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What is a Penta Prism?

Prism is a piece of triangular glass or plastic. Prism works because different colors of light propagate at different speeds inside the glass. Because colors of light propagate at different speeds, they will bend different amounts, and then all scatter instead of mixing together.


Introduction to Pentaprism


Pentaprism is a type of five-face reflective prism used to deflect a light beam at a constant 90°, even if the incident light beam is not at 90° to the prism. The light beam reflects twice inside the prism, allowing the image to be transmitted at a right angle without being inverted (i.e. without changing the rotational direction of the image), unlike conventional right-angle prisms or mirrors.


The internal reflection in the prism is not caused by total internal reflection because the light beam enters at an angle smaller than the critical angle (the minimum angle for total internal reflection). Instead, both faces are coated to provide a mirror. The two opposing transmission surfaces are usually coated with anti-reflection coatings to reduce stray reflections. The fifth face of the prism is not optically utilized but truncates the awkward angle connecting the two mirrored surfaces.


A variation of this prism is a rooftop Penta Prism, which is commonly used in the viewfinders of single-lens reflex cameras. The camera lens presents an image that is both vertically and horizontally inverted, and the reflective prism re-inverts it, leaving an image that is horizontally inverted. In this case, when the prism transmits the image formed on the camera's focusing screen, the image needs to be reflected from left to right. This horizontal inversion is achieved by replacing one of the reflective surfaces of a normal Pentaprism with a "roof" section, and the other two surfaces are angled toward each other and intersect at 90° to re-invert the image horizontally to its normal orientation.


A standard Penta Prism can be used as an imaging component and as a laser insertion prism for ranging and advanced monitoring. These optical devices provide stable deviations that do not change when the prism is slightly rotated, making them valuable in situations where precise deviation is required but precise alignment is impossible or difficult. This same characteristic also provides stability and allows it to provide good imaging even with slight movement.


What is the function of a Pentaprism?

A Penta Prism is a type of five-face reflective prism used to deflect a light beam at a constant 90°, even if the incident light beam is not at 90° to the prism. The fifth face of the prism is not optically utilized but truncates the awkward angle connecting the two mirrored surfaces.

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