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Microlens Arrays

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Microlens arrays (MLAs), sometimes called micro-lens arrays or lenslet arrays, are optical components consisting of multiple small lenses (often called lenslets) arranged in a one-dimensional or two-dimensional pattern on a supporting substrate[1][2][3][4]. Each lenslet typically has a diameter significantly less than 10 millimeters, often ranging from tens or hundreds of micrometers down to just a few micrometers, or even sub-micrometer in specialized cases[1][2][5][6]. The array pattern is commonly periodic, such as a square or hexagonal grid, but can also be linear, rectangular, circular, or even random/stochastic for specific applications[1][2][3][6]. An array can contain thousands, millions, or even more individual lenslets[1][7][8].

MLAs are characterized by their potential for miniaturization, integration into complex systems, and considerable design flexibility[6][9]. They are employed across diverse fields, including telecommunications (fiber coupling, optical switches), medical imaging (endoscopy, OCT), solar energy (concentrators), automotive LiDAR, laser beam homogenization and shaping, sensor technology (Shack–Hartmann wavefront sensors, image sensors), and consumer electronics (projectors, cameras, displays)[8][2][7][6][10][11].

In the rapidly evolving fields of Virtual Reality (VR) and Augmented Reality (AR), microlens arrays are emerging as crucial components enabling advancements in Head-Mounted Display (HMD) optics, eye tracking systems, light field display technologies, and environmental sensing[10][8][7][12][13][14][15].

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 polymers such as PMMA, polycarbonate, and PET[5][6][8][13]. 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[8][5][16]. Fused silica, for example, offers excellent transmission from UV (around 193nm) to near-infrared (up to 3µm)[5][7][16]. Silicon is suitable for infrared applications (approx. 1.2µm to 5µm) and integrates well with MEMS fabrication[8][5].
  • Lenslet Shape and Profile: Individual lenslets can have various shapes, including circular, square, or hexagonal footprints[2][6]. Their optical surfaces can be spherical, aspherical, cylindrical, or even freeform[6][8][3][17]. Aspherical and freeform profiles are crucial for minimizing optical aberrations (like spherical aberration and chromatic aberration) and achieving specific beam shaping or imaging performance, especially off-axis[1][8][14]. Cylindrical lenses focus light along a line rather than to a point and are used in applications like barcode scanners or laser line generation[8][11]. Lens profiles can be continuous surface type or stepped (diffractive)[6].
  • Array Pattern and Fill Factor: Common arrangements include square and hexagonal grids, which offer regular packing[1][2]. The arrangement influences the fill factor, defined as the ratio of the optically active area (the sum of lenslet areas) to the total array area. High fill factors (e.g., up to 98% or more for square arrays, potentially >99% for gapless hexagonal arrays) are often desired to maximize light throughput, ensure uniform illumination, and avoid energy loss or undesirable diffraction effects like zero-order hotspots in homogenization applications[2][7][18][3]. Some fabrication methods allow for "gapless" arrays where lenslets meet edge-to-edge[3]. Random or stochastic arrangements are also possible for specific diffusion or security applications[3].
  • Pitch: The center-to-center distance between adjacent lenslets is known as the pitch. This can range widely, from millimeters down to a few micrometers or even less, depending on the application and fabrication limits[1][6][19].
  • 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[6][20][19]. The NA describes the range of angles over which the lens can accept or emit light.
  • Coatings: Anti-reflection coatings are frequently applied to the MLA surfaces (often both sides) to minimize reflection losses and maximize light transmission within the desired spectral range[1][5][13]. Other coatings might be applied for filtering or environmental protection.
  • Dimensional tolerances and Quality: Key quality parameters include the accuracy of lenslet shape (surface form error, often specified in fractions of a wavelength), surface quality (scratch-dig), lenslet positioning accuracy (centration, pitch uniformity), focal length uniformity across the array, and overall array flatness[21][11]. Positional accuracy better than 1 µm can be achieved[11].

Fabrication Methods

Manufacturing microlens arrays requires specialized techniques capable of creating microscale optical structures with high precision and often in large volumes:

  • Photolithography and Etching: This is a foundational technique borrowed from the semiconductor industry. A pattern is defined using a photomask and photoresist on a substrate. Subsequent etching processes (e.g., wet etching or dry etching like reactive ion etching (RIE)) transfer the pattern into the substrate material, creating the lens structures[8][18][7][22][5]. Multiple etching steps can create multi-level diffractive structures[6].
  • Photoresist Reflow: In this method, cylindrical pillars of photoresist are first patterned using lithography. The substrate is then heated above the resist's glass transition temperature, causing the pillars to melt and reflow. Surface tension naturally pulls the resist into a spherical cap shape (lenslet). This resist lens pattern can then be transferred into the underlying substrate (e.g., fused silica) using RIE, or the reflowed resist structure itself can serve as a mold for replication[3][22].
  • Laser Direct Writing (LDW): High-precision lasers are used to directly shape the lenslets. This can involve selectively hardening a photosensitive material (like a photopolymer) or ablating material from the substrate surface[8][7][9]. LDW offers great flexibility in creating complex, aspheric, or freeform profiles but can be slower and more expensive than mask-based methods, though it is used for creating high-quality masters for replication[8][9].
  • Nanoimprint Lithography (NIL) / Hot Embossing: These are replication techniques. A master mold (stamp) containing the negative pattern of the MLA is created (e.g., using LDW or etching). This mold is then pressed into a softened thermoplastic material (hot embossing) or a UV-curable resin (UV-NIL) coated on a substrate. After hardening (cooling or UV curing), the mold is removed, leaving the MLA structure replicated on the substrate[8][3][7][22]. These methods are suitable for cost-effective high-volume production[3].
  • Injection Molding: Similar to NIL, this is a replication method suitable for mass production. A mold insert containing the negative MLA structure is placed in an injection molding machine. Molten optical-grade plastic (or sometimes specialized glass - Precision Glass Molding) is injected into the mold cavity. After cooling and solidifying, the finished MLA part is ejected[8][3][9][17]. Precision Glass Molding (PGM) uses polished glass preforms (blanks) heated to their transition temperature and pressed into shape by molds, offering high precision for glass MLAs[11][22].
  • Diamond Turning: Ultra-precision lathes equipped with diamond cutting tools can directly machine the MLA structures onto suitable substrate materials (metals for molds, or some IR materials like silicon or polymers directly)[5]. It's highly accurate but generally used for prototyping or creating master molds due to its serial nature.

Applications in VR/AR

Microlens arrays provide enabling capabilities for next-generation VR and AR systems, helping to address critical challenges related to form factor, field of view (FOV), visual quality (resolution, brightness, depth perception), and power consumption.

Display Optics

  • 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[23][24][25]. Each lenslet magnifies a portion of the microdisplay image. This architecture holds the potential for significantly thinner and lighter HMDs[26][27].
  • Wide Field of View (FOV) Systems: To achieve ultra-wide FOVs (e.g., 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[23][26][27]. In such wide-FOV systems, many lenslets are viewed significantly off-axis. To manage image quality (e.g., reduce distortion, maintain eye box size) across the entire FOV, heterogeneous MLAs are crucial. In these arrays, the properties (e.g., shape, focal length, tilt) of the lenslets are custom-designed and vary systematically across the array[26][27]. Optimization algorithms are used to design these complex heterogeneous lenslet profiles[26][27].
  • Light Field Displays: MLAs are a cornerstone technology for creating near-eye light field displays[28][29][30][31]. By placing a precisely aligned MLA over a high-resolution microdisplay, the light rays originating from different pixels under each lenslet can be controlled in direction[28][31]. Each lenslet projects a "micro-image" (sometimes called an elemental image or hogel) composed of pixels underneath it, and effectively acts as a projector sending different information in different directions[30][32]. This allows the display to reconstruct a light field that approximates the light rays that would emanate from a real 3D scene. Crucially, this enables the viewer's eye to naturally change focus to different depths within the virtual scene, potentially resolving the vergence-accommodation conflict (VAC) that plagues conventional stereoscopic displays and causes eye strain[33][34]. This technique is closely related to integral imaging[31][33]. A key challenge is the trade-off between spatial resolution (image sharpness) and angular resolution (number of views/depth cues)[31][30][34].
  • Efficiency and Brightness Enhancement: MLAs can improve the overall light efficiency of display systems. In projectors or backlit displays like LCDs, MLAs can be used to focus light specifically onto the active (transmitting) area of each pixel, reducing light absorption by the surrounding pixel structure (e.g., thin-film transistors)[2][1]. This increases brightness and reduces power consumption[13][9]. 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 polarizers and half-mirrors, but this process typically results in very low light efficiency (often cited as 10-25%)[24][35][36][37][38]. Novel freeform MLA designs claim much higher efficiencies (e.g., 80%) while also achieving thin profiles[38].

Sensing and Tracking

  • Eye Tracking: Tunable microlens arrays, such as those based on electrowetting liquid lenses, 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[33]. This could enhance the realism of light field displays or potentially correct for individual user refractive errors.
  • 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 (e.g., a "top-hat" profile) of infrared light onto the scene[12][13]. 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[12][3]. 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[12][3].
  • Wavefront Sensors: The Shack–Hartmann wavefront sensor uses an MLA placed in front of an image sensor (CCD or CMOS). An incoming optical wavefront is divided by the MLA into multiple beamlets, each focused onto the sensor. Deviations of the spot positions from a reference grid reveal the local slope of the wavefront, allowing its overall shape (including aberrations) to be reconstructed[1][7][6][11]. While primarily used in optical metrology and adaptive optics, this principle could potentially be adapted for HMD calibration or real-time measurement of the eye's aberrations for personalized display correction.
  • Light Field Cameras / 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[1][2][32]. 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[1][8][7][9].
  • High-Resolution Wide-FOV Imaging: Research demonstrates that combining ultrathin MLAs (potentially with wavelength-scale thickness using metasurface concepts) with computational reconstruction algorithms can achieve high-resolution imaging across a wide field of view within an extremely compact system[14]. This could lead to highly integrated, high-performance cameras for AR glasses or VR headset pass-through modes[14].

Advantages in VR/AR

  • Reduced Form Factor: MLA-based optics offer a pathway to significantly thinner and lighter HMDs compared to systems relying on single, thick conventional lenses or even Fresnel lenses[26][27][23][24][25]. Curved MLAs combined with curved displays can further enhance compactness, particularly for wide FOV designs[26][23]. MLA systems can be thinner than pancake optics[38].
  • Wide Field of View (FOV): Advanced MLA designs (curved, heterogeneous) are a key enabling technology for achieving ultra-wide fields of view (approaching or exceeding 180° horizontally) that better match human peripheral vision, enhancing immersion[26][27][23].
  • Light Field Rendering / Improved Depth Perception: MLA-based light field displays can generate more natural depth cues, allowing the eye to focus correctly and potentially mitigating the vergence-accommodation conflict, leading to greater visual comfort[28][33][34].
  • Higher Optical Efficiency: Compared to polarization-based folded optics like pancake lenses, MLA systems can potentially offer significantly higher light throughput, leading to brighter displays or reduced power consumption for the same brightness[38][24].
  • Aberration Correction: The ability to design individual lenslets with aspheric, freeform, or heterogeneous profiles allows for sophisticated, spatially-varying aberration correction across the field of view, potentially leading to sharper images[1][26][14][9].
  • Miniaturization and Integration: The inherent nature of MLAs facilitates integration into compact modules for sensing and imaging functions within the HMD[8][6].

Challenges and Considerations

  • Manufacturing Complexity and Cost: Fabricating MLAs with the required precision (sub-micron tolerances for shape and position), especially for complex designs (aspheric, freeform, heterogeneous, high fill factor, large area), remains challenging and can be expensive, particularly for achieving high yields in mass production[23][22][8]. Mold fabrication for replication techniques is a critical and costly step[8][11].
  • Resolution Trade-offs (Spatial vs. Angular): In light field display applications, there is a fundamental trade-off: increasing the angular resolution (more views, smoother depth) typically requires allocating more display pixels per lenslet, which reduces the overall spatial resolution (perceived sharpness) of the image, and vice versa[31][30][34]. High-resolution microdisplays are essential.
  • Diffraction Limits: As lenslet sizes shrink, diffraction effects become more pronounced, potentially limiting the achievable resolution or sharpness (spot size)[6] (Implied by size limits).
  • Chromatic Aberration: Like single lenses, simple MLA lenslets made from standard materials exhibit chromatic aberration (color fringing). This can be particularly noticeable in light field displays[32]. Correction requires achromatic lenslet designs (e.g., using multiple materials or diffractive features) or sophisticated computational correction algorithms[14].
  • Stray Light and Ghost Images: Multiple surfaces in an MLA-based optical system can lead to internal reflections, potentially causing stray light (reducing contrast) or noticeable ghost images, similar to issues encountered in multi-element lenses or pancake optics[24][9] (Mentions reducing glare/ghosting as advantage).
  • Computational Load: Rendering content for light field displays requires specialized algorithms (calculating the view for each direction from each lenslet) which can be significantly more computationally demanding than standard stereoscopic rendering[29][31][30]. Computational imaging techniques associated with some MLA sensors also require processing power[14].
  • Eye Box Size and Alignment Sensitivity: Designing an MLA system that provides a sufficiently large eye box (the volume within which the user's pupil can move without losing the image or experiencing significant degradation) can be challenging, especially for wide FOV designs. Misalignment between the eye, the MLA, and the display can lead to image artifacts[26][27].
  • Image Quality Artifacts: Depending on the design and quality, MLA-based systems can exhibit artifacts like visible seams between lenslet views, Moiré patterns (if interacting with display pixel structure), or non-uniform brightness/sharpness across the field.

Future Directions

The development of microlens arrays for VR/AR is an active area of research and innovation:

  • Advanced Manufacturing: Continued improvements in fabrication techniques (e.g., wafer-level optics, new materials, higher precision molding and lithography) are needed to enable cost-effective mass production of complex, high-performance MLAs[3][11][22].
  • Freeform and Heterogeneous Designs: Further exploration of freeform surfaces and heterogeneous lenslet optimization to simultaneously maximize FOV, eye box, resolution, and efficiency while minimizing aberrations and form factor[38][26][27].
  • Light Field Display Enhancement: Overcoming the spatial/angular resolution trade-off, reducing computational requirements, and improving image quality (e.g., reducing chromatic aliasing) for light field displays[34][32].
  • Tunable and Active MLAs: Development and integration of dynamic MLAs using technologies like liquid crystals or electrowetting for real-time focus adjustment, aberration correction, or gaze-contingent rendering[33].
  • Hybrid Optics: Combining MLAs with other advanced optical technologies like metasurfaces, diffractive optics (DOEs), holographic optical elements (HOEs), or polarization gratings to achieve novel functionalities or enhanced performance[14][22][24].
  • Computational Co-Design: Increasingly sophisticated computational tools that co-optimize the optical design of the MLA with the required image processing and rendering algorithms to achieve system-level performance targets[26][27][14].
  • Environmental Robustness: Developing MLAs with enhanced durability and resistance to environmental factors like humidity, for example through superhydrophobic surface treatments[39].

See Also

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Microlens arrays – fabrication, parameters, applications - RP Photonics
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Microlens Array - Shanghai Optics
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 Microlens Arrays, MLA - temicon
  4. ↑ Microlens array - Photonics Dictionary
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Microlens Arrays | Single Point Diamond Turning - Syntec Optics
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 Standard Microlens Array - Bön Optics (appears to be distributor for brand, original mfg unclear)
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 Introducing Microlens Arrays - Avantier Inc.
  8. ↑ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 Detailed Insights in Microlens Array Products - OPTICAL COMPONENTS
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Injection-molded microlens arrays - Apollo Optical Systems
  10. ↑ 10.0 10.1 Microlens Arrays: Versatile and Efficient Optical Solutions - Bote Optics Singapore
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 Efficient and precise production of microlens arrays using precision glass molding - GD Optics (PDF)
  12. ↑ 12.0 12.1 12.2 12.3 Apple Invents an optical system with Microlens Array Projectors to advance time-of-flight sensing for Face ID, delivering more realistic AR/VR features+ - Patently Apple (July 21, 2022)
  13. ↑ 13.0 13.1 13.2 13.3 13.4 AR-VR - Augmented and Virtual Reality - BrightView Technologies, Inc.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 (2024-03-19) Imaging with high resolution and wide field of view based on an ultrathin microlens array - AIP Publishing
  15. ↑ Types of Micro Optics - Avantier Inc.
  16. ↑ 16.0 16.1 Microlens Arrays - Edmund Optics
  17. ↑ 17.0 17.1 lens array - Isuzu Glass
  18. ↑ 18.0 18.1 Microlens Arrays - Newport
  19. ↑ 19.0 19.1 Lens Arrays | Microlens Array - MEETOPTICS
  20. ↑ MALS18 Micro Lens Array - Newport
  21. ↑ (Source appears generic, refers to Avantier as example) Microlens arrays – fabrication, parameters, applications - RP Photonics (mentions high accuracy vital)
  22. ↑ 22.0 22.1 22.2 22.3 22.4 22.5 22.6 (2024-12-09) Fabrication of Microlens Array and Its Application: A Review - ResearchGate (PDF)
  23. ↑ 23.0 23.1 23.2 23.3 23.4 23.5 (2023-05-10) Advanced Study of Optical Imaging Systems for Virtual Reality Head-Mounted Displays - Photonics
  24. ↑ 24.0 24.1 24.2 24.3 24.4 24.5 (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
  25. ↑ 25.0 25.1 Compact near-eye display system using a superlens-based microlens array magnifier - Optica Publishing Group (mentions MLA as magnifier)
  26. ↑ 26.00 26.01 26.02 26.03 26.04 26.05 26.06 26.07 26.08 26.09 26.10 (2024-10-22) ThinVR: Heterogeneous microlens arrays for compact, 180 degree FOV VR near-eye displays - ResearchGate
  27. ↑ 27.0 27.1 27.2 27.3 27.4 27.5 27.6 27.7 27.8 ThinVR: Heterogeneous microlens arrays for compact, 180 degree FOV VR near-eye displays - Ronald Azuma (PDF of paper)
  28. ↑ 28.0 28.1 28.2 Nvidia Near-Eye Light Field Display - LightField Forum
  29. ↑ 29.0 29.1 Supplementary Material: Near-Eye Light Field Displays - Research at NVIDIA (PDF)
  30. ↑ 30.0 30.1 30.2 30.3 30.4 Light-field Display Technical Deep Dive - Texas A&M College of Architecture (PDF)
  31. ↑ 31.0 31.1 31.2 31.3 31.4 31.5 Design and simulation of a light field display - ResearchGate
  32. ↑ 32.0 32.1 32.2 32.3 (2021-09-29) Examining the utility of pinhole-type screens for lightfield display - Optica Publishing Group
  33. ↑ 33.0 33.1 33.2 33.3 33.4 Fabrication of an electrowetting liquid microlens array for a focus tunable integral imaging system - Optica Publishing Group
  34. ↑ 34.0 34.1 34.2 34.3 34.4 VR IN FOCUS - Creal (PDF)
  35. ↑ (2024-12-09) Fabrication of Microlens Array and Its Application: A Review - ResearchGate (PDF)
  36. ↑ (2024-07-05) Catadioptric Imaging System Based on Pancake Lenses - LightTrans
  37. ↑ What are pancake lenses? - Reddit (Sept 13, 2022)
  38. ↑ 38.0 38.1 38.2 38.3 38.4 (2022-05-27) LIMBAK's freeform microlens array is thinner and much more efficient than pancake lenses for VR and MR - Reddit
  39. ↑ (2022-11-17) Flexible Superhydrophobic Microlens Arrays for Humid Outdoor Environment Applications - ACS Publications
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