High-speed photography


High-speed photography is the science of taking pictures of very fast phenomena. In 1948, the Society of Motion Picture and Television Engineers defined high-speed photography as any set of photographs captured by a camera capable of 69 frames per second or greater, and of at least three consecutive frames. High-speed photography can be considered to be the opposite of time-lapse photography.
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 strobe 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

The first practical application of high-speed photography was Eadweard Muybridge's 1878 investigation into whether horses' feet were actually all off the ground at once during a gallop. The first photograph of a supersonic flying bullet was taken by the Austrian physicist Peter Salcher in Rijeka in 1886, a technique that was later used by Ernst Mach in his studies of supersonic motion. German weapons scientists applied the techniques in 1916, and The Japanese Institute of Aeronautical Research manufactured a camera capable of recording 60,000 frames per second in 1931.
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 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 camera capable of running 35 mm and 70 mm film in the 1960s. Visible Solutions introduced the Photec IV 16 mm camera in the 1980s.
In 1940, a patent was filed by Cearcy D. Miller for the rotating mirror camera, theoretically capable of one million frames per second. The first practical application of this idea occurred during the Manhattan Project, when Berlyn Brixner was hired into Los Alamos in July 1943. He joined the Optical Engineering Group led by Professor Julian Ellis Mack. The group assisted in providing optical and camera support within the Manhattan Project. Brixner was also the head photographer for the Trinity test and built the first known fully functional rotating mirror camera. This camera was used to photograph early prototypes of the first nuclear bomb. However, the camera was replaced in 1944 by a faster rotating mirror camera invented by Professor Mack, the Mack Streak Camera.
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

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. One such device was the EG&G Microflash 549, which is an air-gap flash. Also see the photograph of an explosion using a Rapatronic camera.
Advancing the idea of the stroboscope, researchers began using lasers to stop high-speed motion. Recent advances include the use of High Harmonic Generation to capture images of molecular dynamics down to the scale of the attosecond.

High-speed film cameras

A high-speed camera is defined as having the capability of capturing video at a rate in excess of 250 frames per second.
There are many different types of high-speed film cameras, but they can mostly all be grouped into five different categories:
  • Intermittent motion cameras, which are a speed-up version of the standard motion picture camera using a sewing machine type mechanism to advance the film intermittently to a fixed exposure point behind the objective lens,
  • Rotating prism cameras, which run film continuously past an exposure point and use a rotating prism between the objective lens and the film to impart motion to the image which matches the film motion, thereby canceling it out,
  • Rotating mirror cameras, which relay the image through a rotating mirror to an arc of film, and can work in continuous access or synchronous access depending on the design.
  • Image dissection cameras, which can use a rotating mirror system, and
  • Raster cameras, which record a "chopped up" version of an image.
Intermittent motion cameras are capable of hundreds of frames per second, rotating prism cameras are capable of thousands to millions of frames per second, rotating mirror cameras are capable of millions of frames per second, raster cameras can achieve millions of frames per second, and image dissection cameras are capable of billions of frames per second.
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, which enhanced the strength and allowed it to be pulled faster. The Estar was also more stable than acetate allowing 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 magazine is typically the longest available for the 35 mm and 70 mm cameras. A magazine is typical for 16 mm cameras, though magazines are available. Typically rotary prism cameras use 100 ft film loads. The images on 35 mm high-speed film are typically more 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 more square rather than rectangular. A list of ANSI formats and sizes is available.
Most cameras use pulsed timing marks along the edge of the film produced by sparks or later by LEDs. These allow accurate measurement of the film speed and in the case of streak or smear images, velocity measurement of the subject. These pulses are usually cycled at 10, 100, 1000 Hz depending on the speed setting of the camera.

Intermittent pin register

Just as with a standard motion picture camera, the intermittent register pin camera actually stops the film in the film gate while the photograph is being taken. In high-speed photography, this requires some modifications to the mechanism for achieving this intermittent motion at such high speeds. In all cases, a loop is formed before and after the gate to create and then take up the slack. Pulldown claws, which enter the film through perforations, pulling it into place and then retracting out of the perforations and out of the film gate, are multiplied to grab the film through multiple perforations in the film, thereby reducing the stress that any individual perforation is subjected to. Register pins, which secure the film through perforations in final position while it is being exposed, after the pulldown claws are retracted are also multiplied, and often made from exotic materials. 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 they 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 continuously past a rotating prism which is synchronized to the main film sprocket such that the speed of the film and the speed of the prism are always running at the same proportional speed. The prism is located between the objective lens and the film, such that the revolution of the prism "paints" a frame onto the film for each face of the prism. Prisms are typically cubic, or four sided, for full frame exposure. Since exposure occurs as the prism rotates, images near the top or bottom of the frame, where the prism is substantially off axis, suffer from significant aberration. A shutter can improve the results by gating the exposure more tightly around the point where the prism faces are nearly parallel.
  • 16 mm rotary prism – Redlake Hycam cameras are capable of 11,000 frame/s with a full frame prism, 22,000 frame/s with a half-frame kit, and 44,000 frame/s with a quarter-frame kit. Visible Solutions also makes the Photec IV. For a more rugged solution, Weinberger made the Stalex 1B, which frames at up to 3000full frames per second, and had the ability to be mounted on board for car crash testing. Fastax cameras can achieve up to 18,000 frames per second with an 8-sided prism.
  • 35 mm rotary prism – Photo-Sonics 4C cameras are capable of 2,500 frame/s with a full frame prism, 4,000 frame/s with a half-frame kit, and 8,000 frame/s with a quarter-frame kit.
  • 70 mm rotary prism – Photo-Sonics 10B cameras are capable of 360 frame/s with a full frame prism, and 720 frame/s with a half-frame kit.