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Kerr cell[′kər ‚sel]
an electrooptical device based on the Kerr effect and used as an optical shutter or light modulator. It is the fastest-acting device for controlling the intensity of a light flux (its actuation time is 10-9 to 10-12 sec).
A Kerr cell consists of a vessel with transparent windows that is filled with a liquid in which the Kerr effect takes place. Two electrodes forming a flat capacitor are immersed in the liquid, and a beam of light passes between the electrodes. The vessel is placed between a polarizer and a light analyzer, which are arranged crosswise. The direction of the electric field E in the capacitors is at a 45° angle to the direction of the electric fields of the polarized light oscillations. In the absence of an electric field the analyzer does not pass the light. When an electric field is applied to the liquid, birefringence takes place. As a result, the Kerr cell becomes transparent to the transmitted light. Maximum transparency is achieved at a voltage V across the electrodes of 3–30 kilovolts; the transparency depends on the liquid in the vessel (liquids used for this purpose, such as nitrobenzene, have a large Kerr constant) and on the dimensions of the cell.
Kerr cells were formerly used to record sound on the soundtrack of motion-picture film (the tagefon, a Soviet device for optical recording of soundtracks); however, they were later replaced by other devices. They are also used in high-speed still and motion-picture photography, in optical telephony and ranging, in geodetic range-finding apparatus, and in laser control circuits.
The quick response of a Kerr cell makes it suitable for measuring the velocity of light under laboratory conditions: after having passed through the Kerr cell, the light is reflected by a mirror and again passes through the cell in the opposite direction, with a delay that depends on the path length from the cell to the mirror and back. This method has historical significance and is effective as a lecture demonstration. In a number of uses the liquid Kerr cell is replaced by a crystal cell, whose operation is based on the Pockels effect.
REFERENCESTager, P. Iacheika Kerra. Moscow-Leningrad, 1937.
Vysokoskorostnaia kinofotos”emka ν nauke i tekhnike. (Translated from English and French.) Edited by P. G. Tager. Moscow, 1955.
V. A. ZAMKOV