The last article in the series on reflecting prismsAbbé-König. Prism
Named for Ernst Abbe and Albert König, the Abbe-König prism is used for 180° inversion and is commonly used in binoculars and some types of KTVs. It is made up of two glass prisms fixed together to form a symmetrical dwarf V shape. The light enters perpendicular to a face, is totally reflected on an inclined plane of 30°, then is reflected again at the level of the “roof” part at the base of the prism. The light is then reflected on the opposite inclined plane of 30° and exits perpendicular to the face of the prism.
Total reflections cause the image to flip both vertically and horizontally. As a result, the image rotates 180° without tilting from left to right. Therefore, the Albert König prism system is used as an inversion system. Unlike the conventional double Porro prism system, the Albert König prism does not cause the emerging ray to move relative to the incident ray. This is useful in some tools. In addition, this prism system is also more compact than the double Porro system.
Abbe-König prisms are sometimes referred to as “roof prisms”, although this is confusing as there are many different types of roof prisms. A variant of the Abbe-König prism replaces the “roof” element with a reflective coated surface. This type of prism. This is a type of prism that flips the image vertically, but not horizontally, and changes the left-right orientation of the image.
The Schmidt-Pechan prism is used to rotate the image 180° and is often used in binoculars as an inversion system. Compared to binoculars using double Porro or Abbe-Koenig prisms, Schmidt-Pechan prism binoculars are more compact.
The Schmidt-Pechan prism is a combination of the two prisms of Schmidt and Pechan. The Pechan prism is made up of 2 prisms separated by an air gap. It has the function of inverting or flipping the image depending on the orientation of the prism, but not both at the same time. By replacing the second prism of the Pechan system with a Schmidt prism, the Schmidt-Pechan prism can simultaneously invert and flip the image and thus act as an image rotation device.
The Schmidt-Pechan prism consists of two prisms separated by an air gap. The second prism, Schmidt, simultaneously inverts and flips the image, but shifts the image by 45°. The first prism corrects by deflecting the ray 45° before entering the Schmidt prism. The two prisms are designed so that the incident ray and the emerging ray are coaxial if the system is correctly collimated axially.
The lower prism applies a total reflection once, followed by a reflection from the mirrored surface to direct the beam to the second Schmidt prism, so that the direction of rotation of the image remains constant. The prism above the image island is equal to 3 times the total reflection in the longitudinal plane passing through the axis of the roof. The “roof” unit flips the image horizontally thanks to 2 total reflections on 2 roofs in the horizontal plane. This pair of total reflections can be considered as a total reflection in the longitudinal plane. Image inversion and image flip combined will rotate the image 180°. The left-right orientation of the image remains unchanged.
The reflection at the base of the first prism is not due to total reflection because the angle of incidence is less than the critical angle. This is different from other types of roof prisms such as the Abbe-Koenig prism. This side of the Schmidt-Pechan prism requires a reflective coating.
The problems encountered:
Glass-Air transition layer
All incident and exposed beam surfaces should be optically coated to minimize light loss. The type of coating must be chosen with care because the same face of the prism can act both as a passing light ray (requires a good antireflection coating) and as a total reflector (requires a reflective coating (maximum radiation). Research carried out by Konrad Seil at Swarovski Optik in his article “Advances in Binocular Design” show that a single anti-reflective coating gives images with the best contrast.
Loss due to reflection
When the light ray strikes the glass-air interface with an angle of incidence less than the critical angle, there is no total internal reflection. To overcome this problem, people cover the surfaces with mirrors, usually aluminum (87-93% reflectivity) or silver (95-98% reflectivity). The light transmission of the prism can be improved by using a dielectric coating instead of a metallic mirror coating. The surface of the prism then acts as a dielectric mirror. The multilayer dielectric coating increases the reflectivity of the prism surface by acting as a distributed Bragg reflector. A well-designed dielectric coating can provide over 99% reflectivity across the visible light spectrum, a significant improvement over aluminum or silver coatings, and the quality of a Schmidt-Pechan prism is comparable to that of a prism. Porro or Abbe- Koenig glasses.
The requirement for a mirror coating makes Schmidt-Pechan prisms more expensive than other types of inversion systems that rely solely on total reflection, such as Porro or Abbe-Koenig prisms.
Multiple total reflection causes a phase shift of transmitted light as a function of polarization, similar to that of rhombohedral Fresnel crystals. This should be mitigated by multi-layer phase correction coatings placed on a roof surface, to avoid unwanted interference effects as well as loss of image contrast.
In a roof prism without a phase correction coating, s-polarized and p-polarized light reach different geometric phases as they pass through the top prism. When the two polarizing components are recombined, the interference between the s and p polarized light produces a different intensity distribution between the direction perpendicular to the roof edge and along the roof edge. This effect reduces the contrast and resolution of the image perpendicular to the edge of the roof, resulting in lower image quality than the Porro prism.
The roof edge diffraction effect can also appear as a diffraction spike perpendicular to the roof edge, produced by bright spots in the image. In addition, this effect is manifested by an elongation of the Airy disc in a direction perpendicular to the roof. This is the diffraction due to the discontinuity at the top of the roof.