tn muzzle flash detection optro 2012

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Fast uncooled MWIR imagers applied toast uncooled MWIR imagers applied toMan Portable Weapons’ muzzle fl ash Man Portable Weapons’ muzzle fl ash

detection and explosion characterizationdetection and explosion characterizationToday, the countermeasure of man portable weapons (aka MPW) is still a complex task with important but unsolved challenges. There is general consensus that future MPW countermeasure (CM) systems will be based on the combination and fusion of data from independent but complementary techniques. However, the phenomenology associated with the

events produced during MPW shots turns the development of uncooled MWIR imagers in a key research activity.(Paper published at OPTRO 2012 - 5th International Symposium on Optronics in Defense and Security, Paris, 8 - 10 February 2012)

1.1. Introduction

The technological gap existing among modern democracies and their opponents increases quickly. As a consequence, the opponents adopt asymmetric strategies to balance their evident disadvantage derived from their poorer technological development. Most of the times, this asymmetry lies on radical differences with the established political order, disregarding the principles of stability, proportionality, rule of law, democracy and life.

As a result, traditional warfare has evolved towards new and reduced plausible scenarios where opponents take advantages of existing and accessible technologies, acting in opaque circumstances, concealing themselves among civil population, under tightly constraining rule of engagement and media scrutiny.

Present and future military missions will be carried out in complex and demanding scenarios. It will be necessary to prevent and avoid terrorist attacks, fi ght against sophisticated guerrilla warfare, protect expeditionary forces and participate in frequent urban operations.

Man Portable Weapons (MPW) have become key instruments in asymmetric strategies because of their wide proliferation, poor control, easy concealment and transport, and can be lethal to a extend range of uses. Today, the development of technologies focused to countermeasure MPW is considered a priority for the most modern military and security forces in the world.

Taking into account the volume of MPW units deployed in the world, there are three main families which have special interest to be detected and countermeasured:

• Small/Medium Caliber guns - Urban warfare poses a different set of challenges most of them related with the use of small/medium caliber guns. Sniper detection and localization is one of the most critical tasks to be performed in modern confl icts where complex and fully cluttered scenarios bring shot detection to the limits, in terms of diffi culty and reliability.

• Rocket Propelled Grenades (RPGs) and Explosive Formed Projectiles (EFPs) - RPGs are considered the most important threat for terrestrial vehicles and helicopters. Currently there are a hundred million units produced and widely proliferated, available at the black market for less than 1 k€. These weapons have infl icted more than 50% of casualties in recent confl icts.

• Man Portable Air Defense Systems (MANPADS) - They are considered the most important threat for commercial and military aircrafts. At the present time, more than one million units have been produced and proliferated and it is possible to fi nd on the black market for less than 12 k€. Since 1970 MANPADS have successfully attacked at least 45 civil aircraft with almost 1000 casualties. They show killing probabilities above 60%.

In all the cases, the existing solutions for detection of these types of threats either do not cover the operational requirements in a complete way, or their elevated cost prevent them from an extensive use. Detecting MPW is still an unsolved challenge waiting for better solutions and, therefore, it is today a research fi eld plenty of activities and initiatives.

Developing technologies focused to solve the problems related with the detection of MPW is considered a priority for the most advanced nations. Systems and sensors are key elements to be improved. In this work, the advances achieved in the development of a specifi c uncooled MWIR imager, which characteristics and performance fi t with the MPW detection application requirements, will be presented.

1.2. Sensors. Uncooled MWIR imagers.

Sensors are one of the key components of a MPW countermeasure system. Their quality and performances dictate the system effi ciency and reliability. At the present time, most mature technologies used for detection of MPW are based on acoustic and radiation sensors. However, all of them have performance constraints and show limitations to achieve a reduction of the false alarm rate, an increment of the range of detection as poor


Technical note - Fast uncooled MWIR imagers applied to Man Portable Weapons’ muzzle fl ash detectionand explosion characterization

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immunity to be counter measured.

Today it is widely accepted that the optimum solution to the MPW detection problem will be a combination of different detecting techniques and sensors, where both, acoustic and light detection sensors, will play a prominent role. Fig. 1 shows a chart with data coming from an IR sensor and an acoustic sensor in a shot detection experiment. The difference in the time of arrival between the infrared and the acoustic signatures is used for range determination. In addition to the ranging function, acoustic detections can be used to reject false muzzle fl ash detections caused by phenomena such as sun glints which may affect the optical sensor.

Photon MPW sensors fi elded so far have typically operated in the UV ‘solar blind’ region of spectrum. However, the detectors available in this spectral region are expensive, show problems of high false alarm rates, low angular resolution and short detection range due to attenuation of radiation. IR detectors have demonstrated to be excellent alternative to UV detectors. In particular, numerous studies have demonstrated that sensors working in the MWIR spectral region (1-5 microns) are an excellent option for the detection of MPW. Their advantages reside on the fact that MPW shooting is usually accompanied by the emission of an intense radiation in this particular region of the IR spectrum, which is higher and different emission than the radiation coming from clutter, resulting in high SNRs and, as a consequence, high probabilities of detection and low false alarm rates.

Compared with acoustic detection, the MWIR detection approach provides more accurate location determination and can unambiguously handle multiple shots. However, an IR detection system integrated with an acoustic detection has demonstrated a robust way of rejecting spurious glint events for operations in a high clutter environment. Additionally, for single-shot events, a combination IR-acoustics would provide passive ranging calculation using time difference between the fl ash and the sound.

High SNRs is not the unique parameter to be taken into account for detecting MPW. Good angular resolution and fast responses are also key parameters for fulfi lling operative requirements. The existing MWIR imagers are able to provide very good fi gures in those aspects; however, most of them still fail in key factors such as cost and robustness, principally due to the need of cooling; as a consequence, these systems are expensive, fragile and costly of maintaining.

MPW detection requires further development of uncooled and affordable photonic MWIR imagers. Section 2 of this article will be focused on the challenges associated with the detection of muzzle fl ashes produced during small weapons shooting (assault rifl es) whereas in section 3 the IR time resolved spectroscopy applied to characterize high energetic events, such as explosions, will be studied. In section 4 of this article, the fi rst camera based on a low density VPD PbSe FPA showing uncooled performance and sensitive to MWIR radiation will be presented. The sensor is able to work at room temperature without any type of stabilization and capable to provide more than 1700 fps, and its characteristics fi t with most of the requirements demanded by the application, making it a candidate technology for the development of MPW CM systems. Section 5 will show some experimental results obtained during a trial campaign using the MATRIX 1024 CAMERA sensor to prove the performance of the system for the MPW CM systems operational requirements.

2. Small guns’ muzzle fl ash detection using MWIR sensors

Signifi cant challenges involved in the development of real-time, automated, MWIR detection, location and identifi cation of hostile gunfi re include:

• Detecting a short-duration event (few ms)• Rejecting natural and man-made false alarms• Wide-area real-time coverage

Approximately, an amount of 30% of the chemical energy released from the propellant used in a conventional gun is converted into kinetic energy of the projectile. The remaining energy is mainly contained in the propellant gasparticle mixture which escapes from the muzzle of the gun in a few milliseconds. This sudden discharge produces a blast wave because of the rapid displacement of the air surrounding the gun. In addition to this, these gases are generally fuel-rich and they mix turbulently with air from the surroundings. Combustion of this mixture causes gun muzzle fl ash, usually associated with the formation of a secondary blast wave. The size and power of both the blast and the fl ash are dependent upon the caliber of the gun, its performance and the charge design.

The temperature of the gases and particles leaving the muzzle has been measured to be in the range from 1000 K to 1500

Figure 1. Muzzle fl ash experiment. Range calculation using the information from an IR sensor and an acoustic sensor



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K. According to Plank’s Law, materials at these temperatures have the majority of their energy in the mid-wave infrared (MWIR, 1-5 μm) part of the spectrum. Also, objects at these temperatures emit considerably more energy than most of the scenario materials, which are closer to ambient temperature, i.e. 300 K. Assuming for these hot gases and particles an arbitrary small emissivity of 0.05-0.1, it is possible to estimate the fl ash radiance of 0.04-0.25 watts/cm2/sr in the MWIR spectrum for these small weapons. Muzzle fl ash size for an assault rifl e is 0.7 m length and 0.3 m width [4]. This area can be represented by an ellipsoid with a volume equal to 0.04 m3. This results in a radiant intensity of 24 to 60 watts/sr (assuming a temperature of 1000 K and an emissivity of 0.05).

On the other hand, glints in the MWIR spectrum could have radiance above 20 watt/cm²/sr and can be modeled as a circle with a small radius likely to be unresolved at the sensor. Although glint is less intense than a fl ash, it is still a bright source and can cause false alarms. However, the temporal evolution of glints and muzzle fl ash intensities on the sensor might be different. That is the reason why time resolved spectroscopy becomes a powerful tool for the identifi cation of both events and, therefore, the reduction of false alarm rates. As a consequence, systems with high frame rates that allow a fast temporal sampling are required.

As a summary, two key characteristics of the MPW muzzle fl ash are short persistence in time (only a few milliseconds) and radiation in the MWIR region (1 – 5 microns). Consequently, the sensors aimed to the detection of the MPW muzzle fl ash must operate with 100% stare effi ciency (duty factor), and have detection in this band of the infrared. Other considerations for the operational requirements include adaptability for spectral and temporal fi ltering to reduce false targets, as the most effective ways to improve detection and discrimination performance are the increase of the sensor bandwidth via improvement of the spectral discrimination, the frame rate and the spatial resolution.

3. Time resolved spectroscopy for explosion characteriza-tion

Fast imagers are also useful tools to detect and discriminate the type of weapon and explosives used. The detonation of a high explosive results in a luminous fi reball. Infrared spectral emissions may reveal key explosive characteristics since most of the absorption and emission features are associated with the presence of molecular gaseous species produced during explosion.

Knowledge of the temporal behavior of these byproducts produced during the detonation may be used to infer the type of material used —for example, the quantity of H2O, CO2 and other gases liberated upon detonation will depend on the type of hydrocarbon explosive material used. The temporal evolution of these by-products will be driven by the kinetic pathways

governing the oxidative decomposition of the starting material, and their temporal signature may provide additional clues about the starting material.

Visible and MWIR spectra of radiation coming from high explosive fi reball can be used to determine some facts, as for example if the explosive was metalized, by the presence or absence of characteristic metal oxide emissions in this spectral region. Explosive size might be best estimated using imagery, and may be inferred from other measured characteristics of the fi reball.

Fast MWIR imaging devices can be also useful tools to study variations in the emissivity of fi reball during explosion. Such a distinction is important for an improved phenomena understanding, as temporal changes in emissivity are connected to chemical processes (e.g., soot oxidation) whereas area changes are linked to the underlying fl ow fi elds.

On the other hand, the temporal evolution of the radiation intensity from fi reball gives information of the amount of explosive used [4]. The rate of decay is highly dependent on the high explosive weight. Larger amounts of explosives produce fi reballs that took much longer to cool down than those smaller in a way that time resolved spectroscopy can be used for stand-off detection and discrimination of different types of explosives and weapons in an easy and feasible way.

4. The VPD PbSe and the MATRIX 1024 technology

As exposed in the previous sections, there is currently a need of fast and affordable MWIR imagers for its use in MPW detection systems. The existing imagers deployed are based on cooled IR detectors (InSb, CMT etc.), which provide excellent performance but are expensive and poor reliable in terms of robustness, power consumption and maintenance costs. On the other hand, emerging technologies for MWIR uncooled detectors such as HOT CMT or Type II supper lattices are still immature and far for being considered volume and affordable technologies for imagers.

Until now, low cost uncooled MWIR detection has been traditionally dominated by lead salts (mainly PbSe). However, it has been traditionally relegated to single-element detectors and small linear arrays due to the technological limitations imposed by its standard manufacturing process, which is based on a Chemical Bath Deposition technique (CBD) and was developed more than 40 years ago.

Recently, an innovative method for processing PbSe detectors, based on a Vapor Phase Deposition (VPD) technique, has allowed the manufacturing of 2D arrays (FPA) of polycrystalline PbSe with good electro-optical characteristics. This new processing method is an all silicon technology and it is compatible with standard CMOS circuitry. VPD PbSe becomes then a perfect



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candidate to fi ll the gap of uncooled IR imaging sensors, MWIR sensitive and fast photodetection. The perspectives and potential applications are important and multiple, converting the VPD PbSe devices into a serious alternative to other uncooled technologies in the IR detection market. The most remarkable advantages of VPD PbSe manufacturing technology are:

• Good reproducibility and uniformity in big areas• Long term stability• Fully compatible with plain (no textured) Si substrates• Affordable, simple and mature technology• Low demanding packaging (not vacuum)• Compatible with CMOS• Produces detectors with uncooled performance at room

temperature, photodetection and MWIR sensitive• High detectivity at room temperature: D*λpk (500 K, 300

Hz, 1.2 Hz) = 3x109 cm·Hz1/2·W-1• Response time of a few μs

The VPD PbSe manufacturing technology has overcome some of the most important inconvenience associated to the classic Chemical Bath Deposition (CBD) method of processing PbSe. Based in this technology, NIT has brought into the market the fi rst sensor based on VPD PbSe FPA, which are self-manufactured by the company. This is a 32x32 camera, known as MATRIX 1024 CAMERA, which characteristics are listed in the Table 1.

Based on the FPA described above, NIT has designed and manufactured an innovative imager. The camera was specifi cally developed to take advantage of the detector’s short time constant and can provide frame rates as high as 1700 fps @ full frame resolution. This sensor is unique because of the combination of MWIR sensitivity, imaging FPA, affordability (uncooled) and high velocity of response, all of them requirements requested by MPW detection applications. Fig. 2 shows a picture of the MATRIX 1024 CAMERA, which is the system used during trials and which results are presented in section 5 of this work. The main characteristics of the sensor are shown in table 2.

5. Test results

In this section some experimental results obtained with the sensor described before are presented. Section 5.1 describes the results obtained during detection trials of muzzle fl ashes coming from a rifl e caliber 30-06 (equivalent to 7.62 mm). They are very promising and in accordance with other results obtained during a detection test trial campaign with real AK47 assault rifl es carried out by the Military University of Technology, aprestigious military institution in Europe, with the same imager [5]. Section 5.2 describes the measurements and experimental results of small explosions using time resolved spectroscopy technique.

5.1. Muzzle fl ash detection results

The aim of the test campaign was to demonstrate the capability of the MATRIX 1024 CAMERA to detect muzzle fl ashes at different ranges, overcoming the challenges of uncooled detection and fast sampling rate.

The trial was carried out in Madrid (650 m height) using a hunting rifl e, caliber 30-06 (7.62 mm). Meteorological conditions during the test were completely unfavorable for the IR transmission: cold (10 C), cloudy and drizzling (RH>95 %). The camera was confi gured to use an integration time of 15 μs,

Feature Value FPA dimensions 24 x 24 mm

Detector material VPD PbSe Format 32 x 32 Pixel size 100 x 100 μm2

Readout method Rolling frame Time constant < 3 μsec Spectral response MWIR (1 – 5 μm) Peak wavelength 3.7 μm Avg. D*pk(500K,1.2 Hz,330 Hz) 3 x 109 Jones

Table 1. Characteristics of the VPD PbSe FPA used as detector in the uncooled MWIR imager

Features Value Dimensions (mm) 120x120x150 Weight (kg) 1.5 Thermal sensitivity <150 mK Frame rate >1700 fps Start-up time 20 sec Integration time Selectable (> 7.36μs) Data output Raw data, I16 format Data communications Ethernet 10/100 Optics FoV 7.6º / FoV 3.8º

Table 2. Main characteristics of the uncooled MWIR imager MATRIX 1024 CAMERA

Figure 2. The MATRIX 1024 CAMERA sensor. Based in a 32x32 VPD PbSe FPA, it is able to provide more than 1700 fps in uncooled operation.



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which provided a frame rate of 1050 Hz (the frame-to-frame time was smaller than 1 ms), and the distances between the shooter and the camera ranged between 50 and 120 m.

4 rounds of shooting with a total of 10 shots were made: 5 at 50 m range, 2 at 75 m range and 3 at 100 m range. Performing threshold analysis, the muzzle fl ashes of all the shots were detected. Fig. 3 shows the temporal sequence of a muzzle fl ash associated with a shot recorded at 50 m range during trials. It is possible to observe the duration of the muzzle fl ash for this specifi c rifl e, which is estimated to be in the order of 1.5 – 2 ms. The same duration has been observed in most of the shots, with the recording of 1 or 2 frames for each shot.

Flash duration is very dependent on the weapon and on the munitions used. This fact converts the camera in an excellent instrument for detecting and classifying weapons by transient time spectroscopy. In addition to the detection event, the FPA provides spatial information that can be used for localization purposes (azimuth and elevation information).

A more extended and detailed study (calculations and fi eld tests) using real assault rifl es, AK 47 and SVD Dragunov (both caliber 7.62 mm) has been performed by the Institute of Electrooptics at the Military University of Technology [5], demonstrating that the MATRIX 1024 CAMERA is able to detect shots from these weapons to distances as far as 240 meters using integration times as short as 7.36 microseconds and frame rates over 1600 Hz.

Table 3 shows calculated detection ranges following the Johnson criteria on the basis of geometrical relations between standard muzzle fl ash sizes, distance to camera, array size and lens parameters, according to the simulation calculated by the group mentioned before.

The results obtained in our experiment correlate with the simulation results, showing both theoretically and practically that the VPD PbSe based camera is a valid system for this application. In fact, the maximum detection range is basically limited for the FPA resolution (32x32).

In a future, bigger FPA formats will allow extended detection and identifi cation ranges, with performances fully compatible with the highest operational requirements.

5.2. Explosion characterization

In section 3, the need of developing fast MWIR imagers for energetic events characterization was described. Many parameters related with the dynamics of an explosion, such as temperature and pressure distributions, gaseous species, etc., are tightly linked to the temporal evolution of the radiation emission during defl agration. In this sense, fast MWIR imagers become extremely useful tools not only for detection but for discrimination of explosive events because of their capacity for carrying out time resolve spectroscopy measurements.

With the objective of testing the performance of the camera for this particular application, two experiments were carried out with the MATRIX 1024 CAMERA, aimed to show the capability of the system for time resolved spectroscopy applications. In both cases the camera was set to allow a frame rate of 1300 Hz using an integration time of 14 μs.

In the fi rst test, the ability to discriminate complex events (multiple explosions separated by short periods of time) was tested. Figure 4 shows the temporal IR signal intensity measured at 50 m of distance corresponding to the explosion of several small pyrotechnic artifacts (explosive charge per device: ~1 g) using a FoV of 7.6º. The total duration of the event was 200 ms and the camera was able to detect the sequence of eight explosions with different intensities and duration.

Figure 3. Sequence of images recorded during the 30-06 caliber rifl e shot trials. Results for the 50 m threshold analysis

Table 3. Calculated distances resulted of applying the Johnson criteria to the MATRIX 1024 CAMERA characteristics.

Figure 4. IR intensity temporal results during the explosion of multiple small charges separated in short periods of time. Total event duration was 200 ms.



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In the same way, Fig.5 shows a sequence of images corresponding to an explosion of 2 devices, 2 g of explosive charge each, happening in a shorter period of time than before. After the analysis, the time gap between the events was determined to be less than 10 ms (see Fig. 6).

The camera was placed at a distance of 65 m, which correlates with the time delay with the signal recorded with an acoustic sensor in the same position (data shown in Fig.1 corresponds also with this experiment).

On the other hand, it was pointed out in section 3 that the temporal evolution of the radiation intensity from fi reball provides information of the amount of explosive used. To prove the capability of the sensor for this application, two measurements of explosions with two different amounts of explosives, 1.5 g and 2 g, have been taken and analyzed. The results are shown in Fig. 7, where it is possible to observe the difference in the time of decay depending on the amount of explosives used. Even though the small charge of explosive of the devices used, it is possible to observe clear differences in the decay of the IR intensity. This shows the sensitivity of the IR time resolved spectroscopy technique for this particular application. The images were recorded with a frame rate of

1300 Hz.

Fig. 8 shows the results of the observation of the complete 2 g device explosion. 70% of the radiation is emitted in the fi rst 3 ms.

The dynamic of explosive events is a source of important information about the type of explosive and weapons used. The results shown above demonstrate the potential of the MATRIX 1024 technology to be a useful tool for detecting, discriminating and characterizing high energetic events such as explosions.

6. Conclusions

Man portable weapons represent a serious threat for civil and military security. Countermeasure systems and technologies are still immature and/or present low effectiveness/cost ratios. There is a real need of developing new devices and technologies able to provide good performance at an affordable cost.

Figure 7. IR signal decay corresponding to two different types of explosive device according the explosive charge: 1.5 g (triangles) and 2 g (squares).

Figure 8. IR signal vs time curve corresponding to the defl agration of 2 g explosives

Figure 5. Temporal sequence of frames recording the explosion of two explosive charges happening in a short period of time (frame rate is 1300 Hz; samplingtime between frames is ~0.77 ms)

Figure 6. Temporal intensity of the 2 explosion events. Time gap between the events is approximately 7 ms



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It is widely accepted that uncooled MWIR sensors will be a key component of the future MPW CM Systems. The existing technologies are still very immature or expensive except for the polycrystalline PbSe. Its standard manufacturing technology (CBD method) presents limitations for processing image FPAs. However, a new processing method based on a vapor phase deposition (VPD) of PbSe outcomes most of the important limitations and makes possible to process imaging FPAs monolithically integrated with CMOS readout electronics.

Based on a small format VPD PbSe FPA (32x32), New Infrared Technologies has developed the fi rst uncooled MWIR camera of the market, known as MATRIX 1024 CAMERA. The sensor has outstanding performances in terms of velocity of response and sensitivity, and accomplishes all the requirements for being considered a good candidate to fi ll the existing gap in uncooled MWIR sensors domain.

Several tests have been conducted in order to show the performance of the MATRIX 1024 technology concerning the main requirements related to MPW CM: MWIR detection, fast sampling rates and uncooled performance.

• Muzzle Flash detection. The camera was able to detect shots from assault (AK 47 and SVD Dragunov) and hunting rifl es (caliber 30- 06) at ranges as far as 250 m with frame rates over 1000 Hz in real uncooled operation using a MATRIX 1024 FPA (low density, 32x32 FPA). In addition to detection, the imaging FPA provides spatial information which can be used for localization of the shot and to study spatial signatures. Larger FPAs will provide better performances.

• Explosion characterization. The camera was able to record sequences of small explosions with frame rates at 1700 Hz. The measurements show the utility of the technique for characterizing high energetic events obtaining time resolved spectroscopy curves, and detecting events occurring in a time gap as short as 7 ms. The time resolution allows the study of the temporal signature and the identifi cation of different types of explosive charges.

7. References

1. ‘VPD PbSe technology fi lls the existing gap in uncooled, low cost and fast IR imagers’. Proc. SPIE 8012-146 (2011)

2. ’VPD PbSe technology: the road towards the industry maturity’. Proc. SPIE 7660-92 (2010)

3. ‘Polycrystalline lead selenide: the resurgence of an old IR detector’. Optoelect. Rew. 15 (2) (2007) 110-117

4. ‘Phenomenological model for Infrared Emissions from high explosive detonation fi reballs’. Report from Air Force Institute of Technology (2007)

5. ‘Fast uncooled module 32x32 array of polycrystalline

PbSe used for muzzle fl ash detection’. Proc. SPIE Vol 8019 (2005)

tn muzzle flash detection optro 2012

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