Introduction
Optical prisms are transparent solid geometric elements with flat polished faces that redirect, deviate, or disperse light through refraction and total internal reflection (TIR). Unlike mirrors, prisms can perform complex beam manipulation without reflective coatings, making them robust and alignment-stable components in optical systems. This guide covers the four most common prism types, their working principles, and practical applications.
Right-Angle Prisms
The right-angle prism is the most fundamental and widely used prism type. It has a 90° angle between two perpendicular faces and 45° angles to the hypotenuse. When light enters one of the perpendicular faces and strikes the hypotenuse at 45° (beyond the critical angle for typical glass), it undergoes total internal reflection and exits through the other perpendicular face, deviating the beam by exactly 90°.
Working Principle
For BK7 (n = 1.517), the critical angle for TIR is arcsin(1/1.517) ≈ 41.2°. Since the 45° incidence angle at the hypotenuse exceeds the critical angle, TIR occurs with 100% reflectivity — no metallic or dielectric coating is needed. This makes right-angle prisms more efficient and durable than 90° turning mirrors in many applications.
Applications
- Beam deviation by 90° in periscope and viewfinder assemblies
- Image inversion and reversion in optical viewfinders
- Porro prism pairs for image erection in binoculars
- Compact beam folding in laser cavities and optical delay lines
When used in reverse (light entering the hypotenuse), a right-angle prism can split a beam into two outputs — functioning as a beamsplitter. PhotonEdge offers BK7 Right-Angle Prisms in sizes from 5 mm to 50 mm with λ/4 surface flatness and 40-20 surface quality.
Penta Prisms
A penta prism has two reflecting surfaces at 45° to each other, which deviates the beam by exactly 90° regardless of the incidence angle (within the acceptance range). This angle-constant property is the defining characteristic of penta prisms and distinguishes them from right-angle prisms.
Working Principle
The two internal reflections are at 22.5° each to the prism faces. Because the two reflecting surfaces are fixed at 45°, the deviation is always 2 × 45° = 90° by geometry, independent of the input beam angle. Unlike TIR-based prisms, the 22.5° incidence angle is below the critical angle, so the reflecting surfaces must be coated with a metallic or dielectric reflective coating.
Applications
- SLR camera viewfinders: The angle-constant deviation ensures the viewfinder image does not shift when the camera is tilted slightly
- Optical tooling and alignment: Used in autocollimators and alignment telescopes where precise 90° deviation must be maintained regardless of small angular errors
- Laser beam steering: When a beam must be turned exactly 90° even if the input angle varies
PhotonEdge manufactures Penta Prisms with precise 90° deviation tolerance (±5 arcsec or better) and protected Al or dielectric coatings on the reflecting surfaces.
Dove Prisms
A dove prism is a truncated right-angle prism with an apex angle typically of 45°. Light enters and exits at an angle, traveling through the prism along the length of the base. The key property of a dove prism is that it rotates the image by twice the rotation angle of the prism about its longitudinal axis.
Working Principle
When a dove prism is rotated by angle θ about its long axis (the axis parallel to the base), the transmitted image rotates by 2θ. This 2× image rotation property makes dove prisms ideal as image rotators in optical systems. The beam undergoes one TIR at the base, so the prism inverts the image in one axis even at zero rotation.
Applications
- Image rotation: Compensating for image rotation in scanning systems (e.g., aerial reconnaissance cameras where the aircraft heading changes)
- Interferometry: Rotating the fringe pattern in Twyman-Green and Fizeau interferometers for alignment
- Polarization optics: In conjunction with waveplates, dove prisms can create complex polarization transformations
Note that dove prisms introduce significant chromatic dispersion because the beam enters and exits at non-normal angles. They are best used with monochromatic or narrowband sources. PhotonEdge offers Dove Prisms optimized for visible and NIR wavelengths.
Corner Cube (Retroreflector) Prisms
A corner cube prism (also called a retroreflector or cat's eye retroreflector) has three mutually perpendicular reflecting surfaces. Any beam entering the prism is reflected off all three surfaces and exits parallel to the input beam, traveling in the opposite direction — regardless of the incidence angle (within the acceptance cone).
Working Principle
Each of the three TIR reflections reverses one component of the beam direction. After three reflections, all components are reversed, and the beam exits anti-parallel to the input. The retroreflected beam is laterally displaced but always parallel to the input, making corner cubes self-aligning retroreflectors.
Applications
- Laser ranging: Corner cubes are placed on satellites (e.g., lunar laser ranging retroreflectors on the Moon) and spacecraft for precise distance measurement by time-of-flight
- Optical interferometry: Used in Michelson interferometers where self-alignment eliminates mirror tilt errors
- Autocollimators: Corner cube retroreflectors return the beam to the source regardless of tilt, used for alignment verification
- Road reflectors: Mass-produced plastic corner cubes in road reflectors and safety markers
PhotonEdge's Corner Cube Prisms are available in BK7 and UV fused silica with angular deviation less than 3 arcsec, suitable for precision metrology and laser ranging applications.
Prism Selection Considerations
When selecting a prism for your application, consider these factors:
| Factor | Consideration |
|---|---|
| Material | BK7 for visible/NIR; UV fused silica for UV/high-power; CaF2 for IR |
| Surface flatness | λ/4 for general use; λ/10 for interferometry and metrology |
| Angular tolerance | Tight angles (±3 arcmin or better) are critical for penta and corner cube prisms |
| Coating | TIR surfaces need no coating; other surfaces may need AR or reflective coatings |
| Beam aperture | Ensure prism aperture accommodates full beam diameter with margin |
| Dispersion | Dove and wedge prisms introduce dispersion; limit to narrowband sources |
Conclusion
Optical prisms are versatile components that perform beam deviation, image rotation, and retroreflection through TIR and refraction, often without requiring reflective coatings. Right-angle prisms provide efficient 90° beam turning via TIR. Penta prisms deliver angle-constant 90° deviation for alignment-critical applications. Dove prisms rotate images at twice the prism rotation angle. Corner cube prisms retroreflect beams regardless of incidence angle, enabling self-aligning interferometry and laser ranging. By understanding each prism type's unique properties, you can select the right component for your optical system. PhotonEdge offers all four prism types in multiple materials with precision angular and surface specifications.