Beam splitter

A beam splitter (also written beamsplitter) is an optical component that divides an incident beam of light into two or more beams, which may or may not carry the same optical power. The same device can be run in reverse to combine two beams into one. Common forms include partially reflecting plates, cubes made from two cemented prisms, thin pellicle membranes, polarizing beam splitters and dichroic (wavelength-selective) mirrors.

In virtual reality (VR) and augmented reality (AR) hardware, beam splitters are used wherever two light paths have to share the same space in front of the eye. A partially reflecting surface was the see-through element of Ivan Sutherland's 1968 head-mounted display. Beam splitters are the basis of many free-space optical combiners such as birdbath optics, they illuminate reflective LCoS microdisplays, they fold the light path in pancake lenses for VR headsets, and as infrared "hot mirrors" they let eye tracking cameras view the eye from outside the user's field of view.

The price of sharing a path is lost light. Every pass through a partially reflecting surface sends part of the light in the wrong direction, so designs in which the image meets a beam splitter twice deliver only a small fraction of the display's output to the eye. This loss, and the ghost images created by stray reflections, are recurring drawbacks of beam splitter based XR optics.

Reviewed 11 October 2026. Every claim checked against its cited source (RP Photonics, MacNeille and Google patents, Guinness, Guttag, Tom's Hardware, and the Xiong, LaRussa, Wong, Stengel, Dunn, Ding and Luo papers, with Crossref bibliographic data). About review dates.

How it works

A beam splitter is described first by its splitting ratio, the share of the incoming power it reflects versus transmits. Other properties matter as much in practice: whether the device is polarizing or non-polarizing, the wavelength range and range of incidence angles over which it works, its optical losses, the geometry of its output ports, and its surface flatness and quality, which affect wavefront distortion and scattering in imaging systems.

Losses depend strongly on the coating. Beam splitters with metallic coatings absorb a significant share of the light; metallic plate beam splitters such as those with Inconel coatings split light over a broad band with little polarization dependence, but at the cost of absorption losses on the order of 30 percent. Devices with dichroic (dielectric) coatings may have negligible losses, so that the two outputs together carry nearly all of the input power. A half-silvered mirror, in which a metal coating is made thin enough to be partially reflective, is the simplest example.

Any beam splitter can in principle also be used to combine two beams into one. Whether the combined output equals the sum of the inputs depends on interference between them, which does not occur when the beams differ in wavelength or polarization. This reversibility is what makes beam splitters useful as combiners in see-through displays, where one input is light from a display and the other is light from the real world.

Types

Type	Construction	Characteristics
Plate	A partially reflecting mirror on a glass substrate, often used at 45 degrees so that one output is deflected by 90 degrees	Wide range of splitting ratios through coating design; the transmitted beam is shifted sideways by an amount that depends on substrate thickness and index; the back surface can create a ghost reflection, reduced with an anti-reflection coating or a wedged substrate
Cube	Two right-angle prisms cemented at their hypotenuse faces, with the splitting coating at the internal 45-degree interface	Minimal sideways offset of the transmitted beam; usable for beams carrying images in cameras and projectors; generally lower power handling than plates; the glass path adds dispersion for ultrashort pulses
Pellicle	A thin membrane used as the substrate	Minimizes the beam offset; reflections from the two closely spaced surfaces can interfere
Polarizing	Multilayer dielectric coating between two prisms, or birefringent crystal prisms such as Wollaston and Glan-Thompson types	A common prism-cube design transmits p-polarized light and reflects s-polarized light at 90 degrees
Non-polarizing cube	Multilayer coating designed to minimize polarization dependence	Specified by the difference between s and p reflectance, for example below a few percent within the design band; does not necessarily preserve the input polarization
Dichroic	Dielectric mirror whose reflectance depends strongly on wavelength	Separates spectral components of a beam; the basis of infrared "hot mirrors"
Geometric (aperture splitting)	A mirror inserted partly into a beam, or a pattern of reflecting stripes or dots such as a polka dot plate	Little wavelength dependence, but changes the beam's intensity profile, so it is generally unsuitable for imaging

Other forms include fiber-optic couplers, micro-optic and diffractive splitters that produce several outputs, and waveguide splitters in photonic integrated circuits.

History

Beam splitters are standard parts of interferometers, cameras, projectors and laser systems. A polarizing design was patented by Stephen M. MacNeille of Rochester, New York, who assigned it to Eastman Kodak. His application, titled "Beam splitter", was filed on 1 April 1943 and granted on 9 July 1946 as US patent 2,403,731. The patent describes a compound prism with multilayer thin films cemented between its two halves, arranged so that light meets the layers at Brewster's angle and the reflected and transmitted rays are polarized at right angles to each other. MacNeille wrote that each of the two rays is at least 95 percent polarized, and named range finders as an intended use.

Sutherland's 1968 head-mounted display, which Guinness World Records lists as the first AR headset, also relied on partial reflection. Describing the system (see Ivan Sutherland's head-mounted 3D display), Sutherland wrote: "Half-silvered mirrors in the prisms through which the user looks allow him to see both the images from the cathode ray tubes and objects in the room simultaneously." He added that displayed material could either "hang disembodied in space" or coincide with real objects such as maps, desk tops, walls or the keys of a typewriter.

Curved beam splitters later appeared in simulator displays. In a 1978 SPIE paper, Joseph A. LaRussa and Arthur T. Gill described the Farrand Optical Co. PANCAKE WINDOW optical simulator, a fast, large-aperture magnifier that presented a displayed image at optical infinity using reflective rather than refractive elements, and its latest improvement, which incorporated a spherical holographic beam-splitter mirror. The "pancake" name is also used for the folded lens configuration of VR headsets described below.

Applications in VR and AR

Optical see-through combiners

In an optical see-through display, the optical combiner has to reflect the display image toward the eye while transmitting the real-world view. The simplest free-space combiner is a partial mirror, also called a half mirror. In a 2021 review in Light: Science & Applications, Jianghao Xiong and colleagues at the University of Central Florida estimated that a geometric combiner with a 50 percent transmitting half mirror, and 50 percent efficiency for the remaining optics, reaches about 4,200 nits per lumen, high compared with waveguide combiners. They also noted that such designs struggle to reach the shape of ordinary flat glasses, because the half mirror has to be tilted.

Birdbath optics pair a flat beam splitter with a curved partial mirror. The review describes the beam splitter as folding the optical path, which reduces the form factor to some extent, while the extra optics on the display side leave room for aberration correction. In a 2017 analysis on his KGOnTech blog, Karl Guttag explained why this layout is inefficient: the image light passes the beam splitter twice, once by reflection and once by transmission. With a nominal 50/50 splitter that reflects and transmits about 48 percent each, only about 23 percent of the light survives both passes, and a nominal 80/20 splitter leaves about 14 percent. In his worked example with a curved combiner reflecting 11 percent and transmitting 75 percent, about 2.5 percent of the display's light reached the eye and about 36 percent of the real-world light passed through. Guttag also described double images from the two faces of a plate beam splitter, which the double pass makes worse, and counted at least four ghost images when photographing a menu through the ODG R-9.

Some combiners use a curved partially reflecting surface on its own. For Leap Motion's open-source Project North Star reference headset, revealed in April 2018, Tom's Hardware reported that the ellipsoidal reflectors were milled from optical-grade acrylic and coated on one side with a thin silver layer that reflects 50 percent of light and transmits the other 50 percent, so the wearer sees the display image and the room at the same time.

Partial mirrors also appear inside some waveguides. A geometric (or reflective) waveguide couples light out toward the eye through a series of partial mirrors embedded in the glass, which gives better image sharpness and color uniformity than diffractive designs but leaves gaps in the eye box and is costly to manufacture, since the mirrors are coated on cut pieces that are then reassembled. Guttag described the approach of Lumus as in effect cutting one thick beam splitter into multiple smaller ones to make the glass thinner, with larger light losses than a single beam splitter.

Polarizing beam splitters and LCoS illumination

Reflective liquid crystal on silicon (LCoS) panels rely on an illumination system with an LED or laser light source, and they require linearly polarized light. A polarizing beam splitter can send polarized light onto the panel and then redirect the reflected image light, whose polarization the panel has rotated, toward the viewing optics.

Guttag described Google Glass as a polarized variant of the birdbath. In his description, LED light passes through a polarizing beam splitter to the LCoS panel; the image light then reaches a second polarizing beam splitter that also acts as the see-through combiner, and a quarter-wave film in front of a spherical mirror rotates the polarization so that the returning light is reflected toward the eye. Because the optics are embedded in a block of glass, there are fewer stray reflections than with a thick air-spaced plate, but Guttag estimated that only about 45 percent of real-world light reaches the eye, noted that the polarizer complicates viewing LCD monitors, and wrote that the design cannot be scaled up to larger fields of view without the glass block becoming large, heavy and expensive. A Google patent filed on 20 July 2011 by Xiaoyu Miao, Adrian Wong and Babak Amirparviz, and granted in 2013 as US 8,508,851, describes a compact see-through display built from a display panel, an image former and two beam splitters; in the embodiment it details, both beam splitters are polarizing, quarter-wave and half-wave plates control the polarization, and the image former includes a concave mirror. The patent also describes a camera coupled to the distal beam splitter so that it receives part of the outside light seen along the viewing axis.

Pancake lenses

Pancake lenses fold the optical path between the display and the eye using polarization optics and a half mirror, a partially reflecting beam splitter. In a 2017 SPIE paper, Timothy L. Wong and colleagues described how polymeric birefringent reflective polarizers have been used to produce compact mid-field-of-view eyepieces and wide field-of-view optics for VR head-mounted displays in the pancake configuration.

The half mirror sets the efficiency ceiling. Because the light meets it twice, the maximum system efficiency is 25 percent for polarized input and 12.5 percent for unpolarized input. Stray light from surface reflections and polarization leakage can also produce ghost images, so pancake headsets usually show darker images with lower contrast than headsets with refractive lenses. A 2024 study in Optics Express traced the light paths of these ghost images experimentally and in simulation. In 2024 Yuqiang Ding and colleagues proposed removing the half mirror altogether, using a nonreciprocal polarization rotator; their demonstration used a commercial Faraday rotator and reflective polarizers, and they reported that close to the theoretical 100 percent efficiency could be approached with two high-extinction-ratio reflective polarizers.

Eye tracking with hot mirrors

A dichroic beam splitter that reflects infrared and transmits visible light is called a hot mirror. In a prototype eye-tracking headset presented at ACM Multimedia 2015, Michael Stengel and colleagues used two flat dichroic mirrors. The mirrors reflect wavelengths longer than 730 nm and transmit wavelengths shorter than 720 nm, so infrared light reflected from the illuminated eyes is redirected to the integrated cameras while visible display light passes through, and gaze can be tracked without obscuring the user's field of view. The mirrors measured 80 by 80 by 2 mm and were tilted at 19.5 degrees; the authors wrote that 45 degrees would give the best view of the eye, but steeper angles would require a larger screen distance and so a larger, heavier display. The prototype used Oculus Rift DK1 lenses and cost about US$450 to build, and the authors cited the EyeSeeCam wearable eye tracker as the design closest to theirs; it also used dichroic mirrors to reflect infrared light from the eyes to cameras outside the field of view.

Research

Several research directions change the beam splitter itself rather than the optics around it. At the University of North Carolina at Chapel Hill, David Dunn, Henry Fuchs and colleagues replaced the static beam splitter of a see-through display with a deformable membrane whose curvature changes the focal distance of the reflected image while leaving the view of the real world unchanged, an approach to the vergence-accommodation conflict. According to the 2018 follow-up paper, the first prototype controlled the membrane's convexity with air pressure and reached a depth range of 0 to 10 diopters. The second prototype dropped all additional optical elements and reported a monocular field of view of 75.5 by 70.6 degrees, focal distances from infinity to less than 10 cm, a head-mounted volume of 5.5 by 12.5 by 15.2 cm and a weight of 452 g. See Varifocal display and Deformable mirror for related approaches.

The 2021 review by Xiong and colleagues describes other variations. In one pin-mirror design, an array of tiny mirrors replaces the beam splitter of a birdbath, which can shrink the system volume and give pin-light images with a large depth of field. In Maxwellian-view displays a half mirror could in theory serve as the combiner, but holographic optical elements are generally preferred because they allow an off-axis, eyeglass-like position and reflect less ambient light. Beam splitters and gratings can also act as ray deflectors or splitters for pupil duplication and pupil steering in such systems.

See also

• Optical combiner
• Birdbath optics
• Pancake lenses
• Reflective waveguide
• Waveguide display
• Optical see-through display
• Liquid crystal on silicon
• Eye tracking
• Birefringence
• Optical aberration