With the ever increasing threat of improvised explosive devices, both in the military arena and the civilian realm, there is a growing demand for technologies with the ability to detect explosives and their precursors from a safe stand-off. While a wide variety of technologies have been investigated for this application, laser-based spectroscopic techniques designed to detect chemical traces on personnel, vehicles and other objects have garnered a lot of attention. These laser-based techniques include Raman spectroscopy, laser induced breakdown spectroscopy (LIBS), diffuse reflectance spectroscopy (DRS), and photothermal spectroscopy (PTS), among others. Laser-based approaches concentrate a large amount of power on a single target location, which enables reasonable signal-to-noise ratio (SNR) to be obtained despite the 1⁄R2 drop-off in return signal strength (where R represents stand-off distance).

Figure 1. Risley Prism Beam Steering
Regardless of which laser-based spectroscopic approach is used, explosive detection maps provide more useful information than point sampling approaches. Such a capability is achieved by coupling these laser based spectrometers with a scanner.

In the interest of adapting these standoff explosive detection technologies to the widest number of applications and platforms, the ideal scanner would be compact, lightweight, low power and vibration insensitive. Further benefit is achieved with a scanner that can both continuously scan the field of regard to look for potential explosives and then rapidly point to a suspected location and confirm the existence of an explosive using high resolution spectroscopic information. Potential approaches include gimbal type mirrors, galvo scanners, fast steering mirrors, and Risley Prism scanners.

While gimbal scanners are used for a wide variety of scanning applications, their carried axis designs typically result in much larger, heavier designs requiring more power to drive. Non-carried axis systems (such as galvanometer scanners) are challenged when large apertures are required. Fast steering mirrors can provide the necessary response time and aperture but they are generally limited to small fields of view. Oftentimes a better solution to these scanner requirements is the Risley prism scanner, which can achieve all of the requirements in a smaller package requiring less operating power.

Standoff Trace Explosive Detection

Figure 2. Risley Prism Scanner Opto-Mechanical Arrangement
A potential real application involves the evaluation of vehicles entering a facility. Trace explosive levels (10’s of μg/cm2) are typically found in fingerprint residues left on a car door handle. A number of requirements exist for an effective standoff explosive detection system in this application, including the ability to maintain a modest vehicle throughput rate (vehicles per hour) as well as the ability to operate both autonomously and with little cooperation from the entering vehicles. From a scanner perspective this requires a wide field of coverage; a typical car door handle is about 25 cm in length and has a separation between handles of approximately 1.25 m, which results in an angular field of 120 degrees. It also requires good spatial resolution – a typical fingerprint size of 5 cm2 at a 0.5 m standoff results in an angular resolution of better than 5 milliradians. Fast acceleration and scanning capability are also necessary – at a maximum of 5 seconds to scan a vehicle (or 2.5 s per handle with one system for each side of the vehicle), a scan velocity of 20 deg/sec results in a dwell time per fingerprint area on a handle of 25 ms and is supported by an acceleration of 20,000 deg/sec2 (negligible amount of time to point from handle to handle of less than 100 ms). Finally, explosive materials exhibit unique spectral signatures – so-called spectral fingerprints – in the mid-infrared (MIR) region of the spectrum spanning 350 – 4000 cm-1 (approximately 2.5 – 28 μm), which requires optical materials that provide good transmission in this waveband.

Risley Prism Scanner Design for Explosive Detection

As shown in Figure 1, two identical prisms rotating about a common optic axis comprise a Risley prism pair. Maximum deviation occurs when the prisms are in alignment (a) and no net deviation occurs when they are in opposition (c). As a result, any point in the conical field of view can be addressed by an intermediate angle between the pair. Mechanical Arrangement A Risley Prism scanner is realized in practice with the optical-mechanical arrangement shown in Figure 2. Hollow core brushless motors are ideal for providing high torque (acceleration), smooth scanning (electronically controlled commutation), and low operating power since the shaft (i.e. prisms) is thin and close to the axis of rotation with a resulting low moment of inertia. In practice, peak powers of 10’s of watts can be obtained for 25mm and larger clear aperture systems that have full field response times on the order of 100 milliseconds. Duplex bearings minimize axial play and provide high pointing accuracy, which is supported with optical encoder-based position sensors to provide high-resolution angular measurement. For example, 20,000 count encoders are easily obtained in practice and provide submilliradian level pointing resolution.

Figure 3. Sample scan patterns that can be achieved by setting the prism rotation velocities to a constant value, resulting in power savings.
In the MIR, a number of material options exist for the prisms that provide high transmission and include zinc sulfide, zinc selenide, silicon, and germanium. Additionally, the high refractive index of these materials results in a small prism to achieve a large field of regard: a 120 deg full angle field of regard can be achieved with a pair of 7.6 deg wedge angle germanium prisms. Matching the prism pair angles to within an arc-minute is easily achievable with standard optical shop fabrication methods and results in a so-called Nadir error (region about the optical axis that cannot be pointed within) less than a milliradian.

Risley Prism systems can be used in either a steering or scanning configuration, depending on the speed of the spectroscopic technique being utilized. For spectroscopic techniques requiring longer integration times, the RP would typically be used in a straightforward step-stare configuration, tracing out a predetermined pattern. For techniques with shorter integration time requirements, the RP can be used in a scanning configuration. Oftentimes combining scanning and step-stare operation is the ideal approach for a search/confirm operating scenario. Constant prism rotation angles minimize system power requirements while providing flexible scanning patterns. Figure 3 shows the spiral pattern and rosette patterns that can be achieved in a scanning mode of operation: the spiral scan is accomplished by rotating the two prisms in the same direction with a small velocity difference between the prism pair, while the rosette is accomplished by counter-rotating the prisms. Figure 4 shows a Risley Prism assembly that embodies this design. The 50 mm clear aperture system measures 130 mm in diameter by 116 mm long and weighs 2.8 kg. Recently, a standoff DRS trace explosive detection system used a Risley Prism scanner to achieve wide field of coverage in an overall compact package.


Figure 4. Risley Prism Scanner Assembly (Photo: OPTRA, Inc.)
Laser based spectroscopic methods have shown excellent potential for standoff detection of explosive materials. The integration of a scanner with the spectrometer can provide wide field of coverage and extend these traditional point sampling systems into two-dimensional field mapping systems. A number of laser scanning systems exist and include Risley Prisms along with gimbal, galvo, and fast scanning mirrors. Risley Prism scanning systems can be adapted for a wide variety of spectral ranges, field angles and scanning configurations to optimize performance based on the attributes of the selected spectroscopic approach. Regardless of the specific system parameters, the Risley Prism scanner’s inherent combination of large aperture, wide field of regard, pointing accuracy, and fast beam deflection in a compact opto-mechanical assembly that requires low-power to operate, make it uniquely suited for standoff detection applications.

This article was written by Craig Schwarze, Principal Systems Engineer, OPTRA, Inc. (Topsfield, MA). For more information, Click Here .