Editing Beam splitter (section)

==Designs==
In its most common form, a cube, a beam splitter is made from two triangular glass [[prism (optics)|prism]]s which are glued together at their base using polyester, [[epoxy]], or urethane-based adhesives. (Before these synthetic [[resin]]s, natural ones were used, e.g. [[Canada balsam]].) The thickness of the resin layer is adjusted such that (for a certain [[wavelength]]) half of the light incident through one "port" (i.e., face of the cube) is [[reflection (physics)|reflected]] and the other half is transmitted due to [[Total internal reflection#Frustrated_TIR|FTIR (frustrated total internal reflection)]]. [[polarizer|Polarizing beam splitters]], such as the [[Wollaston prism]], use [[birefringent]] materials to split light into two beams of orthogonal [[polarization (waves)|polarization]] states.

[[File:Flat metal-coated beamsplitter.png|thumb|left|Aluminium-coated beam splitter.]]
Another design is the use of a half-silvered mirror. This is composed of an optical substrate, which is often a sheet of glass or plastic, with a partially transparent thin coating of metal. The thin coating can be [[aluminium]] deposited from aluminium [[vapor]] using a [[physical vapor deposition]] method. The thickness of the deposit is controlled so that part (typically half) of the light, which is incident at a 45-degree angle and not absorbed by the coating or substrate material, is transmitted and the remainder is reflected. A very thin half-silvered mirror used in [[photography]] is often called a [[pellicle mirror]]. To reduce loss of light due to absorption by the reflective coating, so-called "[[Swiss cheese (North America)|Swiss-cheese]]" beam-splitter mirrors have been used. Originally, these were sheets of highly polished metal perforated with holes to obtain the desired ratio of reflection to transmission. Later, metal was [[sputtering|sputter]]ed onto glass so as to form a discontinuous coating, or small areas of a continuous coating were removed by chemical or mechanical action to produce a very literally "half-silvered" surface.

Instead of a metallic coating, a [[dichroic]] [[optical coating]] may be used. Depending on its characteristics ([[thin-film interference]]), the ratio of reflection to transmission will vary as a function of the [[wavelength]] of the incident light. Dichroic mirrors are used in some [[ellipsoidal reflector spotlight]]s to split off unwanted [[infrared]] (heat) radiation, and as [[output coupler]]s in [[laser construction]].

A third version of the beam splitter is a [[dichroic prism|dichroic mirrored prism]] assembly which uses [[dichroism|dichroic]] [[optical coating]]s to divide an incoming light beam into a number of spectrally distinct output beams.  Such a device was used in three-pickup-tube color [[television camera]]s and the three-strip [[Technicolor]] movie camera. It is currently used in modern three-CCD cameras. An optically similar system is used in reverse as a beam-combiner in three-[[LCD]] [[Image projector|projectors]], in which light from three separate monochrome LCD displays is combined into a single full-color image for projection.

Beam splitters in [[Passive optical network|PON networks]] are often made with [[single-mode optical fiber]], by exploiting [[evanescent wave]] coupling between a pair of fibers to share the beam between them.<ref>{{cite web |last1=Paschotta |first1=Rüdiger |title=Beam Splitters |url=https://doi.org/10.61835/mjw |website=RP Photonics Encyclopedia |publisher=RP Photonics AG |access-date=15 March 2025 |doi=10.61835/mjw}}</ref> The splitter is constructed by fusing together the two parallel bare fibers at one point.<ref>{{cite web |last1=Paschotta |first1=Rüdiger |title=Tutorial: Passive Fiber Optics, Part 8: Fiber Couplers and Splitters |url=https://www.rp-photonics.com/tutorial_passive_fiber_optics8.html |website=RP Photonics Encyclopedia |publisher=RP Photonics AG |access-date=15 March 2025}}</ref>

Arrangements of mirrors or prisms used as camera attachments to photograph [[stereoscopy|stereoscopic]] image pairs with one lens and one exposure are sometimes called "beam splitters", but that is a misnomer, as they are effectively a pair of [[periscope]]s redirecting rays of light which are already non-coincident. In some very uncommon attachments for stereoscopic photography, mirrors or prism blocks similar to beam splitters perform the opposite function, superimposing views of the subject from two different perspectives through color filters to allow the direct production of an [[anaglyph 3D]] image, or through rapidly alternating shutters to record [[Active shutter 3D system|sequential field 3D]] video.