Comprehensive Summary of Optical Prism Types (Part 2)
2025.01.21
2. Summary of Common Optical Prisms
2.1 Bauernfeind Prism
In this type of prism, the light beam enters perpendicularly to one surface, is reflected twice inside the prism, and exits perpendicularly from another surface. There are two usage modes: forward use and reverse use. In forward use, the light enters through the inclined surface and exits from the short surface. In reverse use, the light enters through the short surface and exits through the inclined surface.
The characteristic of this type of prism is that the angle δ between the outgoing light and the incident beam is equal to the base angle α of the prism, and the other base angle β of the prism is half of α. By using this type of prism, the incident beam can be deflected by angles of 30°, 45°, or 60°. Since there are two reflections inside the prism, attention must be paid to energy loss of the light beam. If necessary, a reflective coating should be applied to the reflective surfaces.
2.2 90°-Bauernfeind Prism
When the light beam enters through the short surface, and both the angle of incidence and the base angle of the prism are 45 degrees, the outgoing beam will be emitted in a direction perpendicular to the incident beam. At this point, the outgoing beam will not be perpendicular to the exit surface.
The characteristic of this type of prism is that as long as the light beam undergoes two reflections inside, the outgoing beam will remain perpendicular to the incident beam, even if there is a slight change in the angle of incidence. It is important to note that since the incident and outgoing beams are not perpendicular to the entry and exit surfaces, this type of prism may introduce some aberrations and dispersion during use.
2.3 Dove Prism
The cross-section of a Dove prism is trapezoidal. The light beam enters through the inclined surface on one side of the trapezoid, is reflected once inside the prism, and exits through the inclined surface on the opposite side.
An upright image is inverted by 180 degrees after passing through the Dove prism. Therefore, the Dove prism is commonly used as an image rotator. Since the angle of incidence is not perpendicular to the entry surface, when the Dove prism is unfolded along the reflective surface, it behaves like an inclined parallel plate. The light beam undergoes dispersion inside the prism, and different colors of light will emerge in parallel at the exit end.
2.4 Wollaston Prism
In the Wollaston prism, the light beam enters perpendicularly to the incident surface, is reflected twice, and exits perpendicularly to the exit surface. The deflection angle between the outgoing beam and the incident beam is 90 degrees.
The difference from the pentaprism is that the light beam inside the Wollaston prism undergoes total internal reflection, allowing for total reflection without the need for additional coatings on the prism surface. In practice, two Amici prisms can be cemented together to form a Wollaston prism, as shown in the right diagram above. The base angle of the Amici prism is 67.5 degrees.
In addition to the two reflections mentioned above, when the height of the incident beam is raised, the beam can undergo four reflections inside the Wollaston prism and exit perpendicularly to the exit surface, with a deflection angle of 90 degrees.
2.5 Sprenger–Leman Prism
This prism is named after its inventor. After the light beam enters perpendicularly to the surface, it undergoes three reflections inside the prism and exits in the original direction. The exit direction is perpendicular to the exit surface, with a longitudinal shift relative to the incident beam.
The acute angle at the point where the light enters the prism is 30 degrees. In this case, the displacement distance v of the light beam is twice the beam diameter D.
2.6 Huet Prism
In the Huet prism, the light beam still enters perpendicularly to the incident surface and exits perpendicularly to the exit surface, with the outgoing beam aligned in the same direction as the incoming beam. The light undergoes five reflections inside the prism, allowing for a greater displacement of the light beam.
2.7 Corner Cube Prism
The corner cube prism, also known as a retroreflector, consists of three mutually perpendicular right-angle surfaces, resembling a corner cut from a cube. It allows an incoming light beam to be reflected three times inside the prism and then exit in the direction exactly opposite to the incoming beam, effectively deflecting the light by 180 degrees. The reflections inside the prism are total internal reflections, resulting in no energy loss. The physical appearance of the corner cube prism is shown in the diagram below. It is commonly used in fields such as laser ranging and projection.
2.8 Constant Deviation Prism
By choosing an appropriate incident direction, a regular three-sided prism can provide a constant beam deviation angle. Below is an example of a constant deviation prism:
In the prism shown above, the angles between the long side and the two short sides are α-β and α+β, respectively. When the light beam enters through the short side at an angle of α-β, the deviation angle δ between the outgoing beam and the incident beam is independent of the incident angle and remains constant at 180°-2α. However, neither the incident nor the outgoing light is perpendicular to the entry or exit surface. Examples of this type of prism include the Abbe prism and the Pellin-Broca prism.
2.9 Littrow Prism
As a special case of the constant deviation prism, the Littrow prism can return the incident beam along its original path, exhibiting autocollimation properties. The optical path is shown in the diagram below:
2.10 Wedge Prism
A wedge prism is a prism element with a specific wedge angle, being thick at one end and thin at the other, as shown in the diagram below.
Using a wedge prism, the transmitted light beam can be deflected in direction. Another common application of the wedge prism is beam separation. When a beam of light passes through a wedge prism, it is divided into two beams: one is reflected, and the other is transmitted. The angle of beam separation can be controlled by adjusting the angle of the prism or by changing the refractive index of the material used to make the prism, which makes wedge prisms widely used in laser systems.
3. Conclusion
In this article, building on the previous one, we have summarized and introduced the structures of ten additional optical prisms, including the Bauernfeind prism, Dove prism, and corner cube prism, among others. It is recommended that readers save this article so they can refer back to it for targeted information when encountering these prisms in their work. It is worth noting that the prisms introduced above are all single-element prisms. In the next article, we will discuss integrated prisms composed of two or more prisms, which also have many applications in optical systems. Stay tuned!
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