High Speed Photography

shake by carli davidson

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 128 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 (extremely long exposures).

In common usage, high speed photography may refer photographs taken in a way as to appear to freeze the motion, especially to reduce motion blur, or to  a series of photographs taken at a high sampling frequency or frame rate. The former requires a sensor with good sensitivity and either a very good shuttering system or a very fast strobe light. The latter requires some means of capturing successive frames, either with a mechanical device or by moving data off electronic sensors very quickly.

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 1886, a technique that was later used by Ernst Mach in his studies of supersonic motion. 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 (30 m) 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 was during the Manhattan Project, when Berlin Brixner, the photographic technician on the project, built the first known fully functional rotating mirror camera. This camera was used to photograph early prototypes of the first nuclear bomb, and resolved a key technical issue, that had been the source of an active dispute between the explosives engineers and the physics theoreticians.

Harold Edgerton is generally credited with pioneering the use of the stroboscope to freeze fast motion. He eventually helped found EG&G (Edgerton, Germeshausen, and Grier, a US defense contractor), 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 (a photographic light source capable of producing sub-microsecond light flashes). 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 (for context, an attosecond is to a second what a second is to about 31.71 billion years, or twice the age of the universe).

High-speed motion pictures started in 1916 by German weapons scientists. There are three types of high-speed film camera: ‘Intermittent’ motion camera (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 (pull a long reel of 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); and ‘Rotating mirror’ cameras, which relay the image through a rotating mirror to an arc of film, and can only work in a burst mode. Intermittent motion cameras are capable of hundreds of frames per second. Rotating prism cameras are capable of thousands of frames per second. Rotating mirror cameras are capable of millions of frames per second.

Rotating Mirror cameras can be divided into two sub-categories; pure rotating mirror cameras and rotating drum, or Dynafax cameras. In pure rotating mirror cameras, film is held stationary in an arc centered about a rotating mirror. As such, these cameras typically do not record more than one hundred frames. This means they record for only a very short time – typically less than a millisecond. Therefore they require specialized timing and illumination equipment. Rotating mirror cameras are capable of up to 25 million frames per second, with typical speed in the millions of fps. The rotating drum, or Dynafax, camera works by holding a strip of film in a loop on the inside track of a rotating drum. The image is still relayed to an internal rotating mirror centered at the arc of the drum. The mirror is multi-faceted, typically having six to eight faces. Rotating drum cameras are capable of speed from the tens of thousands to hundreds of thousands of frames per second.

Rotating mirror camera technology has more recently been applied to electronic imaging, where instead of film, an array of single shot CCD or CMOS cameras is arrayed around the rotating mirror. This adaptation enables all of the advantages of electronic imaging in combination with the speed and resolution of the rotating mirror approach. Speeds up to 25 million frames per second are achievable, with typical speeds in the millions of fps. Commercial availability of both types of rotating mirror cameras began in the 1950s with Beckman & Whitley, and Cordin Company. Beckman & Whitley sold both rotating mirror and rotating drum cameras, and coined the ‘Dynafax’ term. Cordin Company sold only rotating mirror cameras. In the mid 1960’s, Cordin Company bought Beckman & Whitley and has been the sole source of rotating mirror cameras since. An offshoot of Cordin Company, Millisecond Cinematography provided drum camera technology to the commercial cinematography market.

For the development of explosives the image of a line of sample was projected onto an arc of film via a rotating mirror. The advance of flame appeared as an oblique image on the film, from which the velocity of detonation was measured. 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 essentially one dimension of spatial information recorded continuously over time. ‘Streak photography’ is therefore a space vs. time graphical record. The image that results allows for very precise measurement of velocities. It is also possible to capture streak records using rotating mirror technology at much faster speeds.

Motion compensation photography (‘Smear Photography’) is a form of streak photography. When the motion of the film is opposite to that of the subject with an inverting 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/smear photograph is a photograph of time. When used to image high-speed projectiles the use of a slit produce very short exposure times ensuring higher image resolution. The use for high-speed projectiles means that one still image is normally produced on one roll of cine film. From this image information such as yaw or pitch can be determined. Because of its measurement of time variations in velocity will also be shown by lateral distortions of the image.

The introduction of the CCD revolutionized high-speed photography in the 1980s. The staring array configuration of the sensor eliminated the scanning artifacts of vacuum tube based systems. 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 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.

The Kodak MASD group developed the first HyG (rugged) high-speed digital color camera called the RO that replaced 16-mm crash sled film cameras. Many new innovations and recording methods were introduced in the RO and further enhancements were introduced in the HG2000, a camera that could run at 1000 frame/s with a 512 x 384 pixel sensor for 2 seconds. Kodak MASD group also introduced an ultra high-speed CCD camera called the HS4540 that was designed and manufactured by Photron in 1991 that recorded 4,500 frame/s at 256 x 256. The HS4540 was used extensively by companies manufacturing automotive air bags to do lot testing which required the fast record speed to image a 30 ms deployment.

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. This enables high-speed CMOS cameras to have broad flexibility in trading off speed and resolution. Current high-speed CMOS cameras offer full resolution framing rates in the thousands of fps with resolutions in the low megapixels. But these same cameras can be easily configured to capture images in the millions of fps, though with significantly reduced resolution. The image quality and quantum efficiency of CCD devices is still marginally superior to CMOS.

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 Center (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.

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.

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