Ghost image

A ghost image (also called ghosting) is a faint secondary image formed in an optical system by unwanted reflections, usually light that reflects back and forth between lens or element surfaces before reaching the eye or sensor. Ghost images lower contrast and resolution, and optical designers reduce them with anti-reflection coatings, careful choice of element shapes and spacings, and, in polarization-based optics, tight control of polarization.

In virtual reality (VR) and augmented reality (AR) headsets, ghost images are a known side effect of folded optics. Pancake lenses deliberately reflect light several times between a half mirror and a reflective polarizer, and any light that escapes the intended path can form ghosts; a 2024 Optics Express study by researchers at the University of Central Florida attributed these ghosts to "multiple surface reflections and imperfect polarization control inside the optical system". AR combiners such as birdbath optics, reflective waveguides and windshield head-up displays face the same problem, because every partially reflecting surface can send a displaced copy of the image toward the viewer.

The optical ghost image discussed here is a separate effect from display "ghosting" caused by slow pixel response, and from crosstalk ("ghosting") between the left-eye and right-eye images of a stereoscopic display (see Other uses of the term).

Reviewed 13 October 2026. Checked each citation against its source (DOI metadata and abstracts for the papers, full text of the Quest 2 SID and windshield HUD papers, the Meta patent, Kopin, UploadVR, MIXED, KGOnTech, Edmund Optics and Magic Leap pages) and corrected quotes, author lists and claims. About review dates.

Definition and causes

Edmund Optics describes a ghost image as "a faint second image caused by reflections within an optical component", for example when some of the light passing through an imaging lens is reflected, bounces back, and comes to focus on a camera sensor. The underlying mechanism is Fresnel reflection. According to Edmund Optics, about 4% of light is reflected at each air-glass interface of an uncoated glass element, so a single uncoated element transmits only about 92% of the incident light. Part of every reflected ray is reflected again at each further interface it reaches, and reflections that travel backward through a system can "create ghost images".

Because a ghost must end up travelling toward the image like the main beam, it is formed by an even number of reflections, the simplest case being two. In a 2009 SPIE paper, Rania Abd El-Maksoud and José Sasián of the University of Arizona modelled ghosts formed by two reflections between the surfaces of a multi-element lens in the paraxial regime. In a 2011 Applied Optics paper they extended the method to ghosts formed by any even number N of reflections. Their analysis treats each ghost as an aberrated image with its own sagittal and tangential image surfaces rather than as a uniform spot, gives an expression for the ghost irradiance at the nominal image plane, and provides tools for picking out the most troublesome ghosts, which are those that come to focus. A sharply focused ghost appears as a recognizable duplicate of a bright object. A defocused ghost spreads its light over a wider area, where it is less distracting but still lowers contrast.

Ghost images are one kind of stray light, the general term for any light that reaches the image by an unintended path. Edmund Optics lists stray light, flare and veiling glare as related topics.

Analysis and mitigation

Ghost analysis is part of lens design; José Sasián's textbook Introduction to Lens Design (Cambridge University Press, 2019) has a chapter on it. Designers generate the possible double-reflection paths, trace rays along each one, and estimate how large each ghost is at the image plane and how much light it carries; the origin of each ghost can then be traced back to the components that produce it. For folded VR optics, researchers simulate stray light with illumination software such as LightTools and rank ghost paths by spot size and energy.

The usual countermeasure is an anti-reflection (AR) coating on each lens surface. Edmund Optics states that AR coatings increase transmission, enhance contrast and eliminate ghost images, and that they matter most in systems with many transmitting elements; its visible-band "VIS 0°" coating, for example, is specified for an average reflection of 0.4% or less over 425-675 nm. Coatings add cost, which Edmund Optics notes as the main drawback. Other methods change the geometry so that ghost light is spread out instead of focused, for example by changing the focal power along a ghost path or by moving, tilting or curving an element.

Ghost images in VR optics

Pancake lenses

A pancake lens, also called a polarization-based catadioptric lens, uses a half mirror to fold the optical path three times, with polarization optics (quarter-wave plates and a reflective polarizer) deciding which pass of the light is allowed through to the eye. Because of the half mirror, the theoretical maximum optical efficiency of a conventional pancake lens is 25%. Any light that leaks through at the wrong pass, or that reflects from an uncoated surface, takes a different number of bounces and arrives as a ghost. A 2023 SPIE paper by researchers at 3M called reducing the intensity of ghost images and halos "one of the major challenges" of folded optics. Published studies include:

Study	Venue and year	Finding
Peng, Gao, Ding, Zhao and Chi	Virtual Reality & Intelligent Hardware, 2020	Simulated the stray light of a polarized catadioptric VR system in LightTools, selected three major ghost paths, and suppressed them by changing the focal power along those paths. After optimization the ghost spot area grew by 40% and the ghost energy ratio fell by 40%.
Hou, Cheng, Wang and colleagues	Optics Express, 2022	Analyzed several types of stray light in a pancake VR head-mounted display and built a prototype with stray light below 2.3%, a 96° diagonal field of view, and a 10 mm exit pupil at 11 mm eye relief.
Hao, Le, Aastuen, Kotz, McGrath and Kent (3M)	SPIE AR, VR, MR conference, 2023	Divided ghosts into "local ghosts" (halos near the image that directly lower contrast) and "distant ghosts" (faint images located away from the main image), and traced each ghost back to the optical component that produces it.
Luo, Ding, Yang and Wu	Optics Express, 2024	Identified the light paths of individual ghosts by experiment and simulation, and measured how the incident angle changes the intensity of each ghost.
Ding, Luo, Borjigin and Wu	Opto-Electronic Advances, 2024	Replaced the lossy half mirror with a Faraday rotator and reflective polarizers to approach 100% theoretical efficiency; ghost images measured with a micro-OLED panel could be "suppressed to undetectable level" when the optics had anti-reflection coatings.

Material and assembly choices also matter. A Meta (Facebook Technologies) patent filed in 2019 and granted in 2021 states that Fresnel reflections from the top of a polarization stack laminated onto the display "may create multiple near-focus ghost artifacts" that reduce contrast, color fidelity and sharpness. It proposes mounting the quarter-wave plate between the display and the lens with air gaps on both sides, and optionally tilting or curving it, so that the ghosts are defocused. Plastic lens elements are another source of trouble because plastics can be birefringent, which disturbs the polarization state the design depends on. In June 2021 Kopin Corporation said that earlier pancake optics "needed at least one spherical glass lens to avoid image artifacts" for this reason, and announced an all-plastic two-element design that it said has "virtually no birefringence". UploadVR summarized the issue as one element having to be glass "to avoid an image 'ghosting' effect", and reported Kopin's claim that its P95 lens had almost eliminated the problem.

In consumer headsets

Reviewers have reported ghosting with high-contrast content. In a September 2023 hands-on with the Meta Quest 3 before its release, MIXED's Benjamin Danneberg wrote that the headset's pancake lenses "show significant ghosting, i.e., reflections of bright objects such as white lettering on a black background", that he noticed it only on loading screens and menus while playing, and that the Pico 4 had shown the same problem. He described the lenses of the Meta Quest Pro as "perfect" in his experience and wrote that he had been told the Quest 3 did not use the same pancake lenses.

Ghost images in AR displays

Optical see-through AR displays combine the virtual image with the real world through partially reflecting surfaces, which makes them prone to ghosts.

• Birdbath combiners. In a 2017 analysis of birdbath optics on his KGOnTech blog, Karl Guttag explained that a plate beam splitter can produce "a double image off of each face", with the unwanted light slightly displaced depending on the thickness of the beam splitter, and that because image light passes the beam splitter twice there are "two opportunities to cause double images". He counted "at least 4 ghost images" in a photo of a menu taken through the ODG R-9.
• Reflective (geometric) waveguides. A 2021 SPIE paper on geometric waveguides found that improper tilt angles and differences in parallelism between the units of the partially reflective mirror array "may cause stray light or overlapping ghost images", and that the refractive indices of the glue and the glass affect the appearance of further ghosts. By adjusting the mirror tilt angle and matching the glue (1.500) to the glass (1.502), the authors designed a 1.8 mm thick waveguide with stray light below 1%, a 40° field of view and a 10 mm by 10 mm eye box. Magic Leap, which uses diffractive waveguides, states that the stacked-mirror design of reflective waveguides "has the potential to introduce ghosting, double reflections, and other visual artifacts".
• Windshield head-up displays. In an automotive head-up display the projected image reflects from both the inner and outer surfaces of the laminated windshield, which can produce a double image. Head-up display windshields normally use a polyvinyl butyral (PVB) interlayer whose thickness varies across the glass, giving a "wedge angle" that makes the two reflections overlap. A 2022 article in Transactions on Transport Sciences by researchers at the University of Chemistry and Technology, Prague, describes this double image (ghosting) as the most common defect found in head-up display windshields, and notes that the wedge works well for a certain driver position but that image quality usually deteriorates as the driver moves the seat up or down. Ábel Sulyok and Pál Koppa of the Budapest University of Technology and Economics proposed optimized polarization-correcting optics attached to the head-up display projector, and reported that this increased the ghost-free viewing area of polarization-type head-up displays more than fourfold.

Other uses of the term

In display engineering, "ghosting" also describes trailing images caused by pixels that do not switch fast enough, which is unrelated to optical reflections. Meta engineers describing the fast-switch LCD of the Oculus Quest 2 wrote that the liquid crystal must settle before the backlight is turned on, and that if the settling time is not sufficient "users could notice visual artifacts, such as ghosts and tailing effects in swift motion". The Quest 2 display uses fast panel scan-out, fast liquid crystal response and low-persistence backlight illumination to suppress such artifacts. See persistence for the related problem of motion blur.

In stereoscopic 3D displays, "ghosting" is a common name for crosstalk, in which each eye sees part of the image intended for the other eye, so that the picture looks doubled. Andrew Woods' 2012 review of the subject notes that high crosstalk makes stereoscopic images hard to fuse.

See also

• Stray light
• Anti-reflection coating
• Pancake lenses
• Polarizer
• Birefringence
• Optical combiner
• Birdbath optics
• Reflective waveguide
• God rays
• Optical aberration