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Streak and Strip Photography - Streak Photography, Strip Photography, Photofinish cameras, Panoramic cameras, Peripheral cameras, Synchroballistic cameras, Aerial strip cameras

film image slit subject

RMIT University

Rochester Institute of Technology

Streak and strip photography are generic terms for a variety of specialized imaging techniques that capture a continuous record of events over time, along one dimension the resultant photograph. This is accomplished either by exposing a piece of film as it moves past a narrow slit located at the image plane or by repeatedly imaging a linear array of pixels in a digital detector. Both of these situations result in the camera recording only a single line of the subject image at any moment in time. Exposure time for a film camera is governed by the slit width and the speed of film motion. With digital cameras, exposure is controlled by the scan rate assigned to the pixel array. In either method, the final photograph is a chronological compilation of the individual line images continuously recorded by the camera during an event by introducing a time dimension to the image in a direction at right angles to the orientation of slit or pixel array. Streak and strip photographs display time as a visual component in a single “still” image and are effectively a link between normal still photographs and motion picture photography. These
images are always intriguing, often informative, and sometimes quite beautiful.

Streak and strip cameras share a common fundamental principle. In their application, the image is continuously focused along the film or when being captured by the detector as a single, long streak. Either the film is exposed continuously by a slit, with the film effectively moving behind the slit during the exposure, or the image is built up rapidly by repeating image capture with a linear photo sensor array, it is the specific application of the camera that sets the different categories apart. It is the specific application of the camera that sets the different categories of photography apart. The differences between streak photography and strip photography are most clearly defined by the direction of subject motion relative to the slit orientation. In streak photography subject images remain relatively stationary over the slit or move parallel to the slit orientation (and thus at right angles to the direction of film motion) during photography. In this situation, images will be reproduced as a continuum of streaks that look nothing like the original subject.

In strip photography recognizable records of subjects are made by making their image move over, or across, the camera’s slit parallel with the direction the film is moving. When strip cameras photograph subjects whose images at the camera’s slit are stationary, the camera is more properly called a streak camera.

Streak Photography

A streak camera, sometimes called a velocity recording camera, is related to strip or smear cameras, and there is no universally accepted nomenclature for these instruments. At the fundamental level they simply move film past a slit located at the image plane. For recording very rapid events, the image of a slit containing the subject image is moved over stationary film. In the fastest streak cameras, the whole image-recording portion of the camera is electronic in nature, and a camera is simply used to make a photograph of a fluorescent screen or a cathode-ray tube. An electronic image intensifier may be used to amplify the intensity up to 10,000 times before the image is recorded photographically.

Streak cameras record events that occur over a period of time, and thus do not produce images that resemble the subject. The slit built into these cameras limits the field of view to the length of the slit, which defines the spatial dimension of the image in this direction. The spatial dimension at right angles to the slit orientation; however, is effectively eliminated by the narrow slit, the width of which, together with the velocity of the film, governs the temporal resolution. Continuous capture of the many adjacent, dimensional slit images as the film moves past the slit builds the time or temporal dimension of the image along the direction of this motion. The resulting photograph is a continuous record of the image disposition along the slit axis over time. In effect, streak cameras behave as strip-chart recorders, but instead of using mechanical pens to draw on moving paper they record the color and gradation of optical images, with time as the third dimension.

One of the greatest advantages of streak records, when compared to still photographs or motion picture records, is the low cost and the ease with which uninterrupted timing and velocity information can be obtained.

A major characteristic by which streak cameras are classified is their ability to resolve small units of time. This is determined by the relative rate of movement of the film and by the slit width. The simplest streak camera designs transport a roll of film past a fixed slit from a supply spool to a take-up spool. For higher film transport speeds, the film may be attached to the outside of a drum that rotates past the slit.

Such drum-type streak cameras need no synchronization devices to record a single, brief self-luminous event. However, since film length is limited with drums, synchronized shutters are necessary to prevent film overwriting with longer or multiple events. At very high speeds, mechanical limitations of film strength and film transport mechanisms dictate that the film be kept stationary. Instead, an image of the fixed slit containing the subject image is wiped onto the film surface by a rotating mirror system. Again, complex timing, synchronization, and shuttering schemes are usually required to make sure that the event behavior is recorded along a relatively short length of film, and that the film is not rewritten by continued rotation of the mirror.

Ultra high speeds are attained with electronic counterparts of the basic film systems. A photocathode at one end of an electron tube produces electron analogs of the optical image. The electrons are accelerated and focused by rapidly changing electrostatic fields, and swept across a large phosphor screen at the other end of the electron tube to produce a self-luminous streak image. Temporal resolutions of 10 -14 seconds can be achieved with these instruments.

Streak cameras are often used to study the simultaneity of events in the fields of detonics and ballistics, and they are also particularly suited for applications requiring subject velocity information.

Strip Photography

Strip photography uses the same camera design as streak photography and like streak photography, it depends on a recording medium that is in motion behind an exposing slit placed at the image plane of the camera or a digital equivalent. However, unlike streak photographs, in which the original subject is unrecognizable, strip photographs produce an image that has some resemblance to the subject.

The essential feature of strip cameras is that the image they record must move across the slit assembly installed in the camera, rather than along it. This is the result of the translational or rotational motion of the subject. Strip photography and strip cameras are typically referred to by their specific applications rather than the generic strip term. Strip photography and cameras can be subdivided into several categories, such as linear-strip, panoramic, and peripheral photography, and within categories systems are often referred to by their particular function, such as synchroballistic and photofinish cameras. The strip method of making photographs is associated with several well-known pictorial, industrial, scientific, and military photographic camera systems.

Photofinish cameras

The most widely recognized application of strip cameras is probably their use at racetracks as photofinish cameras. The camera is placed to view the entire width of the track at the finish, and the slit in the strip camera is carefully aligned with the finish line. The film is moved at the expected velocity of the images of the subjects participating in the race. Because the visual content of the image is comprised solely of objects exactly at the finish line, and the time dimension is recorded along the film as it moves, photofinish cameras provide an almost indisputable record of the order of finish in a race. Only tampering with the alignment of the camera relative to the finish line could result in misleading results. The misalignment would have to be considerable to actually create a photograph that was an incorrect record of time.

In digital photofinish cameras, the slit and moving film mechanism is replaced by a linear charge-coupled device (CCD) photosensor, which is typically 1-3 pixels wide by 1300-1500 pixels long. The sensor is read out at rates between 100 and 10,000 times per second, depending upon the speed of the participants in the race. When the camera is properly aligned, each line image is of the finish line only, so race participants are recorded only as they cross the finish line. All images have individual timing information automatically attached, and are saved to a computer system where the software builds a continuous composite of the line images in chronological order to give an overall image of the finishing order of all race participants. Because each individual line image is indexed with its discrete time from the start of the race, it is a simple matter to determine the time of each individual finisher as well. A digitally introduced line that is parallel with all of the individual images (and therefore the finish line) can be moved along the composite image, the time of each individual image can be displayed. The hairline can be moved within the image, but cannot be tilted or otherwise manipulated, making errors in timing virtually impossible. Digital photofinish cameras have almost completely replaced film cameras in horse racing, greyhound racing, cycling, rowing, track and field, and motor sports.

Panoramic cameras

Strip cameras are also widely used as panoramic cameras. The camera rotates about the rear nodal point of the lens and scans the surrounding scene with the slit built into the camera. At the same time the film is advanced through the camera at the velocity of the image passing by the camera slit. With these cameras it is quite easy to cover angles of view of up to 360 degrees or more. The length of film exposed per 360-degree revolution is a function of the lens focal length ( f ), and is given by the formula 2? f .

The earliest panoramic cameras to be commercially manufactured were off shoots of the panoramic cameras patented by William J. Johnston in 1904 and David A. Reavill in 1905. The commercial version of this camera was patented by Frederick Brehm in 1905 and eventually manufactured by various divisions of the Eastman Kodak Company under the name of Cirkut. Modern cameras such as the Globuscope, Hulcherama, Alpa Rotocamera, Spinshot, and Roundshot are spin-offs of the basic strip panoramic camera principles introduced by the Cirkut camera. Two electronic versions of these cameras were placed on the surface of Mars by each Viking Lander in 1976 and transmitted stereo views of the Martian landscape back to earth.

Peripheral cameras

Several methods exist for recording rotational motion to make photographs of the outside surfaces of subjects. Strip cameras can make peripheral photographs by rotating a subject in front of the camera with the camera slit aligned to the center of rotation of the subject. The film velocity is matched to the velocity of the subject’s image, and a continuous 360-degree view of the surface of a regular cylindrical subject can easily and accurately be reproduced. Most strip cameras can only accurately reproduce subjects of one given diameter at a time. Larger and smaller diameters on the subject are compressed and elongated, respectively, as all subject circumferences on any subject are reproduced on the same length of film. Nevertheless, the advantage of reproducing all subject surface features on one flat sheet of film or paper often makes up for this shortcoming.

Synchroballistic cameras

Synchroballistic cameras, also known as ballistic-synchro and image-synchro cameras, are nothing more than photofinish cameras, but they are used to photograph missiles and cannon rounds once fired. Because the synchroballistic camera moves the film at the expected velocity of a missile’s image rather than trying to use a short exposure time to freeze motion, continuous light sources can be used to make blur-free images. Excellent spatial resolution can be obtained and information that could not be visualized by other means can be recorded relatively easily. By placing regular, pre-measured markings on the missile it is possible to determine velocity and, in certain cases, even spin rate and other in-flight missile characteristics.

Aerial strip cameras

Aerial strip cameras, such as the stereoscopic Sonne camera, are flown over terrain while moving the film at the same velocity as the image of the ground moves within the camera. In early cameras, the camera operator made this adjustment manually.

Modern designs rely on electronic sensors linked to forward-looking radar to match the two velocities. Military versions of these cameras are sometimes used for reconnaissance purposes because image velocities are often considerable and normal aerial camera shutter speeds are not fast enough to produce adequately sharp images for interpreters. The strip aerial-reconnaissance camera records may be distorted because of lateral movement of the image while it travels across the slit, and they may even be slightly stretched or compressed, but often they contain more detail than an instantaneous photograph which may have motion blur. A special version of the aerial strip camera has been used to record the condition of long stretches of highway.

Circular strip cameras

A strip camera developed by Professor Andrew Davidhazy from the Rochester Institute of Technology uses film that is moved in a circular fashion while the subject is also rotated. This enables the camera to make distortion-free peripheral photographs of conical objects. Because the slit in this camera lies along the radius of the circularly moving film, it can match the different image velocities associated with the changing surface velocity of the cone from the base to the apex. This camera also can correct for differential image motion that occurs along the slit in panoramic photography when the camera is tilted up or down rather than using the rising/falling front movements possibly available on the camera. The records produced by this camera reproduce a 360-degree peripheral view of a cone or a 360-degree panoramic view of a surrounding scene on a portion of the film taking up less than the complete 360-degree film circle.

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