Common Combinatorial Prisms

Common Combinatorial Prisms

Schmidt-Pechan Prism

The Schmidt-Pechan prism is a combination of the Baernfeld prism and the Schmidt prism that we introduced earlier. The incident light beam undergoes five reflections within the combined prism and exits in the original direction, as shown in the figure below: [Image] Since the light beam undergoes multiple reflections inside the combined prism, there will inevitably be a loss of light energy. To maintain the beam's energy, it is necessary to coat the reflective surfaces with internal reflection films or to achieve total internal reflection within the prism. To achieve total internal reflection, an air gap of a certain width needs to be left between the two prisms. A roof surface can also be set at the second prism, causing the exiting beam to have a 180° image rotation relative to the incident beam. Sometimes, multiple total internal reflections can alter the polarization state of the light, causing phase delays between different polarization components. Therefore, special phase correction films need to be applied to improve this, to avoid unnecessary interference phenomena in the image.

Uppendahl Prism

The Uppendahl prism is made up of three prisms glued together, without the need for an air gap. After the third prism is equipped with a roof surface, its function is the same as that of the Schmidt-Pechan prism introduced above, enabling a 180° rotation of the image and exiting in the same direction, as shown in the figure below: 

Daubresse Prism

The Daubresse prism is composed of a pentagonal prism and a right-angle prism. It does not change the propagation direction of the light beam but shifts it a certain distance v before exiting. Its structure and optical path are shown in the figure below, where the pentagonal prism includes a roof surface, so the exiting beam has a 180° rotation relative to the incident beam.

Pellin-Broca Prism

The Pellin-Broca prism is a combination of a reflective prism and a dispersive prism. Its function is similar to the constant deviation prism mentioned in the previous article. The prism structure and optical path are shown in the figure below: [Image] As seen from the figure, since the incident and exiting beams are not perpendicular to the prism surfaces, the exiting wavefront will exhibit some chromatic aberration. In practice, specific optical materials with appropriate Abbe numbers are chosen to reduce the amount of chromatic aberration.

Abbe Prism with Roof Edge

This type of prism is made up of three equiangular Amici single prisms glued together, with the middle prism equipped with a roof surface, as shown in the figure below: [Image] The light beam entering from the left end will undergo a 180° image flip after passing through the prism and exit in the original direction.

Abbe-Koenig Prism

The Abbe-Koenig prism is a reflective optical prism that can achieve image redirection and is often used in binoculars. Its optical path is shown in the figure below: [Image] As seen from the figure, the Abbe-Koenig prism is made up of two glass prisms glued together, forming a symmetrical shallow V-shaped combination. The light beam enters from the left end perpendicular to the surface, undergoes total internal reflection at the 30° inclined surface, is reflected at the other right-angle surface, and then undergoes total internal reflection again at the surface of the second prism before exiting. The direction of the light beam remains unchanged, but the image is inverted up and down. If the bottommost surface is set as a roof surface, a 180° rotation of the image can be achieved.

Rantsch Prism

The Rantsch prism is made up of three separate prisms: one is an equilateral prism, and the other two are right-angle prisms with a 30° acute angle. Its structure and optical path are shown in the figure below: [Image] The light beam enters from the left end, undergoes three reflections within the prism, and exits. The exiting beam has a certain longitudinal shift from the incident beam, and the image of the exiting beam is a mirror image of the incident beam.

Cranz Prism

The Cranz prism still serves to image the light beam without changing its propagation direction. A representative Cranz prism is shown in the figure below: [Image] In the figure, there are two Baernfeld single prisms on both sides. The incident light beam undergoes five reflections inside the prism and exits from the right end. The image of the exiting beam is a mirror image of the incident beam.

Dialyt Prism

This prism is used in a binocular produced by the German company Zeiss. Its structure and optical path are shown in the figure below: [Image] This prism is a combination of the Amici prism and the Baernfeld prism introduced earlier, with a roof surface set on the second prism. The light beam entering from the left end undergoes three internal reflections and is rotated 180° before exiting from the right end in the same direction as the incident beam.

Porro Prism Pair

The Porro prism pair is widely used in binoculars, and its position in the binocular is shown in the figure below: [Image] When observing distant scenes with binoculars, the light beam entering the binoculars reaches the Porro prism pair area after passing through the objective lens. The two prisms have their inclined surfaces facing each other, enabling the light beam to be shifted and bent, thereby being transmitted to the eyepiece to form an image.

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