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High-velocity ballistic objects blazing through the atmosphere, such as fired bullets, are subjected to increased surface temperatures caused by aerodynamic heating and, in some cases, the acceleration process. Measuring the intensity of thermal infrared (IR) emission from ballistic objects uses a variety of application techniques in an array of fields, both military and non-military. These applications include tracking the re-entry of ballistic spacecraft such as the Space Shuttle, rockets, missiles and missile warheads; protecting armored vehicles from tank projectiles and rockets; and detecting snipers by tracing bullets back to their point of origin.
Infrared (IR) imaging techniques are highly useful for experimentally determining the effects of aerodynamic heating on the projectile shapes of bullets. At high velocities, bullets might be adversely affected by such heating if the surface temperature exceeds various values. These effects might include weakening, melting or ablating of the bullet material, all of which may, in turn, lead to shape changes that result in flight instability. An infrared camera calibrated to measure high temperatures has proven to be the best way to accurately determine the in-flight surface heating of bullets, and other projectiles as well. Other thermometry methods require physical contact.
FLIR(Goleta, CA) has developed an infrared camera that is capable of thermal-imaging objects at temperatures of 50° C and above at shutter speeds as fast as 1 µsec. Called Phoenix Mid, this camera uses indium antimonide detectors (InSb) in a 320x256 pixel focal plane array (FPA). This array exploits a low-noise, high-gain-readout integrated circuit (IC) to enable sensitive imaging at 1 µsec exposures.
Recent experiments have demonstrated the camera's capability to generate thermal images of bullets in flight. Figure 1 shows a .30-caliber rifle bullet that has traveled about 3 feet out of the muzzle of a Fabrique Nationale FAL light automatic rifle. The bullet is traveling at 840 m/sec (approximately 1900 mph); yet, it travels just 0.84 mm during the 1-µs exposure, reducing the image blur to about 5 pixels.
Note that the image shows aerodynamic heating at the bullet's tip, as well as frictional heating of the grooves cut into the bullet by the rifling of the gun's barrel. The glowing areas on the tail of the bullet appear to result from two effects. The bright spot on the tail along the bullet's centerline is caused by reflections of the muzzle flash off the bullet's tail, and perhaps also from the flash heating of the tail caused by the powder charge.
Further experiments will attempt to suppress the reflections of the muzzle flash from the bullet image by firing the bullet through a thin sheet of metal that will act as a light shield between the muzzle flash and the point at which the bullet is imaged. In this scenario, only the very small portion of the muzzle flash that shines through the bullet hole in the light shield is expected to reflect off the bullet's tail.
The rifle was secured in a rubber-jawed clamping mechanism so that fired bullets would follow the same trajectory from shot to shot. A soldering iron operating at low power (so that it did not appear obtrusively bright) provided a spatial marker for the camera-based system, as seen in the lower left corner of Figure 1.
Bullet images were first acquired with a 25 mm camera lens; this provided the camera with a wide field-of-view in order to find the bullets relative to the soldering iron tip. Germanium infrared lenses made by Janos Technology Inc. ( Townshend , VT ) delivered 3 to 5-micron imaging.