An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled—elements with only two parallel surfaces are windows, not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, acrylic and fluorite. A dispersive prism can be used to break white light up into its constituent spectral colors (the colors of the rainbow) to form a spectrum as described in the following section. Other types of prisms noted below can be used to reflect light, or to split light into components with different polarizations.
Types
Dispersive
Dispersive prisms are used to break up light into its constituent spectral colors because the refractive index depends on wavelength; the white light entering the prism is a mixture of different wavelengths, each of which gets bent slightly differently. Blue light is slowed more than red light and will therefore be bent more than red light.
Abbe prism Amici prism and other types of compound prisms Féry prism Grism, a dispersive prism with a diffraction grating on its surface Littrow prism with mirror on its rear facet Pellin–Broca prism Triangular prism Spectral dispersion is the best known property of optical prisms, although not the most frequent purpose of using optical prisms in practice.
Reflective Reflective prisms are used to reflect light, in order to flip, invert, rotate, deviate or displace the light beam. They are typically used to erect the image in binoculars or single-lens reflex cameras – without the prisms the image would be upside down for the user. Reflective prisms use total internal reflection to achieve near-perfect reflection of light that strikes the facets at a sufficiently oblique angle. Prisms are usually made of optical glass which, combined with anti-reflective coating of input and output facets, leads to significantly lower light loss than metallic mirrors.
Odd number of reflections, image projects as flipped (mirrored) triangular prism reflector, projects image sideways (chromatic dispersion is zero in case of perpendicular input and output incidence) Roof pentaprism projects image sideways flipped along the other axis Dove prism projects image forward Corner-cube retroreflector projects image backwards Even number of reflections, image projects upright (without change in handedness; may or may not be rotated) Porro prism projects image backwards and displaced Porro–Abbe prism projects image forward, rotated by 180° and displaced Perger prism a development based on the Porro–Abbe prism, projects image forward, rotated by 180° and displaced Abbe–Koenig prism projects image forward, rotated by 180° and collinear (4 internal reflections [2 reflections are on roof plains]) Bauernfeind prism projects image sideways (inclined by 45°) Amici roof prism projects image sideways Pentaprism projects image sideways Schmidt–Pechan prism projects image forward, rotated by 180° (6 reflections [2 reflections are on roof plains]; composed of Bauernfeind part and Schmidt part) Uppendahl prism projects image forward, rotated by 180° and collinear (6 reflections [2 reflections are on roof plains]); composed of 3 prisms cemented together)
Beam-splitting
Various thin-film optical layers can be deposited on the hypotenuse of one right-angled prism, and cemented to another prism to form a beam-splitter cube. Overall optical performance of such a cube is determined by the thin layer. In comparison with a usual glass substrate, the glass cube provides protection of the thin-film layer from both sides and better mechanical stability. The cube can also eliminate etalon effects, back-side reflection and slight beam deflection.
dichroic color filters form a dichroic prism Polarizing cube beamsplitters have lower extinction ratio than birefringent ones, but less expensive Partially-metallized mirrors provide non-polarizing beamsplitters Air gap − When hypotenuses of two triangular prisms are stacked very close to each other with air gap, frustrated total internal reflection in one prism makes it possible to couple part of the radiation into a propagating wave in the second prism. The transmitted power drops exponentially with the gap width, so it can be tuned over many orders of magnitude by a micrometric screw. Biprism (or Fresnel biprism): two prisms joined at their bases, forming a wide vertex angle (~ 180°); used in common-path interferometry.
Polarizing
Another class is formed by polarizing prisms which use birefringence to split a beam of light into components of varying polarization. In the visible and UV regions, they have very low losses and their extinction ratio typically exceeds 10 5 : 1 {\displaystyle 10^{5}:1} , which is superior to other types of polarizers. They may or may not employ total internal reflection;
… excerpt ends here. Continue reading the full article.
