Ultrahigh-Speed Cinematography

Ultrahigh-Speed Cinematography

 

filming at speeds greater than 105 frames per sec. Ultrahigh-speed cinematography is used in various fields of science and technology to investigate phenomena and processes that occur at extremely high speeds, such as explosions, the propagation of shock waves, electrical discharges, and nuclear reactions. It is also used in making educational and popular-scientific films because it permits the viewer to examine in detail all phases of the motion of the object being filmed.

The range of speeds from 105 to 107 frames per sec is covered by using optical compensation, optical commutation, and electrical commutation. In the last method, a sequence of images is formed on a stationary photosensitive material by means of a series of identical objectives or lenses, which are positioned along the path of the moving object being filmed. During the filming, a corresponding number of pulsed light sources are switched on, each serving to illuminate the photographic field of only one lens. In this case, the switching must ensure illumination of the object at the moment when the object is in front of each successive lens.

The highest filming speeds (~109 frames per sec) are achieved by using scanning techniques and image dissection. In scanning photography, the optical image formed by the objective is split by a mechanical or optical raster into separate elements which are distributed in the image plane. By mutually shifting the raster image and the photosensitive material, a scan of the image is obtained on the photosensitive material in the form of a series of strips corresponding to the number of image elements. The width of a strip is equal to the length of the image element in the direction perpendicular to the direction of the scan; the variation in the optical density of each strip over its length depicts the variation in the brightness of a given portion of a frame during the period of filming. Positives are printed from the negative of the expanded image by reversing the path of the light rays. In order to obtain a sequence of frames after printing each individual frame, it is necessary to shift the negative in the direction of the scan by an amount equal to the cross section of the image element.

In scanning photography, the number of frames exposed is limited by the separation between the image elements on the photosensitive material in the direction of the scan; it does not exceed 300. There is no such limit when using the technique of image dissection, in which a frame is divided into narrow strips, which are projected onto a single line by means of special optical equipment (image dissector). Similar results are obtained by using a system of fine light guides in the form of fibers with a diameter of 0.01 to 0.005 mm. One set of ends is placed close together in the field of the primary optical image, and the other set is placed in a single row along a line perpendicular to the direction of the scan.

REFERENCES

Sakharov, A. A. Vysokoskorostnaias” emka. Moscow, 1950.
Dubovik, A. S. Fotograficheskaia registratsiia bystroprotekaiushchikh protsessov. Moscow, 1964.
Salamandra, G. D. Fotograficheskie melody issledovaniia bystroprotekaiushchikh protsessov. Moscow, 1974.

A. A. SAKHAROV

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