Digital Imaging Systems for Ballistics Testing
- Saturday, 01 September 2012
The mid 1990s saw the introduction of high-speed video (HSV) cameras onto the proofing ranges in place of highspeed framing cameras. The new high speed video cameras were used to look at a multitude of events ranging from firing pin behaviour, barrel flexing, muzzle blast formation, as well as terminal/ impact ballistics. Initially terminal and impact ballistic events were often far too fast for the early HSV cameras, but with modern HSV cameras capable of exceeding 250,000 fps and exposure time of <5μs per frame, the study of these types of events has become more feasible.
The number of frames that a highspeed video can record offers a range of test scenarios which cannot be achieved using a single shot camera. This makes them ideally suited to record longer time frame events not requiring submicrosecond triggering accuracy, given that they have a post event triggering system. Using a single-shot high-resolution still camera, on the other hand, necessitates extremely precise triggering to even guarantee seeing the subject within the field of view!
HSV cameras are very flexible, offering many modes of operation so that the subject can be filmed. However, very often compromises have to be made in order to acquire images at an appropriate exposure and framing rate, which may result in failure to produce the required data from an entire highspeed sequence.
Ultra-high-speed cameras with a limited number of images (such as the SIR3 ballistic range camera or the SIM multiple framing camera) have the ability to capture an image or sequence of images at exactly the time when it is needed, with extremely high resolution, high frame rate and timing accuracy. With frame rates of over 330 Mfps and shutter speeds down to 3ns, these types of camera can remove motion blur of projectiles or fragments travelling at speeds of up to 4000m/s.t
Modern HSV cameras have allowed their users to broaden the scope of ballistic testing, and to consider detailed recording of many aspects of ballistics that were recently beyond the realms of possibility. Unfortunately, this broadening of requirements sometimes means that the capability of the high speed video camera is over-stretched, resulting in poor quality and the inability to extract meaningful data from the recorded sequences. An example of this is where the test engineers want to study a projectile travelling over several meter and yet need at least 10,000fps (100us) to minimise the projectile motion blur to an acceptable level. This field of view will produce a very small image of the projectile in each frame of the video and together with the residual motion blur adding to the uncertainty of projectile position and attitude, this will reduce the accuracy/viability of any data that can be extracted from the sequence.
This has caused the parallel development of ideas to further enhance the ability of users to extract data from fast ballistic events. One example of this is producing a digital streak camera that sweeps the image at a constant speed across a CMOS sensor to produce a long record-time image. This methodology was known as a SMEAR (ballistic-synchro) camera in the days of film. The film was moved across a narrow slit at a speed that matched the predicted image velocity of a projectile as it passed the recording point. The matching of the direction of film travel past the slit and its velocity to the predicted projectile flight path and velocity enabled a very high resolution image of a fast moving projectile to be recorded. This method of recording tended to give a distorted image of the projectile if the film speed was not matched to the projectile image velocity, and the peripheral components (such as sabots, driving rings, pusher plates, etc.) were also usually distorted due to their difference in speed from the main projectile.
Driven by the limited amount of data that could be extracted using this method - Specialised Imaging introduced, in 2006, a Trajectory Tracker system (Figure 2) that allows a HSV camera to record over a large part of the flight path, or any portion of the flight path that is of interest. The Trajectory Tracker employs a triggered scanning mirror that is programmed to scan in synchronism with a passing projectile so that it relays the image of the subject into the HSV camera. Because the mirror is programmed to match the velocity of the projectile, motion blur is eliminated, enabling the high-speed camera to now operate at a much more modest frame rate that only needs to eliminate any vertical movement. Realistic framing rates are typically less than 6000fps - giving far better resolution and sensitivity.
High speed digital imaging cameras, which provide high resolution results and rapid data analysis, have replaced high speed still and film cameras. Modern cameras, such as the SIR3 and SIM, offer much more flexibility and capability for imaging external and terminal ballistics.
The continuous demand for complex and sophisticated data analysis has resulted in rapid advances in imaging technology to give high resolution and better quality images.