Anoptical prism is a transparentoptical element with flat, polished surfaces that are designed torefractlight. At least one surface must be angled—elements with two parallel surfaces arenot prisms. The most familiar type of optical prism is thetriangular prism, which has a triangular base and rectangular sides. Not all optical prisms aregeometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to thewavelengths for which they are designed. Typical materials includeglass,acrylic andfluorite.
Comparison of the spectra obtained from a diffraction grating by diffraction (1), and a prism by refraction (2). Longer wavelengths (red) are diffracted more, but refracted less than shorter wavelengths (violet).
Dispersive prisms are used to break up light into its constituent spectral colors because the refractive index depends onwavelength; 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.
Spectraldispersion 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 inbinoculars orsingle-lens reflex cameras – without the prisms the image would be upside down for the user.
Reflective prisms usetotal internal reflection to achieve near-perfect reflection of light that strikes the facets at a sufficiently oblique angle. Prisms are usually made ofoptical glass which, combined withanti-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
Schmidt–Pechan prism projects image forward, rotated by 180° (6 reflections [2 reflections are on roof plains]; composed of Bauernfeind part andSchmidt 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)
Variousthin-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 eliminateetalon effects, back-side reflection and slight beam deflection.
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 incommon-path interferometry.[1][2]
Another class is formed bypolarizing prisms which usebirefringence to split a beam of light into components of varyingpolarization. In the visible and UV regions, they have very low losses and theirextinction ratio typically exceeds, which is superior to other types ofpolarizers. They may or may not employ total internal reflection;
One polarization is separated by total internal reflection:
Both polarizations stay parallel, but are spatially separated:
polarisation beam displacers, typically made of thick anisotropic crystal with plan-parallel facets
These are typically made of abirefringent crystalline material likecalcite, but other materials likequartz andα-BBO may be necessary for UV applications, and others (MgF2,YVO4 andTiO2) will extend transmission farther into theinfrared spectral range.
Prisms made ofisotropic materials like glass will also alter polarization of light, aspartial reflection under oblique angles does not maintain the amplitude ratio (nor phase) of the s- andp-polarized components of the light, leading to generalelliptical polarization. This is generally an unwanted effect of dispersive prisms. In some cases this can be avoided by choosing prism geometry which light enters and exits under perpendicular angle, by compensation through non-planar light trajectory, or by use of p-polarized light.
Total internal reflection alters only the mutual phase between s- and p-polarized light. Under well chosen angle of incidence, this phase is close to.
Fresnel rhomb uses this effect to achieve conversion between circular and linear polarisation. This phase difference is not explicitly dependent on wavelength, but only on refractive index, so Fresnel rhombs made of low-dispersion glasses achieve much broader spectral range thanquarter-wave plates. They displace the beam, however.
Doubled Fresnel rhomb, with quadruple reflection and zero beam displacement, substitutes ahalf-wave plate.
Similar effect can also be used to make a polarization-maintaining optics.
Depolarization would not be observed for an ideal monochromaticplane wave, as actually both devices turn reducedtemporal coherence orspatial coherence, respectively, of the beam into decoherence of its polarization components.
Other uses
Total internal reflection in prisms finds numerous uses through optics, plasmonics and microscopy. In particular:
Prisms are used to couple propagating light tosurface plasmons. Either the hypotenuse of a triangular prism is metallized (Kretschmann configuration), or evanescent wave is coupled to very close metallic surface (Otto configuration).
Somelaser active media can be formed as a prism where the low-qualitypump beam enters the front facet, while the amplified beam undergoes total internal reflection undergrazing incidence from it. Such a design suffers less from thermal stress and is easy to be pumped by high-power laser diodes.
Other uses of prisms are based on their beam-deviating refraction:
Wedge prisms are used to deflect a beam of monochromatic light by a fixed angle. A pair of such prisms can be used forbeam steering; by rotating the prisms the beam can be deflected into any desired angle within a conical "field of regard". The most commonly found implementation is aRisley prism pair.[3]
Transparent windows of, e.g., vacuum chambers or cuvettes can also be slightly wedged (10' − 1°). While this does not reduce reflection, it suppressesFabry-Pérot interferences that would otherwise modulate their transmission spectrum.
Anamorphic pair of similar, but asymmetrically placed prisms can also change the profile of a beam. This is often used to make a round beam from the elliptical output of alaser diode. With its monochromatic light, slight chromatic dispersion arising from different wedge inclination is not a problem.
Deck prisms were used on sailing ships to bring daylight below deck,[4] since candles andkerosene lamps are a fire hazard on wooden ships.
By shiftingcorrective lenses offaxis, images seen through them can be displaced in the same way that a prism displaces images.Eye care professionals use prisms, as well as lenses off axis, to treat variousorthoptics problems:
Prism spectacles with a single prism perform a relative displacement of the two eyes, thereby correcting eso-, exo, hyper- or hypotropia.
In contrast, spectacles with prisms of equal power for both eyes, calledyoked prisms (also:conjugate prisms,ambient lenses orperformance glasses) shift the visual field of both eyes to the same extent.[5]
^Kaplan, M; Carmody, D. P.; Gaydos, A (1996). "Postural orientation modifications in autism in response to ambient lenses".Child Psychiatry and Human Development.27 (2):81–91.doi:10.1007/BF02353802.PMID8936794.S2CID37007723.
