High Speed Photography is the science of taking pictures of very fast phenomena.
In 1948, the Society of Motion Picture and Television Engineers (SMPTE) defined
high-speed photography as any set of photographs captured by a camera capable of
128 frames per second or greater and of at least three consecutive frames.
In common usage, high speed photography may refer to either or both of the
following meanings. The first is that the photograph itself may be taken in a
way as to appear to freeze the motion, especially to reduce motion blur. The
second is that a series of photographs may be taken at a high sampling frequency
or frame rate. The first requires a sensor with good sensitivity and either a
very good shuttering system or a very fast light. The second requires some means
of capturing successive frames, either with a mechanical device or by moving
data off electronic sensors very quickly.
Other considerations for high speed photographers are record length,
reciprocity breakdown, and spatial resolution.
Early applications and development
Nuclear explosion photographed by rapatronic camera less than 1 millisecond
after detonation. From the Tumbler-Snapper test series in Nevada, 1952. The
fireball is about 20 meters in diameter in this shot. The spikes at the bottom
of the fireball is known as the rope trick effect.
Nuclear explosion photographed by rapatronic camera less than 1 millisecond
after detonation. From the Tumbler-Snapper test series in Nevada, 1952. The
fireball is about 20 meters in diameter in this shot. The spikes at the bottom
of the fireball is known as the rope trick effect.
The first practical application of high-speed photography was Eadward
Muybridge's 1878 investigation into whether horses' feet were actually all off
the ground at once during a trot.
Bell Telephone Laboratories was one of the first customers for a camera
developed by Eastman Kodak in the early 1930s. Bell used the system, which ran
16 mm film at 1000 frame/s and had a 100 foot load capacity, to study relay
bounce. When Kodak declined to develop a higher speed version, Bell Labs
developed it themselves, calling it the Fastax. The Fastax was capable of 5,000
frame/s. Bell eventually sold the camera design to Western Electric, who in turn
sold it to the Wollensak Optical Company. Wollensak further improved the design
to achieve 10,000 frame/s. Redlake Laboratories introduced another 16 mm
rotating prism camera, the Hycam, in the early 1960s.
Photo-Sonics developed several models of
rotating prism cameras capable of running 35 mm and 70 mm film in the 1960s.
These cameras are still in service now in the motion picture industry and can be
serviced by Photo-Sonics
International in the UK. Visible Solutions introduced the Photec IV 16 mm
camera in the 1980s.
The D. B. Milliken company developed an intermittent, pin-registered, 16 mm
camera for speeds of 400 frame/s in 1957. Mitchell, Redlake Laboratories, and
Photo-Sonics eventually followed in the 1960s with a variety of 16, 35, and 70
mm intermittent cameras.
Stroboscopy and laser applications
Doc Edgerton is generally credited with pioneering the use of the stroboscope
to freeze fast motion. He eventually helped found EG&G, which used some of
Edgerton's methods to capture the physics of explosions required to detonate
nuclear weapons. See, for example, the photograph of an explosion using a
Rapatronic camera.
Advancing the idea of the stroboscope, researchers began using lasers to stop
high speed motion.
High speed film cameras
As film and mechanical transports improved, the high-speed film camera became
available for scientific research. Kodak eventually shifted its film from
acetate base to Estar (Kodak's name for a Mylar-equivalent plastic), which
enhanced the strength and allowed it to be pulled faster. The Estar was also
more stable than acetate for more accurate measurement, and it was not as prone
to fire.
Each film type is available in many load sizes. These may be cut down and
placed in magazines for easier loading. A 1200 foot magazine is typically the
longest available for the 35 mm and 70 mm cameras. A 400 foot magazine is
typical for 16 mm cameras, though 1000 foot magazines are available. The images
on 35 mm high speed film are typically rectangular with the long side between
the sprocket holes instead of parallel to the edges as in standard photography.
16 mm and 70 mm images are typically square rather than rectangular. A list of
ANSI formats and sizes is available.
Intermittent pin register
The intermittent pin register camera actually stops the film in the film gate
while the photograph is being taken. In high speed photography, this requires a
complex mechanism for keeping the film moving quickly through the camera from
the supply reel, but then stopping it for imaging, and then starting it again to
move it onto the take-up reel. In many cases, a loop is formed before and after
the gate to create and then take up the slack. Pull-down claws grab the film and
move it into place and then move it back out of the film gate after the
exposure. Register pins secure the film while it is being exposed. In some
cases, vacuum suction is used to keep the film, especially 35 mm and 70 mm film,
flat so that the images are in focus across the entire frame.
* 16 mm pin register: D. B. Milliken Locam, capable of 500 frame/s; the
design was eventually sold to Redlake. Photo-Sonics built a 16 mm pin-registered
camera that was capable of 1000 frame/s, but eventually removed it from the
market.
* 35 mm pin register: Early cameras included the Mitchell 35 mm.
Photo-Sonics won an Academy Award for
Technical Achievement for the 4ER in 1988. The 4E is capable of 360 frame/s.
* 70 mm pin register: Cameras include a model made by Hulcher, and
Photo-Sonics 10A and 10R cameras, capable
of 125 frame/s.
Rotary prism
The rotary prism camera allowed higher frame rates without placing as much
stress on the film or transport mechanism. The film moves smoothly past a
rotating prism which is synchronized to the main film sprocket. Each revolution
of the prism "paints" the same number of frames onto the film as there are faces
on the prism. A shutter also improves the results by only opening as the prism
faces are nearly parallel, and then closing again.
* 16 mm rotary prism - Redlake Hycam and Fastax cameras are capable of 10,000
frame/s with a full frame prism (4 facets), 20,000 frame/s with a half-frame
kit, and 40,000 frame/s with a quarter-frame kit. Visible Solutions also makes
the Photec IV.
* 35 mm rotary prism - Photo-Sonics 4C
cameras are capable of 2,500 frame/s with a full frame prism (4 facets), 4,000
frame/s with a half-frame kit, and 8,000 frame/s with a half-frame kit.
* 70 mm rotary prism - Photo-Sonics 10B
cameras are capable of 360 frame/s with a full frame prism (4 facets), and 720
frame/s with a half-frame kit..
Rotary mirror
The Cordin Dynafax held a strip of film still while a mirror rotated at high
speeds. At the appropriate moment, the capping shutter was opened and the mirror
steered images onto the film. This type of system was capable of 1,000,000
frame/s for a few hundred frames.
Streak, shadowgraph, and motion compensation photography
By removing the prism from the rotary prism cameras, and using a very narrow
slit in place of the shutter, it is possible to take images whose exposure is
proportional to the film speed across the slit. The image that results has
several useful properties. The film advance direction is essentially a measure
of time. If the subject's motion is perpendicular to the slit, it may show
growth or motion perpendicular to the slit.
When the motion of the film is opposite to that of the subject with an
inverting (positive) lens, and synchronized appropriately, the images show
events as a function of time. Objects remaining motionless show up as streaks.
This is the technique used for finish line photographs. At no time is it
possible to take a still photograph that duplicates the results of a finish line
photograph taken with this method. A still is a photograph in time, a streak
photograph is a photograph of time.
By combining this technique with a diffracted wavefront of light, as by a
knife-edge, it is possible to take photographs of phase perturbations within a
homogeneous media. For example, it is possible to capture shockwaves of bullets
and other high-speed objects. See, for example, Shadowgraph and Schlieren
photography.
Video
Early video cameras using tubes (such as the Vidicon) suffered from severe
"ghosting" due to the fact that the latent image on the target remained even
after the subject had moved. Furthermore, as the system scanned the target, the
motion of the scanning relative to the subject resulted in artefacts that
compromised the image. The target in Vidicon type camera tubes can be made of
various photoconductive chemicals such as Sb2S3, PbO, and others with various
image "stick" properties. The Farnsworth Image Dissector did not suffer from
image "stick" of the type Vidicons exhibit, and so related special Image
Converter tubes might be used to capture short frame sequences at very high
speed.
The mechanical shutter, invented by Pat Keller et al at China Lake in the
1980s, helped freeze the action and eliminate ghosting. This was a mechanical
shutter similar to the one used in high speed film cameras, a disk with a wedge
removed. The opening was synchronized to the frame rate, and the size of the
opening was proportional to the integration or shutter time. By making the
opening very small, the motion could be stopped.
Despite these improvements in the image quality, the systems were still
limited to 60 frame/s.
CCD
The introduction of the CCD revolutionized high speed photography in the
1980s. The staring array configuration of the sensor eliminated the scanning
artefacts. Precise control of the integration time replaced the use of the
mechanical shutter. However, the CCD architecture limited the rate at which
images could be read off of the sensor. Most of these systems still ran at NTSC
rates (approximately 60 frame/s), but some, especially those built by the Kodak
Spin Physics group, ran faster and recorded onto specially constructed video
tape cassettes. Eventually, the Kodak group managed to develop the HG2000, a
camera that could run at 1000 frame/s with a 512 x 384 pixel sensor for 2 s.
By adding an image intensifier to a CCD, it is possible to capture a single
frame of a very fast event. Hadland uses this technique for a range of high
speed cameras capable of running at 1,000,000 frame/s, though record lengths are
limited to 8 or 16 images.
CMOS
The introduction of CMOS sensor technology again revolutionized high speed
photography in the 1990s and serves as a classic example of a disruptive
technology. Based on the same materials as computer memory, the CMOS process was
cheaper to build than CCD and easier to integrate with on-chip memory and
processing functions, though the image quality and quantum efficiency of CCD
still compare favourably. The first patent of an Active Pixel Sensor (APS),
submitted by JPL's Eric Fossum, led to the spin-off of Photobit, which was
eventually bought by Micron Technology.
However, Photobit's first interest was in the standard video market; the
first high speed CMOS system was NAC Image Technology's
HSV 1000, first produced in 1990. Vision Research uses a CMOS sensor in the
Phantom v4 camera, with a sensor designed at the Belgian Interuniversity
Microelectronics Centre (IMEC). These systems quickly made inroads into the 16
mm high speed film camera market despite resolution and record times (0.25
megapixel, 4 s at full frame and 1000 frame/s) that suffered in comparison to
existing film systems. IMEC later spun the design group off as FillFactory,
which was later purchased by Cypress Semiconductor. Photobit eventually
introduced a 500 frame/s 1.3 megapixel sensor, a device found in many low-end
(high speed) systems.
Subsequently, several camera manufacturers compete in the high speed digital
video market, including AOS Technologies, Fastec Imaging,
NAC, Olympus, Photron, Redlake, Vision Research,
Weinberger, and others, with sensors developed by Photobit, Cypress, and
in-house designers.
In addition to those science and engineering types of cameras, an entire
industry has been built up around industrial machine vision systems and
requirements. The major application has been for high speed manufacturing. A
system typically consists of a camera, a frame grabber, a processor, and
communications and recording systems to document or control the manufacturing
process.
Infrared
High speed infrared photography has become possible with the introduction of
the Amber Radiance, and later the Indigo Phoenix. Amber was purchased by
Raytheon, the Amber design team left and formed Indigo, and Indigo is now owned
by FLIR Systems. Santa Barbara Focal Plane, CEDIP, and Electrophysics have also
introduced high speed infrared systems.
