Stray light

Stray light is unwanted light in an optical system, such as light that reaches the image plane, detector or viewer's eye by a path the designer did not intend. Common causes are partial reflections between optical surfaces, which form ghost images, and scattering from the roughness of optical elements or from the mechanical housing. It adds unwanted features to the image and causes a loss of resolution. Stray light can appear in any kind of optical instrument, from personal cameras and microscopes to space telescopes. Optical engineers predict it with ray tracing, suppress it with baffles and coatings, and adapt the design process to control it.

In virtual reality (VR) and augmented reality (AR) displays, stray light shows up as several familiar artifacts. In Fresnel VR optics, the lens grooves add diffraction artifacts and stray light, and light scattered by the ridges is linked to the streaks known as god rays. In folded pancake optics, surface reflections and imperfect polarization control create ghost images and lower image contrast. In waveguide AR glasses, the gratings can diffract room light into the wearer's eye as colored "rainbow" streaks. Researchers have described the reduction of ghost images and halos as one of the major challenges of folded VR optics, and several groups have published stray light analyses of specific VR and AR designs.

Reviewed 13 October 2026. Checked the definitions, mechanisms, metrics and XR examples against the full texts of the Sci Rep, Sensors, TVST and Light Sci Appl papers, the Crossref/Semantic Scholar abstracts and bibliographic data of the other papers, the Fest book synopsis, the ISO 9358 and ISO/DIS 9358 listings, two Road to VR articles and the Meta Orion waveguide blog. About review dates.

Definition and terminology

Eric C. Fest's SPIE Press monograph Stray Light Analysis and Control (2013) defines stray light as "unwanted light in an optical system". The book's own example is a photograph taken with the sun in or near the camera's field of view. The publisher's description notes that the cost of stray light depends on the system: in a cheap camera it is a nuisance, but in a space telescope it can mean the loss of data worth millions of dollars. The book covers stray light terminology, radiometry, and the physical mechanisms, such as surface roughness scatter and ghost reflections.

Engineers often sort stray light by where it comes from. A 2022 study of a space camera in the journal Sensors uses three classes:

Class	Description
External non-imaging stray light	Light from sources outside the field of view that reaches the image plane after scattering from mechanical surfaces or structural defects. Sunlight outside the field of view is an example.
Imaging stray light	Part of the image-forming light that reaches the image plane by an abnormal path inside the system, through residual reflections from optical surfaces.
Internal thermal radiation stray light	Infrared emission from the warm parts of the instrument itself; it matters mainly in infrared imaging systems.

An international standard for measuring the effect in imaging systems uses the term veiling glare. ISO 9358:1994 (Optics and optical instruments - Veiling glare of image forming systems - Definitions and methods of measurement) adopts the veiling glare index (VGI) and the glare spread function (GSF) as measures of veiling glare in optical and electro-optical imaging systems, and describes laboratory measurement methods chiefly for the visual spectral range. A revision titled Optics and photonics - Stray light (ISO/DIS 9358), registered as a draft international standard in June 2026, adds point source transmission as a third measure and is set to replace the 1994 edition.

Vision science uses a related term, intraocular stray light: light scattered inside the human eye itself. With cataract, for example, forward scattering by the eye's optics widens the point-spread function and casts a "veiling" luminance over the retina, which reduces perceived contrast. This article covers stray light inside optical instruments and displays.

Mechanisms

Common physical mechanisms include the following.

Mechanism	What happens	Examples in XR optics
Ghost reflections	Partial reflections between lens interfaces send some light to the image plane by an unintended path, where it forms a ghost	Ghost images in pancake lens stacks; suppressed with anti-reflection coatings (see Ghost image)
Scattering	Surface roughness of optical elements and scattering from the housing or mechanical surfaces redirect light, including light from sources outside the field of view	Light scattered by the back faces of Fresnel lens ridges, linked to god rays
Diffraction	Periodic structures send light into unwanted directions or diffraction orders	Fresnel lens grooves; diffraction gratings in waveguides
Polarization leakage	Imperfect polarization control lets light follow paths the polarization optics were meant to block	Ghost images in pancake lenses
Thermal emission	Warm internal parts of an instrument radiate onto an infrared detector	Mainly infrared imaging systems

Analysis and measurement

Stray light control is part of every stage of instrument development. Designers use ray tracing to predict stray light paths and then suppress or attenuate them with baffles or coatings. The finished system is then tested, usually by illuminating it with a point-like source, which produces an image of all stray light components laid over the normal image. Fest's book also covers the use of stray light analysis software and the application of these principles during design, fabrication and testing.

The conventional test cannot separate the individual stray light contributions, so engineers who find too much stray light often have to guess at its origin. In 2021, L. Clermont, W. Uhring and M. Georges demonstrated an ultrafast time-of-flight method that illuminates the system with a pulsed laser and records it with a streak camera. Because each stray light path has a different optical path length, its light arrives at a different time and can be measured on its own and traced to its source. The authors noted that the worst case is a stray light problem discovered only once the instrument is already in space, as happened with the Gaia mission.

Common figures of merit include:

Metric	Definition	Typical use
Point source transmittance (PST)	Ratio of the irradiance that a source at off-axis angle θ produces on the detector to the irradiance of that source at the entrance of the optical system	Rejection of bright sources outside the field of view, such as the sun for space cameras
Veiling glare index (VGI)	Proportion of the stray light energy on the detector relative to all the light energy reaching it	Overall stray light level across the field of view; one of the two measures in ISO 9358
Glare spread function (GSF)	The second measure of veiling glare adopted by ISO 9358	Laboratory characterization of optical and electro-optical imaging systems

The space camera study above reported a simulated PST on the order of 10^-8 and a full-field VGI below 1.31% after adding a light barrier at the third mirror. The head-mounted display studies described below report stray light as a percentage.

In virtual reality headsets

Conventional VR head-mounted displays view the display panel through a transmissive lens with a focal length of about 4 cm, and image contrast depends on both the display panel and stray light. The type of lens affects which stray light artifacts appear.

Fresnel lenses

Fresnel lenses are thinner than conventional lenses, but their grooves add diffraction artifacts and stray light that degrade image quality (the modulation transfer function, MTF). A 2021 review in Light: Science & Applications by Jianghao Xiong, En-Lin Hsiang, Ziqian He, Tao Zhan and Shin-Tson Wu notes this as one reason Fresnel lenses will not be an ideal solution as display pixel density rises. Reviewers associate Fresnel headsets with god rays, streaks of light that radiate from high-contrast elements such as white text on a black background; a Sony patent proposed adding light-absorbing material to the back faces of the ridges to stop them scattering light. In its review of the Meta Quest 3, which uses pancake (non-Fresnel) lenses, Road to VR compared it with the Meta Quest 2 and other headsets of the "Fresnel-era": the "painfully obvious god-rays" that could be seen even in the headset's main menu were gone, and "only subtle glare" remained, even in scenes with extreme contrast.

Pancake optics

Pancake lenses (polarization-based catadioptric optics) use a half mirror and polarization optics to fold the optical path three times, which makes the headset more compact. Their surfaces are smooth, so they produce far fewer diffraction artifacts and less stray light than Fresnel lenses. The fold creates other problems. Light hits the half mirror twice, which limits efficiency to at most 25% for polarized input and 12.5% for unpolarized input. Reflections from the many surfaces in the stack and polarization leakage also produce stray light that can form visible ghost images. As a result, the Xiong review states, pancake headsets usually show darker images and lower contrast than corresponding dioptric (purely refractive) designs.

Several peer-reviewed studies have examined this problem:

• Xu Peng and colleagues simulated stray light in a polarized catadioptric VR optic with LightTools and picked out three major ghost paths, using the size of the ghost's diffuse spot and the energy ratio of the stray light as measures. Changing the optical structure to control the focal power of the ghost paths increased the spot area by 40% and reduced the stray light energy ratio by 40%.
• Qichao Hou, Dewen Cheng, Yongtian Wang and colleagues systematically analyzed the causes and effects of several types of stray light created by the folded path, proposed suppression methods and built a prototype pancake VR headset. The prototype had stray light below 2.3%, a 96° diagonal field of view and a 10 mm exit pupil diameter at 11 mm eye relief.
• Bing Hao and colleagues presented a ghost analysis method at the SPIE AR, VR, MR conference in 2023. They sorted the ghosts of a polarized catadioptric system into "local ghosts", halos close to the image that directly reduce contrast, and "distant ghosts", faint images displaced diagonally that can distract with certain content. Tracing each ghost to the component that causes it allows targeted fixes and can feed constraints back into lens design software.
• Zhenyi Luo, Yuqiang Ding, Qian Yang and Shin-Tson Wu traced the light paths of pancake ghost images by experiment and simulation and studied how the angle of incidence changes the intensity of each ghost. They attribute the ghosts to stray light from multiple surface reflections and imperfect polarization control.
• Ding, Luo, Garimagai Borjigin and Wu proposed replacing the half mirror with a nonreciprocal polarization rotator (a Faraday rotator in their demonstration) to remove the 25% efficiency ceiling. When they evaluated ghost images with a micro-OLED panel, they found the ghosts could be suppressed to an undetectable level if the optics had anti-reflection coatings.

In augmented reality displays

In an optical see-through AR display, ambient light also affects image contrast, which is usually measured as the ambient contrast ratio. Light from the real world can also become stray light, as with the rainbow artifact described below.

Diffractive waveguides

The Xiong review lists three artifacts specific to diffractive waveguides, each caused by unwanted diffraction at the gratings:

• Light leakage: out-coupled image light that exits toward the outside world. Besides wasting light, it gives the wearer an unnatural "bright-eye" look and can be a privacy issue.
• See-through ghost: repeated in-coupling and out-coupling of real-world light by the out-coupler grating, which can create a ghost of a real object shifted in field of view and depth. Higher-efficiency out-couplers tend to produce more see-through ghosting.
• Rainbow: diffraction of environmental light into the user's eye. Because the grating vectors do not cancel for this light, it is spread into colors. Suggested remedies are a higher-index substrate, which pushes the unwanted diffraction orders out of the see-through field of view, and grating designs that suppress the unwanted diffraction.

Meta's description of the waveguides in its Orion AR glasses prototype gives an industry account of the rainbow problem. Meta describes rainbows as "colorful streaks of light that are created when ambient light reflects off of the waveguide". Meta optical scientist Pasqual Rivera recalled "rainbows everywhere" when wearing glasses with glass-based waveguides, and Barry Silverstein, Meta's director of research science, said silicon carbide "gets rid of those rainbows". AR waveguides tech lead Giuseppe Calafiore gave silicon carbide's refractive index as 2.7; according to Meta, the highest-index glass the team had in 2016 was 1.8.

Geometric waveguides and freeform optics

In geometric (reflective) waveguides, which use mirrors instead of gratings as couplers, stray light generated inside the waveguide is one of the factors that limit the field of view, alongside the total internal reflection condition. Qiwei Wang, Dewen Cheng, Qichao Hou, Luo Gu and Yongtian Wang analyzed the stray light of a dual-layer geometric waveguide and designed a structure that suppresses it. Their design had a total thickness of 3.0 mm, stray light below 1%, a 62° field of view in the pupil-expanding direction and a 10 mm exit pupil at 18 mm eye relief.

Dewen Cheng, Yongtian Wang and co-authors have applied stray light analysis to other see-through designs as well. A 2022 freeform-prism head-mounted display with a 38° diagonal field of view, no more than 9.5 mm thick and under 9.8 g per eye, included an investigation of the causes of stray light in the optical-mechanical system and proposed suppression measures. A 2019 integral-imaging AR eyeglass design presented at the IEEE ISMAR conference used an annular aperture array to keep stray light within an imperceptible limit.

See also

• God rays
• Ghost image
• Anti-reflection coating
• Polarizer
• Pancake lenses
• Fresnel lens
• Waveguide display
• Optical aberration
• Optical combiner