Flat focus: Difference between revisions
Xinreality (talk | contribs) Created page with "{{Stub}} {{VR}} {{AR}} {{Optics}} '''Flat Focus''' refers to an optical system design, common in virtual reality (VR) and augmented reality (AR) headsets (HMDs), where the lenses are optimized to bring light originating from the microdisplay (the screen) to a sharp focus at a single, fixed focal plane. This means that regardless of the apparent depth of virtual objects depicted on the screen, the light reaching t..." |
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[[Flat focus]] refers to an [[optical system]] design, common in [[virtual reality]] (VR) and [[augmented reality]] (AR) [[head-mounted display|headsets]] (HMDs), where the [[lens|lenses]] are optimized to bring light originating from the [[microdisplay]] (the screen) to a sharp [[focus]] at a single, fixed [[focal plane]]. This means that regardless of the apparent depth of virtual objects depicted on the screen, the light reaching the user's eye always appears to emanate from this specific, unchanging distance. | |||
This approach contrasts sharply with how the [[human visual system]] naturally perceives the real world. In reality, the [[eye]] employs a process called [[accommodation (optics)|accommodation]], where the [[crystalline lens]] dynamically changes its shape (and thus its focal length) to sharply focus on objects at varying distances. Concurrently, the eyes use [[vergence (optics)|vergence]], rotating inward ([[convergence]]) or outward ([[divergence]]) to align their gaze on the object of interest, providing crucial [[depth perception]] cues through [[binocular disparity]]. | This approach contrasts sharply with how the [[human visual system]] naturally perceives the real world. In reality, the [[eye]] employs a process called [[accommodation (optics)|accommodation]], where the [[crystalline lens]] dynamically changes its shape (and thus its focal length) to sharply focus on objects at varying distances. Concurrently, the eyes use [[vergence (optics)|vergence]], rotating inward ([[convergence]]) or outward ([[divergence]]) to align their gaze on the object of interest, providing crucial [[depth perception]] cues through [[binocular disparity]]. | ||
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== Relevance in VR and AR == | == Relevance in VR and AR == | ||
The flat focus design is prevalent in VR/AR primarily due to its relative simplicity, cost-effectiveness, and ability to deliver wide fields of view with manageable [[aberration (optics)|optical aberrations]] using lens technologies like [[aspheric lens|aspheric]] or [[Fresnel lens|Fresnel lenses]], and more recently, [[pancake lens|pancake lenses]] (which achieve a thinner profile but typically still maintain a fixed focus)<ref name="PancakeOptics">Maimone, A., Georgiou, A., & Kollin, J. S. (2017). Holographic near-eye displays for virtual and augmented reality. ''ACM Transactions on Graphics (TOG) - Proceedings of ACM SIGGRAPH 2017, 36''(4), Article 85, | The flat focus design is prevalent in VR/AR primarily due to its relative simplicity, cost-effectiveness, and ability to deliver wide fields of view with manageable [[aberration (optics)|optical aberrations]] using lens technologies like [[aspheric lens|aspheric]] or [[Fresnel lens|Fresnel lenses]], and more recently, [[pancake lens|pancake lenses]] (which achieve a thinner profile but typically still maintain a fixed focus)<ref name="PancakeOptics">Maimone, A., Georgiou, A., & Kollin, J. S. (2017). Holographic near-eye displays for virtual and augmented reality. ''ACM Transactions on Graphics (TOG) - Proceedings of ACM SIGGRAPH 2017, 36''(4), Article 85, 1-16. doi:10.1145/3072959.3073624 (Discusses various HMD optics including pancake lenses)</ref>. | ||
However, the primary consequence of flat focus is the introduction of the [[Vergence-Accommodation Conflict]] (VAC)<ref name="VACReview">Hoffman, D. M., Girshick, A. R., Akeley, K., & Banks, M. S. (2008). Vergence-accommodation conflicts hinder visual performance and cause visual fatigue. ''Journal of Vision, 8''(3), 33, | However, the primary consequence of flat focus is the introduction of the [[Vergence-Accommodation Conflict]] (VAC)<ref name="VACReview">Hoffman, D. M., Girshick, A. R., Akeley, K., & Banks, M. S. (2008). Vergence-accommodation conflicts hinder visual performance and cause visual fatigue. ''Journal of Vision, 8''(3), 33, 1-30. doi:10.1167/8.3.33</ref>. This conflict arises because the cues the brain receives for depth perception become inconsistent: | ||
* '''Vergence Cues:''' Based on stereoscopic rendering ([[binocular disparity]]), the user's eyes converge or diverge naturally to fuse the image of virtual objects presented at different depths. For a nearby virtual object, the eyes converge significantly. | * '''Vergence Cues:''' Based on stereoscopic rendering ([[binocular disparity]]), the user's eyes converge or diverge naturally to fuse the image of virtual objects presented at different depths. For a nearby virtual object, the eyes converge significantly. | ||
* '''Accommodation Cues:''' Regardless of where the eyes are converged, the light is always coming from the fixed focal plane set by the headset optics. Therefore, the eye's accommodation reflex receives cues (primarily from [[retinal blur]]) indicating that the object is always at that fixed distance, prompting the crystalline lens to remain focused there. | * '''Accommodation Cues:''' Regardless of where the eyes are converged, the light is always coming from the fixed focal plane set by the headset optics. Therefore, the eye's accommodation reflex receives cues (primarily from [[retinal blur]]) indicating that the object is always at that fixed distance, prompting the crystalline lens to remain focused there. | ||
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This mismatch between where the eyes are pointing (vergence) and where they are focusing (accommodation) is unnatural. The human visual system is accustomed to these two mechanisms working in tandem. The conflict can lead to several negative effects: | This mismatch between where the eyes are pointing (vergence) and where they are focusing (accommodation) is unnatural. The human visual system is accustomed to these two mechanisms working in tandem. The conflict can lead to several negative effects: | ||
* [[Visual fatigue]] and [[eye strain]]<ref name="VACReview"/>. | * [[Visual fatigue]] and [[eye strain]]<ref name="VACReview"/>. | ||
* [[Headache|Headaches]]<ref name="Shibata2011">Shibata, T., Kim, J., Hoffman, D. M., & Banks, M. S. (2011). The zone of comfort: Predicting visual discomfort with stereo displays. ''Journal of Vision, 11''(8), 11, | * [[Headache|Headaches]]<ref name="Shibata2011">Shibata, T., Kim, J., Hoffman, D. M., & Banks, M. S. (2011). The zone of comfort: Predicting visual discomfort with stereo displays. ''Journal of Vision, 11''(8), 11, 1-29. doi:10.1167/11.8.11</ref>. | ||
* Difficulty focusing on real-world objects after prolonged VR/AR use. | * Difficulty focusing on real-world objects after prolonged VR/AR use. | ||
* Inaccurate perception of [[depth]] and scale<ref name="Kramida2016"/>. | * Inaccurate perception of [[depth]] and scale<ref name="Kramida2016"/>. | ||
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Research and development efforts are actively exploring solutions to overcome the limitations of flat focus, primarily aiming to resolve the VAC: | Research and development efforts are actively exploring solutions to overcome the limitations of flat focus, primarily aiming to resolve the VAC: | ||
* '''[[Varifocal display|Varifocal Displays]]:''' These systems can dynamically adjust the focal plane of the headset to match the depth of the virtual object the user is looking at. This can be achieved through various methods: | * '''[[Varifocal display|Varifocal Displays]]:''' These systems can dynamically adjust the focal plane of the headset to match the depth of the virtual object the user is looking at. This can be achieved through various methods: | ||
**Mechanically moving the lenses or displays. | |||
**Using [[liquid lens|liquid crystal lenses]] or other electronically tunable optical elements. | |||
**Employing deformable membrane mirrors<ref name="MembraneMirrorVarifocal">Rathnayake, A. U., Nguyen, T., & Zhan, T. (2021). Varifocal near-eye display using a focus-tunable Alvarez lens. ''Optics Express, 29''(19), 30935-30947. doi:10.1364/OE.436385</ref>. | |||
* '''[[Multifocal display|Multifocal Displays]]:''' These designs present images on multiple distinct focal planes simultaneously or in rapid succession, allowing the eye to focus more naturally on the plane closest to the target object's depth<ref name="MultifocalDisplays">Mercier, T., Ito, Y., & Kawahito, S. (2017). Multi-focal augmented reality display using time-multiplexed focal planes. ''Optics Express, 25''(23), 28633-28645. doi:10.1364/OE.25.028633</ref>. | * '''[[Multifocal display|Multifocal Displays]]:''' These designs present images on multiple distinct focal planes simultaneously or in rapid succession, allowing the eye to focus more naturally on the plane closest to the target object's depth<ref name="MultifocalDisplays">Mercier, T., Ito, Y., & Kawahito, S. (2017). Multi-focal augmented reality display using time-multiplexed focal planes. ''Optics Express, 25''(23), 28633-28645. doi:10.1364/OE.25.028633</ref>. | ||
* '''[[Light field display|Light Field Displays]]:''' These advanced displays aim to replicate the way light rays travel in the real world, providing correct focus cues by presenting slightly different information depending on the viewing angle and position of the pupil. The eye can then potentially focus naturally at different depths within the captured light field<ref name="LightfieldVR">Lanman, D., & Luebke, D. (2013). Near-eye light field displays. ''ACM Transactions on Graphics (TOG) - Proceedings of ACM SIGGRAPH Asia 2013, 32''(6), Article 220, | * '''[[Light field display|Light Field Displays]]:''' These advanced displays aim to replicate the way light rays travel in the real world, providing correct focus cues by presenting slightly different information depending on the viewing angle and position of the pupil. The eye can then potentially focus naturally at different depths within the captured light field<ref name="LightfieldVR">Lanman, D., & Luebke, D. (2013). Near-eye light field displays. ''ACM Transactions on Graphics (TOG) - Proceedings of ACM SIGGRAPH Asia 2013, 32''(6), Article 220, 1-10. doi:10.1145/2508363.2508364</ref>. | ||
* '''[[Holographic display|Holographic Displays]]:''' True holographic displays reconstruct the [[wavefront]] of light from the virtual scene, which would inherently contain all necessary focus cues, potentially eliminating the VAC entirely. This remains a significant technical challenge for near-eye displays<ref name=" | * '''[[Holographic display|Holographic Displays]]:''' True holographic displays reconstruct the [[wavefront]] of light from the virtual scene, which would inherently contain all necessary focus cues, potentially eliminating the VAC entirely. This remains a significant technical challenge for near-eye displays<ref name="PancakeOptics"/>. | ||
While flat focus remains the dominant approach in current consumer VR/AR due to its practicality, ongoing advancements in these alternative display and optical technologies promise future HMDs with more natural and comfortable visual experiences. | While flat focus remains the dominant approach in current consumer VR/AR due to its practicality, ongoing advancements in these alternative display and optical technologies promise future HMDs with more natural and comfortable visual experiences. | ||
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== References == | == References == | ||
<references/> | <references/> | ||
[[Category:Terms]] | |||
[[Category:Technical Terms]] | |||
[[Category:Optics]] | |||
[[Category:Display Technology]] | |||
[[Category:Virtual Reality]] | |||
[[Category:Augmented Reality]] | |||
[[Category:Human Visual System]] | |||