Final article in the series explores reflective prismsAbbe-König prism
Named after Ernst Abbe and Albert König, the Abbe-König prism is used to invert images 180° and is commonly used in binoculars and some types of KTV. It is made of two glass prisms tightly attached together to form a symmetrical dwarf V shape. Light enters perpendicular to one face, is completely reflected on a 30° inclined surface, and then continues to be reflected on the “roof” portion at the bottom of the prism. The light is then reflected on the opposite surface inclined at 30° and then exits perpendicular to the surface 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 image 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 very useful in some tools. Additionally, this prism system is also more compact than the dual Porro system.
Prism holder tray
Abbe-König prisms are sometimes called “roof prisms”, although this is confusing because there are many different types of roof prisms. A variation of the Abbe-König prism replaces the “roof” element with a reflectively coated surface. This type of prism. This type of prism flips the image vertically, but not horizontally, and changes the left-right nature of the image.
Schmidt-Pechan prism
Schmidt-Pechan prisms are used to rotate images 180° and are often used in binoculars as an image inversion system. Compared to binoculars using double Porro or Abbe-Koenig prisms, binoculars using Schmidt-Pechan prisms are more compact and lightweight.
The Schmidt–Pechan prism is a combination of the Schmidt and Pechan prisms. The Pechan prism is made up of 2 prisms separated by an air gap. It has the function of inverting the image or flipping the image depending on the direction 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 is made up of 2 prisms separated by an air gap. The second prism, Schmidt, simultaneously inverts and flips the image, but changes the image angle by 45°. The first prism retracts by deflecting the ray up to 45° before entering the Schmidt prism. The two prisms are designed so that the incident and emerging rays are coaxial if the system is correctly collimated on the optical axis.
The lower prism applies total internal reflection once, followed by reflection from the mirror-covered surface to direct the beam to the second Schmidt prism, so that the direction of rotation of the image remains unchanged. The upper prism inverts the image by 3 times the total reflection in the longitudinal plane passing through the roof axis. The “roof” part flips the image horizontally thanks to 2 total reflections on the 2 roofs in the horizontal plane. This pair of total reflections can be considered as 1 total reflection in the sagittal plane. The combination of invert and flip rotates the image 180°. The left-right properties of the image remain unchanged.
The reflection at the lower surface of the first prism is not due to total reflection because the angle of incidence is smaller 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 emerging beam surfaces must be optically covered to minimize light loss. The type of coating must be carefully chosen because the same face of the prism can both play the role of passing light rays (requiring a good anti-reflective coating), and also play the role of total reflection (requiring a reflective coating). ).maximum radiation). Research conducted by Konrad Seil at Swarovski Optik in the article “Advances in Binocular Design” shows that a single anti-reflective coating results in images with the most optimal contrast.
Loss due to reflection
When light rays strike the glass-air interface with an angle of incidence less than the critical angle, there is no total internal reflection. To solve the above problem, people cover these surfaces with mirrors, usually aluminum (87-93% reflection) or silver (95-98% reflection). The light transmission of a 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 multi-layer dielectric coating increases the reflectivity of the prism surface by acting as a distributed Bragg reflector. A well-designed dielectric coating can provide greater than 99% reflectance across the entire visible light spectrum, a significant improvement over aluminum or silver coatings, and the quality of a Schmidt-Pechan prism is equivalent to that of a Schmidt-Pechan prism. Porro or Abbe-Koenig glass.
The requirement for a mirror coating makes Schmidt-Pechan prisms more expensive than other types of image inversion systems that rely solely on total internal reflection, such as Porro prisms or Abbe-Koenig.
Phase correction
Multiple total reflections result in a phase delay in the transmitted light as a function of polarization, similar to that of rhombic Fresnel crystals. This must be limited by multi-layer phase correction coatings placed on one 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 it passes through the top prism. When the two polarization components are recombined, the interference between s- and p-polarized light produces different intensity distributions 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.
Roof edge diffraction effects can also appear as diffraction spikes perpendicular to the roof edge, created by bright spots in the image. In addition, this effect is also manifested by an elongation of the Airy disk in the direction perpendicular to the roof peak. This is diffraction due to a discontinuity at the top of the roof.
