Kerr cell

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The Kerr cell is named after the Scottish physicist John Kerr . He discovered the Kerr effect , also named after him, which belongs to the field of non-linear optics . Then the refraction and polarization behavior of a material is changed by an externally applied electric field .

The Kerr cell uses this effect for the controlled modulation of the polarization of the light passing through, so it forms a polarization modulator . A Kerr cell can be used to convert electrical signals into optical signals. Also called "Karolus cell" or "Kerr-Karolus cell" (after August Karolus ), it was an essential component of the early sound film technology. As a counterpart to the photocell in the projector, the sound track was exposed during film production with the help of the Kerr cell ( optical sound method ).

Kerr cell.svg

The cell (A) is filled with a liquid, usually nitrobenzene (B). An electric field is applied across the direction of the light passing through the liquid via electrode plates (C and D). With the liquid mentioned and the usual cell sizes in the centimeter range, electrical voltages in the range of a few kilo volts (E) are required . The front and back of the Karolus-Kerr cell are translucent made of glass, the metal side walls are electrode plates.

When used for optical sound transmission , the analog audio signal is applied to the electrodes together with a bias voltage. The incoming light is then linearly polarized light, either a laser beam or a conventional light source followed by a linear polarizer ( polarization filter ). The polarization plane is then more or less rotated by the Kerr cell. Another linear polarizer is arranged behind the cell, so that the intensity of the light transmitted depends on the current angle of rotation of the Kerr cell and thus on the modulation of its excitation voltage. On the receiving side of the transmission path, the modulated light can be converted back into an electrical signal by means of a photo receiver . This can be amplified using conventional electronic means and reproduced as sound.