The people at MetroLaser, Inc., have come up with a new digital streak camera design that is capable of capturing full-color images of projectiles traveling at blinding speeds up to 3350 m/s, or 10 times the speed of sound. This system was developed to replace the obsolete film-based streak cameras that are still being used at high-speed test tracks. Film-based streak photography captures the movement of an object as it passes in front of the lens of a camera, while the film moves t the back of a vertical slit aperture during the exposure. The image you get is a long, continuous composite image of the traveling object.
The transition from film to digital has brought challenges to the photography industry, prominent among which is the film for streak photography being unavailable already. This prompted the U.S. Air Force to request Benjamin Buckner and Drew L’Esperance under a Small Business Innovation Research (SBIR) agreement to develop a system that could capture high-quality images, since no digital technology exists that can take images at the ultra high speeds they need. This latest digital technology relies on a precisely controlled mirror to follow the moving object, while freezing the image on the camera.
Buckner is the principal author of a study that describes the new camera design in Optical Engineering. In the study, he points out that the galvo mirror follows the object as it moves past the camera and directs the right section of the object’s image on the right section of the image sensor to be able to create a whole, undistorted image. Having the mirror smartly coordinated to a ballistic item, the principal challenge is to precisely determine the object’s velocity, and compute the swing of the mirror to match that speed exactly. “This has to be done in a few thousandths of a second,” says Buckner.
The digital streak camera is designed to comply with requirements set by operators of rocket sleds, meaning that the camera can resolve an object 1.27 mm in size if the sled is moving a 3350 m/s, and must withstand outdoor conditions, which include dust, heat, moisture and cold. The setup makes use of a commercial portrait quality digital camera as well as standard photographic flash illumination. The software will run optimally on a compact, low-power computer, without compromising a high level of mathematical precision.
“The real power of this approach is that you can take almost any kind of existing camera back and fit it into a rig like this to turn it into a high-speed streak camera,” says Buckner, who along with L’Esperance foresee applications of digital streak imaging to include rocket sled imaging, ballistics, and resolving finishing order in high-speed races too close to call with the naked eye.