An Uppendahl prism is an erecting prism, i.e. a special reflection

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

that is used to invert an image (rotation by 180°). The erecting system consists of three partial prisms made of optical glass with a high

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...

cemented together to form a symmetric assembly and is used in

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of mi ...

as well as in

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes ( binocular vision) when viewing distant objects. Most binoculars are sized to be hel ...

technology. Leitz Trinovid 8x20 compact binoculars 1

In the past the Uppendahl prism system, for example in the

Trinovid Trinovid is the protected model designation of a roof prism binoculars series from the company Leitz (optics) (since 1986 Leica Camera) based in Wetzlar, a German centre for optics as well as an important location for the precision engineering ...

binoculars series from

Leitz Leitz may refer to several German companies: * Esselte Leitz GmbH & Co KG, founded by Louis Leitz in 1896, a German manufacturer of office products ** Louis Leitz (1846–1918), German inventor and founder of Esselte Leitz GmbH & Co KG * Leitz GmbH ...

(since 1986

Leica Leica Camera AG () is a German company that manufactures cameras, optical lenses, photographic lenses, binoculars, rifle scopes and microscopes. The company was founded by Ernst Leitz in 1869 (Ernst Leitz Wetzlar), in Wetzlar, Germany. ...

), was commercially offered in some binoculars. The Trinovid series binoculars were introduced in 1958 and used at the time patented moving internal optical lenses between the ocular lens group and the prism assembly within the housing for

focusing Focusing may refer to: * Adjusting an optical system to minimize defocus aberration * Focusing (psychotherapy), a psychotherapeutic technique See also *Focus (disambiguation) Focus, or its plural form foci may refer to: Arts * Focus or Focus Fe ...

. Like the much more common optical lenses located between the objective lens group and the prism assembly method, this central internal focussing method does not change the volume of the binoculars.

Bausch & Lomb Bausch + Lomb is an eye health products company based in Vaughan, Ontario, Canada. It is one of the world's largest suppliers of contact lenses, lens care products, pharmaceuticals, intraocular lenses, and other eye surgery products. The compan ...

Elite and Browning 7×35 binoculars, both made in Japan during the late 1980s to early 1990s, also used Uppendahl prisms. In the early 2020s the commercial market share of Uppendahl prism type standard binoculars was nil. The Leica Geovid R (laser) rangefinder binoculars series and 7×24 Rangemaster monocular using a (modified) Uppendahl prism system were still commercially available.

Method of operation

The Uppendahl prism system is composed of three cemented prisms, with two glass/air transition surfaces. On its way through the first prism, the bundle of rays (red) is first reflected on a surface that is coated with either a metallic or a dielectric coating (mirroring) and a total internal reflection face just like the one used in a Schmidt-Pechan cluster only the light enters and leaves through the opposite ends as used in the Uppendahl. The other reflections of the beam take place by means of loss-free total internal reflection. The second prism is a 90° reflection and shouldn't need a mirroring coating. In order to achieve a complete reversal of the image, a roof edge is ground into the third prism (green). Furthermore, the beam leaves the inversion system without any axial offset, which is why the Uppendahl prism is counted among the straight-vision

roof prism A roof prism, also called a Dachkanten prism or Dach prism (from German: ''Dachkante'', lit. "roof edge"), is a reflective prism containing a section where two faces meet at a 90° angle, resembling the roof of a building and thus the name. Re ...

s. The net effect of the six reflections (two reflections are on roof plains). Since the light is reflected an even number of times, this produces a 180° image rotation (without changing the image's

handedness In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to it being stronger, faster or more dextrous. The other hand, comparatively often the weaker, less dextrous or simply less subjec ...

) and allows use of the prism as an ''image erecting system'' to flip the image both vertically and horizontally. An advantage of this prism system is that the light beam only passes two transitions between air and glass, which minimizes losses in the form of

Fresnel reflection The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media. They were deduced by Augustin-Jean Fresne ...

s. The relatively strong folding of the beam path in the Uppendahl prism, which is only comparable with the compact Schmidt-Pechan prism system, supports the construction of compact optical instruments with short overall lengths.

Problems with the Uppendahl prism

The Uppendahl roof prism system is from a purely technical point of view a rather complicated roof prism design. Light entering the Uppendahl design reflects more times and less efficient than in the Abbe-König prism design.

Reflection losses

Total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflecte ...

does not occur. To mitigate this problem, a mirror coating is used on a surface. Typically an aluminum mirror coating (

reflectivity The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electroni ...

of 87% to 93%) or silver mirror coating (reflectivity of 95% to 98%) is used. The transmission of the prism can be further improved by using a dielectric coating rather than a metallic mirror coating. This causes the prism surfaces to act as a

dielectric mirror A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickne ...

. A well-designed dielectric coating can provide a reflectivity of over 99% across the visible light spectrum. This

is much improved compared to either an aluminum or silver mirror coating and the performance of the Uppendahl prism is similar to the Porro prism or the Abbe–Koenig prism. The necessary mirror coating not only adds a manufacturing step, but it makes the Uppendahl roof prism lossier than the other image erectors using Porro prism or Abbe–Koenig prism that rely only on total internal reflections. A dielectric mirror coating is comparable in reflection effectivity, but makes the Uppendahl more expensive.

Phase correction

The Uppendahl furthermore shares the phase correction problems with other roof prisms. Uppendahl prism and other roof prism binoculars benefit from phase-correction coatings to minimize these problems and substantially improve resolution and contrast.Why do the best roof-prism binoculars need a phase-correction coating?
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Maximal light transmission

In order to achieve maximum light transmission, the Uppendahl prism system should be provided with an

anti-reflective coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the eff ...

on the incidence and exit surfaces. In addition, a high-quality

ing and phase correction coatings should be used on both roof surfaces.

References