Holographic optical element

A holographic optical element (HOE) is a thin film of material that diffracts light according to a certain shape and pattern. It can be used for augmented reality.

An HOE is a hologram recorded to perform an optical function: the interference pattern of an object wave and a reference wave is stored in a photosensitive medium, and when light resembling the reference wave later strikes the film, the stored fringes diffract it into the recorded object wavefront. Depending on what was recorded, the element can act as a grating, a lens, a lens array or a diffuser. Volume HOEs respond strongly only to light that arrives at the recorded angle and wavelength, so a film can redirect the narrow-band light of a display toward the eye while most light from the surroundings passes through it. That property is why HOEs have been used as see-through combiners in smart glasses such as Intel's Vaunt prototype and Focals by North, and why research groups have used them as flat, lightweight lenses for virtual reality headsets.

Reviewed 27 September 2026. Checked every new claim against the Xiong et al. 2021 Advanced Photonics Research and Light: Science & Applications reviews, Kogelnik 1969, Crossref records for Close 1975, Maimone and Wang 2020 and Jang et al. 2020, the Meta Research blog, CREAL white paper, and the Verge, TechCrunch, Engadget, KGOnTech, 9to5Google, Bosch Sensortec, New Atlas, TV Tech, FRAMOS, Covestro, SIGGRAPH and NVIDIA pages cited. About review dates.

How it works

Recording and reconstruction

Holography is the recording of the complete wave field of interfering coherent beams in a medium, from which the original wave field can later be reproduced. To make an HOE, an object wave and a reference wave are overlapped in the recording material. The material responds to the intensity or the polarization of the combined field and turns the fringe pattern into a physical grating by modulating its transmittance, its refractive index or the orientation of its molecules. When a reference beam later falls on the finished film, the local gratings diffract it and the object wave is reconstructed.

A holographic optical element can replicate a lens in an arbitrarily thin piece of substrate. Xiong and colleagues note that, unlike conventional bulk optics, the optical power of an HOE does not depend on its thickness and is set only by the recording process. A June 2020 Meta Research blog post about Maimone and Wang's VR prototype described the element this way: "The holographic optic bends light like a lens but looks like a thin, transparent sticker."

Bragg selectivity

Because the fringe spacing of an HOE varies slowly compared with the fringe period, each small region can be analyzed as a grating defined by its pitch and slant angle. Diffraction efficiency is highest when the incoming and outgoing light satisfy the Bragg condition, and it falls as the incident angle or wavelength moves away from it. Herwig Kogelnik's coupled wave theory, published in the Bell System Technical Journal in November 1969, analyzes this Bragg diffraction in thick hologram gratings. It gives formulas for the diffraction efficiency and for the angular and wavelength sensitivity of transmission and reflection holograms, and remains valid at high efficiencies where the incident wave is strongly depleted.

How wide the angular and spectral acceptance bands are depends on the refractive index modulation of the material. Using rigorous coupled wave analysis of the same reflective grating (175 nm Bragg pitch, 20 degree slant, glass on both sides), Xiong, Yin, Li and Wu compared three families of HOE:

HOE type	Index modulation used	Spectral bandwidth	Angular bandwidth (in glass)	Thickness for high efficiency
Photopolymer	about 0.02	about 8 nm	15 degrees	about 15 μm
Holographic polymer-dispersed liquid crystal (HPDLC)	0.07	about 25 nm	25 degrees	6 μm
Cholesteric liquid crystal optical element (CLCOE)	equal to the liquid crystal birefringence	about 100 nm	about 60 degrees	2 μm gives more than 90%

The same study found that the reflective photopolymer and HPDLC gratings were insensitive to input polarization, while the CLCOE diffracted circularly polarized light of one handedness efficiently and let the opposite handedness pass with essentially zero efficiency.

Transmission and reflection types

If the two recording beams reach the film from the same side, the fringes form a transmission HOE; if they arrive from opposite sides, the result is a reflection HOE. Reflection HOEs are the more common choice for AR combiners. A transmission HOE in front of the eye also diffracts light from the environment, sending stray light into the eye and producing ghost images of real objects, whereas a reflection HOE sends that stray light backward. The reflective layout also lets the image source sit on the viewer's side of the lens, which shrinks the system. Transmission HOEs are still used in some roles, for example as the input coupler of a waveguide.

Materials

Early holography used intensity-sensitive materials such as silver halide emulsion, dichromated gelatin and photopolymer. In silver halide film the fringes are stored as a transmittance (amplitude) pattern, much like the exposure of photographic film. Phase holograms instead modulate the refractive index; materials include photorefractive crystals, dichromated gelatin, photoresists and photopolymers, and photopolymers are widely used because of their low cost, low scattering, high resolution and simple fabrication. In a holographic photopolymer, monomers polymerize faster in the bright fringes and diffuse in from the dark regions, which leaves a slightly higher density and refractive index in the bright regions. Covestro sells a photopolymer film of this kind, Bayfol HX, consisting of a transparent substrate film with a photoreactive layer; the company names automotive head-up displays and head-mounted displays among its AR applications.

Holographic polymer-dispersed liquid crystal (HPDLC) forms by the same diffusion and polymerization process but also contains liquid crystal droplets, which makes the grating electrically switchable. A second family relies on polarization holography: the recording beams have orthogonal polarizations and the material records the polarization pattern through photoinduced anisotropy. In photoalignment polarization holography only a thin layer of azo dye is exposed, and a liquid crystal coated on top copies the pattern. Using a cholesteric liquid crystal gives cholesteric liquid crystal optical elements (CLCOEs), including polarization volume gratings (PVGs) and polarization volume lenses (PVLs); because the helical structure is stable, a CLCOE can accommodate diffraction angles of about 70 degrees.

Photopolymers can also hold several holograms in one film. If the first exposure does not use up all the monomer, a second exposure can record another element in the same layer, at the cost of splitting efficiency between the recorded functions.

History

Holography was invented by Dennis Gabor in 1948. In the introduction to his 1969 coupled wave paper, Kogelnik wrote that the high light-conversion efficiency attainable with thick dielectric holograms "may make it practical to use holographic optical components (for example, gratings or fly's eye lenses) in a variety of optical systems". D. H. Close's article "Holographic Optical Elements" appeared in Optical Engineering in October 1975. Beyond displays, HOEs have been used in data storage, solar concentration and imaging.

In consumer wearables, CREAL's 2022 white paper names Sony as possibly the first developer of a commercially ready HOE combiner for AR glasses, followed by Intel with Vaunt and North with Focals. Sony announced the development of its SmartEyeglass in September 2014 and said it had used its hologram-optics technology to make a 3.0 mm lens with 85 percent transparency that needed no half mirrors in the user's view.

Academic groups used HOEs for 3D and light field AR displays in the same period. At SIGGRAPH 2016, Seungjae Lee, Changwon Jang, Seokil Moon, Jaebum Cho and Byoungho Lee presented a see-through "additive light field display" that used HOEs as transparent additive layers. The "Retinal 3D" display by Changwon Jang, Kiseung Bang, Seokil Moon, Jonghyun Kim, Seungjae Lee and Byoungho Lee (2017) projected a pupil-tracked light field onto the retina and used an HOE as its image combiner for high transparency in a thin structure. In 2020, Changwon Jang, Olivier Mercier, Kiseung Bang, Gang Li, Yang Zhao and Douglas Lanman published a pipeline for designing and fabricating freeform HOEs with complex, customized wavefronts. Their demonstrations included an aspheric lens, a head-up display lens, a lens array and a full-color caustic projection element.

Applications in VR and AR

Combiner architectures

Xiong and colleagues group HOE-based AR systems into five architectures and summarize their general trade-offs as follows:

Display type	Field of view (diagonal)	Eye box	Form factor	Efficiency	3D capability
Projection (diffuser HOE)	Small (about 20 degrees)	Large	Bulky	Medium	Medium
Free-space (lens HOE)	Medium (about 50 degrees)	Medium	Medium, depends on design	Medium	Medium
Integral imaging (lens-array HOE)	Small (about 20 degrees)	Large	Medium, depends on design	Medium	High
Maxwellian view (retinal projection)	Large (about 100 degrees)	Small	Small	High	Low
Waveguide	Medium (about 50 degrees)	Large	Small	Low	Medium

In a projection system the image is focused onto a diffuser HOE, and the viewer needs to stand about 1 m from it, so the authors see it as suited to fixed installations such as exhibitions rather than near-eye displays. Free-space systems use a lens HOE instead of a partial mirror, which removes the usual trade-off between see-through transmission and image brightness, but the off-axis layout produces large coma and astigmatism, and the HOE's angular selectivity can limit how much of the field of view reaches the eye.

Retinal projection glasses

The strong chromatic dispersion of an HOE usually calls for a laser light source, which tends to lead to a Maxwellian-view design. In such a display, image light is focused through a small point in the eye's pupil, so the picture on the retina stays in focus whatever the eye's accommodation. The lens HOE can be recorded with a spherical reference wave whose focus matches a laser beam scanning source, and Maxwellian AR prototypes with a horizontal field of view as large as 80 degrees have been demonstrated.

They have been used in AR glasses by Intel on the Vaunt project and by North for the Focals. In Vaunt, which The Verge revealed in an exclusive first look in February 2018, electronics in the right temple drove a low-power VCSEL laser that projected a red, monochrome image of roughly 400 by 150 pixels onto a holographic reflector in the right lens, which sent it onto the retina. Intel confirmed in April 2018 that it had ended the project. North's Focals, revealed in October 2018, housed a small projector in the right temple that reflected off an element inside the lens back into the eye. Display analyst Karl Guttag described North's holographic film as acting like a tilted mirror. Google acquired North on 30 June 2020; Focals 2.0 was cancelled and the first-generation Focals were wound down. Bosch Sensortec's Smartglasses Light Drive, shown at CES 2020, uses the same principle: a MEMS-based collimated light scanner scans a holographic element embedded in the lens, and the HOE redirects the beam onto the retina. Bosch said the system weighs less than 10 grams and works with curved and corrective lenses.

The main weakness of this design is its small eye box. Because the viewing point is generally smaller than the pupil (about 4 mm), the eye box of a single Maxwellian view equals the pupil diameter, and a small misalignment makes the image vanish. CREAL's white paper says that with the Intel and North combiners the whole image passed through a tiny area near the pupil and was lost when the pupil moved outside it. It also says the image carried shadows of dust, eyelashes and floaters and was sensitive to mechanical and temperature shifts. Proposed remedies include duplicating the viewpoint with a ray duplicator, switching between several light sources with eye tracking, and recording several focal points into one multiplexed lens HOE. Each of these has to handle ghost images when two viewpoints fall inside the pupil, or gaps when they are too far apart. Kim and Park demonstrated a multiplexed photopolymer HOE with three viewpoints separated by 3 mm.

Waveguides

HOEs are widely used as couplers in waveguide displays because large-angle gratings can be fabricated in them. An in-coupler diffracts collimated light from the microdisplay into a glass plate about 1 mm thick, where it travels by total internal reflection until an out-coupler, usually a reflective grating, sends it toward the eye. Repeated out-coupling enlarges the eye box without shrinking the field of view, but it creates a trade-off between efficiency and uniformity. Because a photopolymer grating accepts a range of only several degrees, several gratings have to be multiplexed to cover the field of view, and red, green and blue channels must be separated to avoid crosstalk, which becomes difficult at larger fields of view and can require more than one waveguide. Sony Semiconductor Solutions' SED-100A display module, offered through distributor FRAMOS in 2019, is one commercial example. It is based on Sony's hologram optics technology: an optical engine with a microdisplay projects into a thin glass waveguide, which delivers the virtual image to the eye through holographic optical elements, and the module transmits more than 85% of outside light.

Light field and 3D displays

A holographic optical element can replicate a microlens array, and be used to make a much thinner light field display than by using a traditional microlens array. In integral imaging AR systems, the lens array can be replaced by a lens-array HOE, and a laser scanning source can be combined with it to avoid the limited depth of field of panel-based designs. Such systems provide correct focus cues but lose resolution in proportion to the number of views. CREAL describes its own "light-field HOE" combiner as reflecting many small exit pupils that together form one large eye box, and claims it reflects up to 50% of the selected wavelengths with up to about 4% total light source-to-eye efficiency, compared with about 0.05% it attributes to diffractive waveguides.

Virtual reality optics

HOEs have also been proposed for VR headsets. In a 2020 paper in ACM Transactions on Graphics, Andrew Maimone and Junren Wang of Facebook Reality Labs presented a class of near-eye displays that combined holographic optics, directional backlighting, laser illumination and polarization-based optical folding. Using only thin, flat films as optical components, their prototypes reached thicknesses under 9 mm and fields of view of more than 90 degrees horizontally. A full-color benchtop version used wavelength-multiplexed holographic lenses. The Facebook Reality Labs blog post about the work notes that "holographic optics compel the use of laser light sources", which are more difficult to integrate than the LEDs used in most VR headsets but provide a richer set of colors.

See also

• Holography
• Optical combiner
• Waveguide display
• Retinal projection
• Diffractive optics
• Eye box
• Light field display
• Near-eye display