Wednesday, August 31, 2016

Cinematography Notes 11



High-speed cinematography


High-speed cinematography

Motion pictures have also been used to study phenomena that occur so fast that they cannot be recorded on normal cameras. An immense amount of ingenuity has been applied to the solution of many problems in this field.

Optical systems outside the camera

Shadowgraph film can show sharp shadows of fast-moving bodies, indicating their speed and any changes of attitude they undergo. With a rapid enough exposure this procedure can also show the sharp shadow of a wave front in air.

Schlieren optics show changes in the condition of a test area of space in the optical path, even when it remains fully transparent. The Schlieren interferometer is a system modification. An apparatus that polarizes the light beam—separating it into two slightly different paths and then reintegrating it into one path—is inserted before and after the area T to bring out details of the phase of the light as it progresses from left to right. These details appear in the form of light and dark phase bands, or fringes. They outline the disturbance of the wave by any nonuniformity existing. Counting the number of fringes displaced gives a measure of how great the nonuniformity is. The arrangement is especially adaptable to detecting and measuring irregular flow phenomena and wave fronts in the air (or another fluid).

A spectrum of a self-luminous subject is often obtained. Sometimes the spectrum varies quickly, as in an explosion, and it is desired to study the variation. The light output from the spectrometer is led into a motion-picture camera that can handle the speed needed, and a film record is taken. The film can then be studied at leisure.


Framing

The normal-speed camera records on the film a succession of frames, each showing a complete picture but taken at successive instants of time. Not all high-speed cameras record in this way, but some do, and the various methods of separating the frames and recording them represent key elements in the technology.

When a cube (or many-sided prism) is rotated in the optical path in a camera, it moves the image periodically past the picture aperture. If the film is moved in the same direction at the same speed, the image, while it is appearing, is fixed on the film. The problems are that, during its appearance, the image does not move with an exactly constant velocity, and the prism introduces some optical distortion of the image. These can be partially remedied by a variety of modifications, but there are still residual imperfections. The rotating prism device is used, however, up to about 10,000 frames per second. A variation, in which an internal mirror drum with many facets is used instead of a glass prism, is usable up to about 40,000 frames per second. In evaluating the performance of the system, the size of the frames and sharpness of the images must be considered, as well as the number of frames per second.


Rotating-mirror systems take a number of forms, showing a reasonably typical arrangement. The objective lens forms a primary image at the rotating-mirror position. The mirror reflects this image through an arc of fixed, individual relay lenses in succession on the stationary film. The relay lenses having the primary and final images both in focus keep the final image fixed at each place in succession on the film as the beam traverses each lens. With some rotating-mirror systems it is possible to go to some 5,000,000–10,000,000 frames per second (again keeping in mind that frame size and image sharpness are important as well). The arrangement wastes light, so that bright original subjects are necessary for a satisfactory exposure. The system has been used to study explosions and plasmas and to study subjects by reflected light with very high-intensity illuminants.

The central idea of the image dissector technique is to cut up a picture in the same way that a television picture is cut into horizontal scanning lines. The bright lines are made very narrow, with black or empty spaces between them, which permits superimposing other, similar pictures—also cut into fine lines separated by empty spaces. A number of pictures can be superimposed in this way without the lines interfering with each other. Lenticulations (embossed lenslike shapes) on the film form the scanning lines and concentrate the incoming light into fine lines on the film, according to where the light comes from on the objective lens.

The method will also work with two sets of lenticulations, one horizontal and the other vertical and superposed. This permits the storing of more pictures on the record film—in this case, as dots instead of thin lines. A number of devices based on these general ideas have been used. The frame speeds achievable vary with the specific arrangements but are generally lower than with the moving-mirror mechanisms.

The number of frames may be increased to infinity by changing the optical system to put the primary image on the film and leaving out the relay lenses or, more simply, to replace the mirror with the film on a rotating drum. This requires compressing the individual frame from a two-dimensional into a one-dimensional image or slit. The equipment becomes a “streak camera.” It is useful when there are one or more distinct demarcations in the one-dimensional view of the subject and it is desired to record their motion along a more or less straight line. This is useful, for example, in recording the motion of the front of an explosion cloud or of a projectile. It leaves no unexplored areas of time, as does a framing record.

Illumination

The need for sufficient light is a most important requirement in high-speed cinematography. This need is so great that it is often undesirable to keep the light on longer than absolutely necessary, so that in many cases the light being turned on and off acts as a shutter. High-intensity, short-time sources have been studied extensively. Most sources consist of electrical discharges or, in some cases, arcs in air or a gas. In most cases they involve sophisticated electronic control methods. In cases in which the original subject is sufficiently self-luminous, such as in explosions, a fast shutter may be required. Since mechanical shutters do not go to very high speeds, various types of electro-optical shutters are used. On some occasions it is necessary to amplify the light after it has left the subject and before it reaches the film.

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