In my last video I looked railguns, now whilst I was reviewing the footage I
started wondering how they filmed the projectiles in flight. These are not the
typical sort of high-speed camera shots where you see a bullet hitting a target
for example, these are tracking the projectile from the barrel down the
firing range. From the footage it looks like the camera is panning around and
following the projectile but that would be impossible, the tank round is
traveling at over 1,500 meters per second and would normally look like this.
For all of you out there who said it's done with mirrors then you are
absolutely correct. It works by having a computer-controlled
high-speed rotating mirror in line of sight of a high-speed camera. The speed
of the rotation of a mirror matches that of the object being followed so the
faster the object is traveling like a railgun projectile the faster the mirror
would turn to keep up with it. Using this method the object can be kept in the
field of view for a hundred meters or so or about ninety degrees of the mirrors
movement. In this example the tracker 2 from specialized imaging you can see the
mirror and to its left where the camera is. Because the mirror is
computer-controlled it can be programmed to follow objects that accelerate even
linearly or non linearly. Now rotating mirrors aren't new in fact they were
some of the first high-speed cameras and are still some of the fastest in the
world capable of up to 25 million frames per second and were used to record atom
bomb blasts. During the Manhattan Project to develop the first atomic bomb they
required cameras that could record the first few microseconds of explosion. In
order to create a nuclear chain reaction and achieve critical mass a
baseball-sized piece of plutonium had to be compressed to about half its size.
This was achieved by using an array of focused high explosive lenses
surrounding the plutonium core. In order to make it work effectively the
explosives 32 of them in all had to be triggered within one microsecond,
if any were delayed then the compression of the core would be unequal and the
reaction would even be much less or may not even happen at all.
Using a super high-speed camera it will be possible to see how effective the
explosive lenses had been just a few microseconds after detonation. At the
time the fastest cameras were Fastax cine cameras and could achieve around
10,000 frames per second or one frame per hundred microseconds, this still
wasn't fast enough though. The first high-speed rotating mirror camera was
the Marley, invented by of a British physicist William Gregory Marley, the
Marley camera used a rotating mirror an array of lenses inside a curved housing
each focused onto a single piece of film around the edge of the case. This could
record a sequence of up to 50 images onto 35 millimeter film at a
100,000 frames per second. But by the time of a Trinity test it was outdated
and too slow to record the ultra quick reaction in the plutonium core. Head of the
photography unit Julian Mack said that the fixed short focus and low quality of
the lenses would probably have made the Marley camera pictures useless. He
helped develop the Mack Streak camera which had a 10 million frames per second
limit, thats one frame every hundred nanoseconds. By the 1950s Harold Edgerton
had developed the Rapatronic camera the name coming from Rapid Action Electronic this used a magneto-optic shutter which allowed it to have an
exposure time as short as 10 nanoseconds thats ten billionths of a second. This was
first used with a hydrogen bomb test of Eniwetok Atoll in 1952.
However they only took one image so to see the first few microseconds of a
nuclear detonation up to 10 were used in sequence with an average exposure
time of three microseconds. The images were then played back and blended
together to give the impression of a film. For the British nuclear tests
the Atomic Weapons Research Establishment created for C4, a huge
rotating mirror camera weighing in at around
2,000 kilograms and was the fastest in the world at the time. This could record
up to 7 million frames per second who have a mirror rotating up to 300,000
revolutions per minute and recorded the first British atom bomb test on the 3rd
of October 1952. The rotating mirror cameras are still in use today
but now they use highly sensitive CCDs to replace the filmstrip. The Brandaris 128
and Cordin model 510 have 128 CCD's and a gas driven turbine mirror
driven by helium to achieve up to 25 million frames per second at a
resolution of 500 x 292 pixels for the brand iris and 616 x 920
pixels of recording. At 25 million frames per second the mirror itself is
running at 1.2 million revolutions per minute that's 20,000
revolutions per second so fast of the atmosphere inside the camera is 98%
helium to reduce for friction and the pressure waves that would occur in
normal air. And so onto something I think you may find rather interesting. It's not
the fastest camera in the world but this one is or it was at the time in 2013 the
fastest real-time tracker of a moving object and was developed by the Ishikawa Oku Lab
at the University of Tokyo. Here it is tracking a ping pong ball and
keeping it in the center of a frame all times both during a game and when it is
being spun around on a piece of string. It does this by moving two mirrors in
front of the camera one for the X movement and Yvon for the Y movement it
then uses software similar to face tracking software to provide feedback to
control the mirrors with a response time of just one millisecond. It can also be
used to control a projector and in this scene it's projecting an image onto the
ping-pong ball whilst it's been bounced on the bat, you can see the little face
change on the ball at the top of its travel.
So anyways I hope you enjoyed this look at some of the equipment behind some of the
most amazing footage recorded to date these aren't the fastest cameras in the
world now but it's still amazing to think what can be achieved by mechanical
means. So as always thanks for watching and don't forget we also have the
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