Camera to capture objects travelling faster than sound
Camera to capture objects travelling faster than sound
Times of India |
Researchers
have designed a new digital streak camera that captures high-resolution
images of projectiles travelling up to 3,350m/s -- 10 times the speed
of sound.
The system was designed to replace
the outdated film-based streak cameras that are still in use at
high-speed test tracks, researchers said.
Film-based
streak photography records the motion of an object as it passes in
front of a camera lens, while the film moves behind a vertical slit
aperture during the exposure. The result is a long, continuous composite
image of the object.
However, the transition
from film to digital has changed the photography industry, and the
specialized film required for streak photography is no longer being
manufactured.
The US Air Force asked Benjamin
Buckner and Drew L'Esperance under a Small Business Innovation Research
(SBIR) contract to design a system that could produce high-quality
magazine-type images, as no existing digital technology exists that can
capture images at the high speeds they require.
The
new digital technology for this camera relies on a precisely controlled
mirror to follow the object and freeze the image on the camera.
Buckner,
the lead author of the paper described in Optical Engineering, said
that the galvo mirror tracks the object as it moves past the camera and
directs the right portion of the object's image onto the right portion
of the image sensor in order to form a complete, undistorted image.
Since
the mirror is synchronized to a ballistic object, the biggest challenge
is to accurately measure the object's speed, and calculate the swing of
the mirror to match that speed precisely.
"This has to be done in a few thousandths of a second," Buckner said.
The
optical design also allows the wave of compressed air pushed along by
the object (the schlieren effect) to be seen in the captured image. The
camera is triggered by the motion of the object.
The
digital streak camera was built to meet specifications established by
operators of rocket sleds, which means that the camera is able to
resolve an object 1.27mm in size when the sled is travelling 3,350m/s,
and must be able to operate outdoors in conditions of heat, cold, dust,
and moisture.
The setup developed at MetroLaser
utilizes a commercial portrait quality digital camera and conventional
photographic flash illumination. The software was designed to run
efficiently on a compact, low-power computer, while maintaining a high
level of mathematical accuracy.
"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," Buckner said.
Buckner and
L'Esperance envision applications of digital streak imaging including
ballistics, rocket sled imaging, and determining finishing order in
high-speed races.
