Reversal

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Reversal

 

in photography, a chemical process used to produce a positive image directly on exposed photographic material. A visible image is produced on the material’s light-sensitive layer by the photochemical action of light and by photographic development. In this image, called a negative, the bright parts of the subject appear dark and the dark parts, light. The negative is developed but not fixed because the silver halide remaining undeveloped in the light-sensitive layer is used to obtain the positive image. After a bleaching bath to remove the metallic silver, the photographic material is exposed a second time and redeveloped, resulting in a positive image.

In reversal processing of color materials, the first development produces a negative silver image and the second development yields a positive image consisting of silver and dyes. After all the silver is removed by bleaching and fixing, only a dye image remains.

After each stage of treating photographic materials in solutions, the materials are rinsed in water to remove soluble substances that could contaminate subsequent solutions or damage the image during storage.

Reversal is called a one-step process to distinguish it from the two-step negative-positive process. Images obtained by the reversal process can be multiplied by printing on reversal cinematographic materials or by duplication. Reversal processing is widely used in professional, amateur, scientific, and instructional photography and cinematography.

REFERENCES

Iofis, E. A. Kinoplenki i ikh obrabotka. Moscow, 1964.
Iofis, E. A. Tekhnika fotografii. Moscow, 1973.

E. A. IOFIS


Reversal

 

(reverse), a change in the fundamental motion of a machine or its working parts to the opposite direction. Reciprocating engines are reversed either by distribution mechanisms (valves or slide valves), which can direct the working fluid to the engine cylinders in a specified sequence, or by an intermediate unit in the transmission that changes the direction of rotation of the driven shaft without altering the motion of the drive shaft. In aircraft engines, reversal (the creation of reverse thrust) is achieved by reversing the propeller blades or deflecting the stream of exhaust gases.

Ships with steam- and gas-turbine power plants use auxiliary turbines for reversal. Ships also use propellers with rotating blades for the same purpose. Electric motors can be reversed by changing the direction of the current in the exciting winding of a DC motor or by switching two phases of the stator winding in AC motors.

M. S. FESTENSHTEIN