Optical wedge

The wedge prism is a prism with a shallow angle between its input and output surfaces. This angle is usually 3 degrees or less. Refraction at the surfaces causes the prism to deflect light by a fixed angle. When viewing a scene through such a prism, objects will appear to be offset by an amount that varies with their distance from the prism.

For a wedge prism in air, rays of light passing through the prism are deflected by the angle δ, which is approximately given by

where n is the index of refraction of the prism material, and α is the angle between the prism's surfaces.

The term "optical wedge" refers to any shallow angle between two plane surfaces of a window. This wedge may range from a few millionths of a degree of perfect parallelism to as much as three degrees of angle. Even though high-precision optics, such as optical flats, may be lapped and polished to extremely high levels of parallelism, nearly all optics with parallel faces have some slight wedge. This margin of error is usually listed in minutes or seconds of arc. Windows manufactured with an intentional wedge are often referred to as wedge prisms, and typically come with wedge angles of one, two, or three degrees. Many applications exist for wedge prisms, including laser-beam steering, rangefinding and variable focusing.

A pair of wedge prisms, called a Risley prism pair, can be used for beam steering. In this case, rotating one wedge in relation to the other will change the direction of the beam. When the wedges angle in the same direction, the angle of the refracted beam becomes greater. When the wedges are rotated to angle in opposite directions, they cancel each other out, and the beam is allowed to pass straight through.

Moving a wedge either closer or farther away from the laser can also be used to steer the beam. When the wedge is moved closer to the target (farther away from the laser), the refracted beam will move across the target. When two wedges in opposite directions slide relative to each other they can be used to provide variable focusing for cameras, allowing objects at vastly different distances to be photographed, in focus, at the same focal plane. This method is common in aerial or space launch-vehicle photography, when the distance to the object is changing very rapidly. Wedges were sometimes used in rangefinding, by combining the image formed by one telescope with the image formed by another.

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Wikipedia