Definition of Abbe-Porro prism

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Photonics Dictionary

Abbe-Porro prism: An Abbe-Porro prism, often referred to simply as a Porro prism, is an optical prism used in binoculars and other optical instruments to invert and reverse an image. It is named after the Italian optician Ignazio Porro who invented it, and Ernst Abbe, who refined and popularized its use.

Features:

Image erection: The primary function of the Porro prism is to rotate an image by 180 degrees. This inverts and reverses the image, which is essential for correcting the orientation of the image seen through optical devices like binoculars.

Design: The Porro prism typically consists of two prisms arranged in a Z-shape configuration. This arrangement allows the light to be reflected multiple times within the prisms, effectively flipping the image.

Light path: In a Porro prism, light enters the first prism, reflects off its surfaces, travels to the second prism, and reflects again before exiting. The multiple reflections cause the inversion and reversal of the image.

Advantages:

Improved depth perception: The Z-shape configuration of Porro prisms allows the objective lenses to be spaced further apart compared to roof prisms, enhancing depth perception and providing a wider field of view.

Optical efficiency: Porro prisms can provide high-quality image transmission with less light loss compared to some other prism types.

Applications: Besides binoculars, Porro prisms are used in various optical instruments where image orientation correction is needed, such as periscopes and some camera viewfinders.

Diagram of a Porro prism: A simple diagram of a Porro prism arrangement would show two prisms aligned in such a way that light enters the first prism, reflects internally, passes to the second prism, reflects again, and exits in the inverted and reversed orientation.

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