Microlens Arrays: Difference between revisions

{{see also|Terms|Technical Terms}}

==Characteristics==

Microlens arrays possess several key parameters that define their function and application suitability:

*'''Materials:''' MLAs can be fabricated from a wide range of optical materials, including various types of [[glass]] (like BK7), [[UV]]-grade [[fused silica]], [[silicon]], [[quartz]], [[zinc selenide]] (ZnSe), [[calcium fluoride]], and numerous optical [[polymer]]s such as [[PMMA]], [[polycarbonate]], and PET<ref name="Syntec"/><ref name="StandardMLA"/><ref name="OpticalComponents"/><ref name="BrightView"/>. The material choice is critical and depends on the target [[wavelength]] range (from deep UV to far [[infrared]]), required [[durability]], thermal stability, compatibility with [[manufacturing]] processes, and cost<ref name="OpticalComponents"/><ref name="Syntec"/><ref name="EdmundOptics">Microlens Arrays - Edmund Optics</ref>. Fused silica, for example, offers excellent transmission from UV (around 193nm) to near-infrared (up to 3µm)<ref name="Syntec"/><ref name="AvantierIntro"/><ref name="EdmundOptics"/>. Silicon is suitable for infrared applications (approx. 1.2µm to 5µm) and integrates well with [[MEMS]] fabrication<ref name="OpticalComponents"/><ref name="Syntec"/>.

** [[Plano-convex lens|Plano-convex]] (flat on one side, convex on the other)

** [[Bi-convex lens|Bi-convex]] (convex on both sides)

*'''[[Focal Length]] and [[Numerical Aperture]] (NA):''' The focal length of the individual lenslets determines their focusing power. Available focal lengths range from sub-millimeter to tens or hundreds of millimeters<ref name="StandardMLA"/><ref name="NewportMALS18">MALS18 Micro Lens Array - Newport</ref><ref name="MeetOptics"/>. The NA describes the range of angles over which the lens can accept or emit light.

*'''[[Optical Coating|Coatings]]:''' [[Anti-reflection coating]]s are frequently applied to the MLA surfaces (often both sides) to minimize reflection losses and maximize light transmission within the desired spectral range<ref name="RPPhotonics"/><ref name="Syntec"/><ref name="BrightView"/>. Other coatings might be applied for filtering or environmental protection.

==Fabrication Methods==

*'''Compact Magnifiers / Eyepieces:''' One key application is replacing bulky single-element eyepiece lenses (like traditional refractive lenses or even Fresnel lenses) with MLAs positioned between the [[microdisplay]] and the user's eye<ref name="PhotonicsArticle">(2023-05-10) Advanced Study of Optical Imaging Systems for Virtual Reality Head-Mounted Displays - Photonics</ref><ref name="PolGrating">(2025-01-02) Field of view and angular-resolution enhancement in microlens array type virtual reality near-eye display using polarization grating - Optica Publishing Group</ref><ref name="SuperlensMLA">Compact near-eye display system using a superlens-based microlens array magnifier - Optica Publishing Group (mentions MLA as magnifier)</ref>. Each lenslet magnifies a portion of the microdisplay image. This architecture holds the potential for significantly thinner and lighter [[Head-Mounted Display|HMDs]]<ref name="ThinVR">(2024-10-22) ThinVR: Heterogeneous microlens arrays for compact, 180 degree FOV VR near-eye displays - ResearchGate</ref><ref name="AzumaThinVR">ThinVR: Heterogeneous microlens arrays for compact, 180 degree FOV VR near-eye displays - Ronald Azuma (PDF of paper)</ref>.

*'''Wide Field of View (FOV) Systems:''' To achieve ultra-wide FOVs (for example 180° horizontally, approaching the human visual system's range) while maintaining a compact form factor, researchers are exploring the use of curved MLAs paired with curved displays<ref name="PhotonicsArticle"/><ref name="ThinVR"/><ref name="AzumaThinVR"/>. In such wide-FOV systems, many lenslets are viewed significantly off-axis. To manage image quality (for example reduce distortion, maintain [[eye box]] size) across the entire FOV, heterogeneous MLAs are crucial. In these arrays, the properties (for example shape, focal length, tilt) of the lenslets are custom-designed and vary systematically across the array<ref name="ThinVR"/><ref name="AzumaThinVR"/>. Optimization algorithms are used to design these complex heterogeneous lenslet profiles<ref name="ThinVR"/><ref name="AzumaThinVR"/>.

*'''Efficiency, Brightness, and [[Screen-door effect]] Reduction:''' MLAs can improve the overall light efficiency and perceived quality of display systems. In projectors or backlit displays like [[LCD]]s, MLAs can be used to focus light specifically onto the active (transmitting) area of each pixel, reducing light absorption by the surrounding pixel structure (for example [[thin-film transistor]]s)<ref name="ShanghaiOptics"/><ref name="RPPhotonics"/>. This increases brightness and reduces power consumption<ref name="BrightView"/><ref name="ApolloOptics"/>. Manufacturers like Sony have applied MLAs over [[OLED]] microdisplays to increase peak luminance<ref name="DisplayDailySony">(Nov 2021) Emerging Display Technologies for the AR/VR Market - Display Daily</ref>. By directing light more effectively or magnifying the apparent pixel area, MLAs can also help reduce the visible "screen door effect" (the dark gaps between pixels)<ref name="Article1Ref_Lanman2013">Lanman, D., & Luebke, D. (2013). Near-eye light field displays. ACM Transactions on Graphics, 32(6), 1-10.</ref>. Furthermore, MLA-based eyepiece designs can offer inherently better light efficiency compared to polarization-dependent folded optical paths used in [[pancake lens]] designs. Pancake lenses achieve thin form factors by folding the optical path using [[polarizer]]s and [[half-mirror]]s, but this process typically results in very low light efficiency (often cited as 10-25%)<ref name="PolGrating"/><ref name="PancakeReview">(2024-12-09) Fabrication of Microlens Array and Its Application: A Review - ResearchGate (PDF)</ref><ref name="LightTransPancake">(2024-07-05) Catadioptric Imaging System Based on Pancake Lenses - LightTrans</ref><ref name="RedditPancake">What are pancake lenses? - Reddit (Sept 13, 2022)</ref><ref name="LimbakReddit">(2022-05-27) LIMBAK's freeform microlens array is thinner and much more efficient than pancake lenses for VR and MR - Reddit</ref>. Novel freeform MLA designs claim much higher efficiencies (for example 80%) while also achieving thin profiles<ref name="LimbakReddit"/>.

*'''[[Waveguide]] Coupling:''' In AR [[waveguide]] displays, microlens arrays can potentially serve as [[in-coupling]] and [[out-coupling]] elements, efficiently directing light from miniature projectors (like [[microLED]] arrays) into thin waveguides and then out toward the user's eyes. Research suggests pixel-level collimating microlenses could narrow microLED emission angles for better waveguide injection, though adding fabrication complexity<ref name="WaveguideReview">(2023) Waveguide-based augmented reality displays: perspectives and challenges - eLight (SpringerOpen)</ref><ref name="Article1Ref_Kress2019">Kress, B. C., & Meyrueis, P. (2019). Applied digital optics: from micro-optics to nanophotonics. John Wiley & Sons.</ref>.

*'''[[Eye Tracking]] and Dynamic Focus:''' Tunable microlens arrays, such as those based on [[electrowetting]] liquid lenses or [[liquid crystal lens]]es, can be integrated into HMDs. Combined with eye-tracking cameras, these systems could dynamically adjust the focus of the displayed image or specific lenslets to match the user's gaze depth in real-time<ref name="ElectrowettingLFD"/>. This could enhance the realism of light field displays, provide variable focus capabilities<ref name="Article1Ref_Muenster2019">Muenster, R., Jaeger, G., Hubner, M., Stetter, M., & Stilla, U. (2019). Liquid crystal tunable microlens array for augmented reality displays. In Digital Optical Technologies 2019 (Vol. 11062, p. 110620J). International Society for Optics and Photonics.</ref>, potentially correct for individual user refractive errors, or even simulate depth-of-field effects by selectively blurring parts of the image<ref name="PatentCNBlur">(B) CN107942517B: VR head-mounted display device with function of relieving visual fatigue based on liquid crystal microlens array - Google Patents</ref>. MLAs are also used in some [[eye tracking]] systems to help collect and direct light for imaging the user's pupil, enabling features like [[foveated rendering]]<ref name="Article1Ref_Kim2019">Kim, J., Jeong, Y., Stengel, M., Akşit, K., Albert, R., Boudaoud, B., ... & Luebke, D. (2019). Foveated AR: dynamically-foveated augmented reality display. ACM Transactions on Graphics, 38(4), 1-15.</ref>.

*'''Depth Sensing ([[Time-of-Flight]], [[Structured Light]]):''' MLAs play a role in the projection modules of active depth sensing systems. In [[Time-of-Flight]] (ToF) sensors, MLAs can shape and homogenize the output beam from illumination sources like [[VCSEL]] arrays, projecting a well-defined pattern (for example a "top-hat" profile) of infrared light onto the scene<ref name="PatentlyApple"/><ref name="BrightView"/>. In [[Structured Light]] systems (like those used in some versions of Apple's [[Face ID]]), MLAs can project a complex pattern of spots or lines onto the target<ref name="PatentlyApple"/><ref name="TEMICON"/>. The distortion of this pattern as seen by a sensor reveals the 3D shape of the target. These capabilities are essential for environmental mapping, hand tracking, [[gesture recognition]], and object recognition in AR/VR<ref name="PatentlyApple"/><ref name="TEMICON"/>. Some HMD patent designs use multiple MLAs combined with parallax barriers for 3D imaging<ref name="PatentUSMultiMLA">(A1) US20140168783A1: Near-eye microlens array displays - Google Patents</ref>.

*'''[[Light Field Camera]]s / Imaging Enhancement:''' Placing an MLA in front of an image sensor enables the capture of light field information (intensity and direction of light rays), creating a [[plenoptic camera]]<ref name="RPPhotonics"/><ref name="ShanghaiOptics"/><ref name="OpticaLFD"/>. This allows computational features like post-capture refocusing, depth map extraction, and perspective shifting. Such capabilities could be valuable for outward-facing cameras on AR/VR headsets for improved environmental understanding or [[computational photography]]. Even in conventional cameras, MLAs are often placed directly on CMOS/CCD sensors (one lenslet per pixel) simply to increase [[light collection]] efficiency (the optical fill factor) by funneling more incident light onto the active photosensitive area of each pixel, improving low-light performance and sensitivity<ref name="RPPhotonics"/><ref name="OpticalComponents"/><ref name="AvantierIntro"/><ref name="ApolloOptics"/>.

*'''High-Resolution Wide-FOV Imaging:''' Research demonstrates that combining ultrathin MLAs (potentially with wavelength-scale thickness using [[metasurface]] concepts) with [[computational imaging|computational reconstruction]] algorithms can achieve high-resolution imaging across a wide field of view within an extremely compact system<ref name="UltraThinMLA"/>. This could lead to highly integrated, high-performance cameras for AR glasses or VR headset pass-through modes<ref name="UltraThinMLA"/>.

* [[Time-of-Flight camera]]

* [[Structured Light]]

* [[Optical Aberration]]

* [[Field of View]]