A Kerr cell is a highly specialised electro-optical shutter device capable of operating at extraordinarily high speeds without the use of any moving mechanical parts. It takes its name from the Scottish physicist John Kerr, who discovered the electro-optical effect upon which the device is based in 1875. The Kerr effect describes the phenomenon whereby certain optically transparent materials change their refractive properties and become birefringent - meaning they affect the polarisation of light passing through them - when subjected to a strong electric field.
In its typical construction, a Kerr cell consists of a transparent cylinder or cell filled with nitrobenzene, a liquid that exhibits a particularly strong Kerr effect, positioned between two crossed polarising filters oriented at ninety degrees to each other. In the absence of an electric field, no light can pass through the device, as the two crossed polarisers block all transmission. When a high voltage electric field is applied across the nitrobenzene filled cell, the liquid becomes birefringent and rotates the polarisation of the light passing through it by ninety degrees, allowing it to pass through the second polarising filter and complete its journey through the device. Removing the electric field instantly returns the cell to its opaque state, effectively opening and closing the shutter with extraordinary speed and precision.
The principal advantage of the Kerr cell shutter is its ability to achieve exposure durations of just nanoseconds or even picoseconds - far beyond the capability of any mechanical shutter system. This makes it invaluable in scientific, technical, and research photography where the capture of extremely rapid phenomena is required, such as the study of explosions, shock waves, electrical discharges, and other high speed events in physics and engineering research. It has also been used in specialised applications such as high speed cinematography and laser pulse control.
Despite its remarkable performance capabilities, the Kerr cell has never found application in mainstream photography due to the high voltages required to operate it, the toxic nature of nitrobenzene, and its considerable complexity and cost compared to conventional mechanical or electronic shutter systems.