High-Speed Photography and Cinematography

Most modern cameras allow exposures with shutter speeds of up to 1/1,000 second. Shorter exposure times can be attained by illuminating the object with a short light flash. In 1931 the American engineer Harold E. Edgerton developed an electronic strobe light with which he produced flashes of 1/500,000 second, enabling him to photograph a bullet in flight. By the use of a series of flashes, the progressive stages of objects in motion, such as a flying bird, can be recorded on the same piece of film. Synchronization of the flash and the moving object is achieved by using a photocell to trigger the strobe light. The photocell is set up so that it is illuminated by a beam of light that is interrupted by the fast-moving object as soon as the object comes into the field of the camera. More recently, high-speed electro-optical and magneto-optical shutters have been developed that allow exposure times of up to a few billionths of a second. Both types of shutter make use of the fact that the polarization plane of polarized light in certain materials is rotated under the influence of an electric or magnetic field. The magneto-optical shutter is made up of a glass cylinder placed inside a coil. A polarization filter is placed at each side of the glass cylinder. Both filters are crossed, and light that passes through the first filter becomes polarized and is stopped by the second filter. If a short electric pulse is passed through the coil, the polarization plane of the light in the glass cylinder is rotated, and light can pass through the system. The electro-optical shutter, built in a similar way, consists of a cell with two electrodes that is filled with nitrobenzene and is placed between the two crossed polarization filters. The polarization plane inside the liquid is rotated by a short electrical pulse at the electrodes. Electro-optical shutters have been used to photograph the sequence of events during the explosion of an atomic bomb. Very fast motion can also be studied by high-speed cinematography. Conventional techniques, in which individual still photographs are taken in a fast sequence, allow a maximum rate of 500 frames per second. By keeping the film stationary and using a fast rotating mirror (up to 5,000 revolutions per second) that moves the images in a sequential order over the film, rates of a million pictures per second can be attained. For extremely high rates, such as a billion pictures per second, classical optical methods are abandoned and cathode ray tubes are used to make the exposures. By Jan Jedlicka

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