Microlens Arrays: Difference between revisions

A [[microlens array]] (MLA) is a planar grid of microscopic refractive elements—typically tens to hundreds of micrometres in diameter—fabricated on a common transparent substrate.<ref name="Paschotta">R. Paschotta, ''RP Photonics Encyclopedia'', article “Microlens Arrays”, accessed 26&nbsp;April&nbsp;2025.</ref> Each lenslet focuses or collimates incoming light, enabling compact optical functions that would otherwise require bulky macroscopic optics. MLAs commonly appear in [[Virtual reality|VR]] and [[Augmented reality|AR]] displays, image sensors, illumination systems, and wavefront sensors.

A typical array arranges circular or hexagonal lenslets in square- or hexagonal-close packing. Key parameters include:

* '''Pitch''' (centre-to-centre spacing, usually 50–300 µm),

* '''Diameter''' (often equal to pitch for circular lenses),

* '''Focal length''' (<math>f</math>≈100 µm–5 mm, depending on design),

* '''Numerical aperture''' (dictated by diameter and <math>f</math>),

* '''Fill factor''' (total lens area ÷ array area; ≈90 % for hexagonal packing).<ref name="Paschotta"/>

Because lenslets are small, geometric aberrations are negligible, but diffraction and interference effects become significant when features approach the optical wavelength.<ref name="eLight">H. Chang&nbsp;et al., “Waveguide-based augmented-reality displays: perspectives and challenges,” ''eLight'' 3 (3), 2023.</ref>

* '''Photoresist reflow''' – patterning cylindrical posts in photoresist, then melting them to form hemispherical lenses.<ref name="Avantier">Avantier Inc., “Introducing Microlens Arrays,” Knowledge Center, 2020.</ref>

* '''Grayscale lithography''' – exposing a resist with spatially varying dose to directly sculpt the lens surface.

* '''Wafer-level glass molding''' – precision pressing of glass into a mould to form hundreds of thousands of lenslets on 150- or 200-mm wafers.

* '''Two-photon polymerisation''' – direct laser writing of free-form lens shapes with sub-micron accuracy.

Common substrates include UV-cured polymers (e.g., PMMA, Ormocer), fused silica, and optical-grade polycarbonate. Anti-reflection coatings are often applied to both air- and substrate-side surfaces.<ref name="Avantier"/>

* '''Image sensors''' – A one-to-one microlens–pixel arrangement funnels light into each photodiode, raising quantum efficiency and mitigating “dead-space” between pixels.<ref name="DisplayDaily">DisplayDaily, “Emerging Display Technologies for the AR/VR Market,” Nov 2021.</ref>

* '''[[Light-field camera|Light-field capture]]''' – An MLA in front of the sensor encodes both spatial and angular radiance, enabling computational refocus and depth extraction.<ref name="NVIDIA">D. Lanman &amp; D. Luebke, “Near-Eye Light-Field Displays,” NVIDIA Research, 2013.</ref>

OLED and LCD micro-displays used in near-eye optics leave dark inter-pixel gaps. Depositing an MLA directly on the colour-filter/top-glass increases pixel aperture ratio, boosting brightness and reducing the “screen-door” effect.<ref name="DisplayDaily"/>

NVIDIA demonstrated a 5-mm-thick 1280 × 720 OLED bonded to a 1.0-mm-pitch MLA (<math>f≈3.3 \text{mm}</math>). The array projects a four-plane light field that lets the eye accommodate naturally, mitigating the vergence–accommodation conflict inherent in stereoscopic HMDs.<ref name="NVIDIA"/>

Patents by Chinese and Korean manufacturers describe electrically tunable liquid-crystal MLAs inside a [[Head-mounted display]] (HMD). By varying lenslet focal length per video frame, the system optically blurs user-defined regions, simulating depth-of-field and reducing eye strain.<ref name="CNPatent">CN107942517B, “Head-mounted display device based on liquid-crystal microlens array,” State&nbsp;Intellectual&nbsp;Property&nbsp;Office&nbsp;of&nbsp;China, granted 2022.</ref>

* '''Sony''' uses on-glass MLAs on its 0.7-inch OLED micro-display line for AR EVF modules.<ref name="DisplayDaily"/>

* '''Apple''' patent US 2014/0168783 describes combining multiple MLAs with a parallax barrier for a switchable 3-D near-eye display.<ref name="ApplePatent">US20140168783A1, “Electronic device with microlens arrays for providing perspective-corrected imagery,” 2014.</ref>