High Speed Imaging for Military Applications

Transcription

1 High Speed Imaging for Military Applications By Andrew Bridges, Photron High-speed imaging of military testing can be broken down into several key applications, including Ballistics, Survivability, Weapons Launch, and Small Arms and Large Caliber. Each of these tend to involve quite different imaging requirements that will be touched upon here. BALLISTICS Ballistics is basically the study of projectiles in flight, whether launched by a handgun, heavy artillery piece, or from a gas gun. The challenge is in recording and understanding the dynamics of the projectile in free space. Today s highspeed video cameras, such as the Photron FASTCAM SA-Z, can record with usable resolutions (128 pixels square and larger) at speeds in excess of 360,000 frames per second (fps). But the frame rate is only half the battle when you re imaging a projectile moving as fast as 8,500 meters per second (or 28,000 feet per second) as achieved by modern gas guns. By being able to reduce the shutter time (the time every pixel can collect light during each frame) we can further reduce the blur. The Photron FASTCAM SA-X2 can shutter down as fast as 293 nanoseconds. Of course, it is critical that ALL pixels be exposed at exactly the same time (via the camera s global electronic shutter); if not, you will see some pretty funky results such as the drooping propeller blades where one line of the image is exposed after another (see Figure 1). Figure 1 Aircraft propeller blades droop when recorded with a camera utilizing a rolling shutter where pixels are exposed one line at a time, instead of all at once as in a high-speed video camera with a global electronic shutter.

2 If the entire trajectory of a projectile is of interest, then mounting the high-speed camera to a device known as a Trajectory Tracker or Flight Follower will bring excellent results. These devices have a programmable mirror, positioned directly in front of the camera, which moves/rotates at a speed corresponding to the projectile s known velocity, possibly updated through optical sky screens or other optical triggering devices. Other areas where high-speed cameras see a good deal of use in ballistics include ballistic gel which is used in determining the damage done by projectiles. Flesh is simulated through the use of blocks of gelatin specifically formulated to closely replicate the density of pig muscle and tissue (see Figure 2). Additionally, impact studies, whether penetration, such as armor piercing, or resistive, such as armored clothing, are also important areas of study that benefit from the use of high-speed imaging technology. Figure mm caliber bullet into gelatin, recorded with the Photron FASTCAM SA1 at 27,000 frames per second with a 4µs shutter. SURVIVABILITY Much time and effort is invested in ensuring the survivability of personnel, equipment, and weapons systems. As soon as the latest armor-piercing projectile is brought to the battlefield, engineers develop systems to prevent their successful deployment. High-speed cameras are used in a variety of applications such as arena tests, where witness panels are positioned around the explosive event; the cameras record the pattern in which shrapnel penetrates the panels (see Figure 3). Reactive armor is another ongoing field of research. Here explosives are positioned on the outside of an armored vehicle, where they explode outwards to dissipate the force of incoming projectiles. The prevalence of IEDs deployed against our forces has resulted in huge efforts being made, with significant results, to improve the survivability of armored vehicles and, more importantly, their occupants. High-speed imaging technology plays a critical role in the assessment of survivability in these instances.

3 Figure 3 IED test showing the exterior explosion, recorded with the Photron FASTCAM SA-X2 at 5,000 fps, and two interior shots, simultaneously recorded with the high-g Photron FASTCAM MH4 at 2,000 fps. WEAPONS LAUNCH Whether it is small arms or high- velocity, large caliber rounds, the way in which the projectile leaves the barrel is always of interest and high-speed imaging is a critical component in studying this phenomenon. Particular attention is given to the projectile s roll, pitch, and yaw, or the way the sabot (a sleeve-like device used to precisely position the projectile within the larger diameter barrel) separates upon firing (see Figure 4). Figure 4 Sabot separation trial, recorded with Photron s FASTCAM SA5, recording at 3,000 fps, mounted in Specialised Imaging s Trajectory Tracker.

4 SMALL ARMS / LARGE CALIBER Lastly, firearms, whether personal or large caliber, receive a good deal of attention when studying specific areas of interest, such as muzzle flash suppression, cartridge loading and ejection, and even safety, in the event the firearms are dropped. All of these events require different high-speed imaging frame rates, ranging from one or two thousand frames per second for cartridge ejection or drop testing, to as fast as you can go ranges of hundreds of thousands to even two million fps. When recording the bullet leaving the muzzle, for example, the highest speed camera available will give the best results. For this, Photron s FASTCAM SA-Z (see Figure 5) is ideal because it provides ultrahigh-speed imaging at megapixel resolution at 21,000 frames per second, with reduced resolution to over 2 million fps. Regardless of the high-speed imaging requirement, Photron has a wide range of high-speed cameras (including the world s fastest) to meet a variety of framing rates, resolutions, light sensitivities, and record-durations requirements. Figure 5 Photron s FASTCAM SA-Z, the world s fastest high speed camera, provides up to 2 million frames per second with superior light sensitivity, excellent image quality and clarity. The enhanced global shutter operates from 1 millisecond down to 159 nanoseconds, freezing the fastest motion, with no blur. About Photron: Photron is a global leader in developing high-speed CMOS sensors to enable the fastest action to be recorded and replayed in detailed slow motion. This sophisticated technology allows the viewer, both engineers and laymen, to see

5 and understand what occurs in the millisecond violence and excitement of a high speed event or phenomena. PRODUCT HIGHLIGHTS INCLUDE: Mega pixel, 1K x 1K, resolution to 12,500 frames per second (fps) 1080 high definition (HD) resolution to 2,000 fps Miniature high-g camera heads for on-board vehicle impact analysis Low-cost, tethered head systems - processor or PC-based PCI card Intensified systems for specialized analysis of low- light-level phenomena at specific wavelengths Frame rates to over 1.5 Million fps Long record systems Photron, Inc 9520 Padgett St, Suite 110 San Diego, CA (858) As originally published on Photonics Online in March of 2015.