu, Dwight L. Woolard, (SPIE, Bellingham, WA, July, 2003), page 38.
• Shu-Zee A. Lo, D. Novotny, E. N. Grossman, and E. J. Heilweil, “Pulsed terahertz bi-directional reflection distribution function (BRDF) measurements of materials and obscurants,” in S.P.I.E Proceedings for Session #8022 entitled "Passive Millimeter-Wave Imaging Technology XIV," Orlando FL, April 25-29, 2011.

Detection Video imaging system for remote detection of hidden threats

http://www.homelandsecuritynewswire.com/dr20131029-video-imaging-system-for-remote-detection-of-hidden-threats

By adapting superconducting technology used in advanced telescope cameras, researchers have built a prototype video imaging system for detecting hidden weapons and other threats at distances up to twenty-eight meters away.

By adapting superconducting technology used in advanced telescope cameras, researchers at the National Institute of Standards and Technology(NIST) have built a prototype video imaging system for detecting hidden weapons and other threats at distances up to twenty-eight meters away.

With further development, the new NIST imager might eventually be used for applications such as detecting suicide bomb vests under clothing at military checkpoints or identifying threats at sea such as ships hidden in fog.

A NIST release reports that NIST’s prototype imager, unveiled at the SPIERemote Sensing Conference last month, has three key features, which may offer advantages over other screening systems such as those used for airport security.

First, the NIST imager has NIST-developed software to weave together high-resolution still images to make video in near real time. Second, the system operates from a longer “standoff” distance, sixteen to twenty-eight meters away from a target, than other similar imaging systems. These combined features make the NIST instrument unique.

Third, unlike screening systems that bombard a target with X-rays or other types of radiation, the NIST imager is passive, which means it merely detects naturally emitted or reflected light. Similar to an infrared camera, the NIST instrument scans a target to detect emissions of terahertz (also called sub-millimeter) light and then generates images based on variations in temperature and reflected light among different target materials, such as biological tissues versus metals or ceramics.

The long-distance operation is made possible by very sensitive detectors —NIST’s transition-edge sensors. These tiny but sensitive thermometers are made of a superconducting metal, which changes resistance in response to very faint light. The system currently has 251 sensors, and the resulting images show detail as small as 1 centimeter in size across a target area about 80 by 60 centimeters. The system produces less detailed images at the farther end of its operating range.

The technology is similar to the NIST camera in the SCUBA-2 telescope in Hawaii (see NIST Tech Beat). Both instruments detect emissions at a wavelength of 850 micrometers, a wavelength of light that passes through both clothing and the atmosphere. Both sets of sensor arrays are packaged with superconducting amplifiers to boost signal strength, using a NISTtechnique that makes large arrays practical. But the new imager’s sensors are made of a different metal, aluminum, which superconducts at higher temperatures of 1.2 Kelvin (minus 272 degrees C). This allows the sensor array to be cooled by a more compact, custom refrigerator.

The NIST imager’s video frame rate is currently six frames per second, which is relatively slow (standard movie format is twenty-four frames per second). NIST researchers say, however, that their system can be developed further to have four times as many sensors (for a total of 1,004), which would produce larger, near-full-body images at faster video frame rates.

System components include two mirrors and a lens that focus light on the sensors, a low-temperature refrigerator containing a wafer of sensors and amplifiers, electronics to combine all the signals, and software that stiches the images together into video. All components were designed and built byNIST except part of the refrigerator, built at the University of Pennsylvania, and the electronics, provided by the University of British Columbia.

— Read more in D.T. Becker et al., “Passive video imaging at 350 GHz with 251 transition edge sensor bolometers (talk and video presented at SPIERemote Sensing Conference, Dresden, Germany, 24 September 2013); and “Powerful NIST Detectors on Hawaiian Telescope to Probe Origins of Stars, Planets and Galaxies,” NIST Tech Beat (6 December 2011)

NIST unveils prototype video imaging system for remote detection of hidden threats

 

 

                       Researcher Dan Becker demonstrates NIST's prototype imaging system for remote
                                    detection of hidden weapons. The inset on the left is the image created by the instrument,
                                    which detects naturally emitted or reflected light at sub-millimeter wavelengths.
                                    The ceramic knife is cooler and reflects cooler light from the room, and therefore appears
                                    darker than the surrounding image. Credit: Becker/NIST

 
http://phys.org/news/2013-10-nist-unveils-prototype-video-imaging.html#jCp
By adapting superconducting technology used in advanced telescope cameras, researchers at the National Institute of Standards and Technology (NIST) have built a prototype video imaging system for detecting hidden weapons and other threats at distances up to 28 meters away.

 With further development, the new NIST imager might eventually be used for applications such as detecting suicide bomb vests under clothing at military checkpoints or identifying threats at sea such as ships hidden in fog.

NIST's prototype imager, unveiled at a conference last week, has three key features, which may offer advantages over other screening systems such as those used for airport security.

First, the NIST imager has NIST-developed software to weave together high-resolution still images to make video in near real time. Second, the system operates from a longer "standoff" distance, 16 to 28 meters away from a target, than other similar imaging systems. These combined features make the NIST instrument unique.

Third, unlike screening systems that bombard a target with X-rays or other types of radiation, the NIST imager is passive, which means it merely detects naturally emitted or reflected light. Similar to an infrared camera, the NIST instrument scans a target to detect emissions of terahertz (also called sub-millimeter) light and then generates images based on variations in temperature and reflected light among different target materials, such as biological tissues versus metals or ceramics.

The long-distance operation is made possible by very sensitive detectors—NIST's transition-edge sensors. These tiny but sensitive thermometers are made of a superconducting metal, which changes resistance in response to very faint light. The system currently has 251 sensors, and the resulting images show detail as small as 1 centimeter in size across a target area about 80 by 60 centimeters. The system produces less detailed images at the farther end of its operating range.

The technology is similar to the NIST camera in the SCUBA-2 telescope in Hawaii. Both instruments detect emissions at a wavelength of 850 micrometers, a wavelength of light that passes through both clothing and the atmosphere. Both sets of sensor arrays are packaged with superconducting amplifiers to boost signal strength, using a NIST technique that makes large arrays practical. But the new imager's sensors are made of a different metal, aluminum, which superconducts at higher temperatures of 1.2 Kelvin (minus 272 degrees C). This allows the sensor array to be cooled by a more compact, custom refrigerator

 

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