Micro-Lens Array (MLA): Efficient Use of Light Using Sub-Micron Lenses Application Note

Micro-lenses are sub-micrometer lenses (often up to 10 microns), typically made with fused silica due to its excellent transmission characteristics of UV to IR rays. MLAs are one or two-dimensional arrays of these micro-lenses (lenslets), patterned in a squared packing order on a wafer. MLAs are commonly manufactured using standard semiconductor processes like photolithography and reactive ion etching (RIE).


The packing of the lenses in an array determines the fill factor. Circular lenslets on a square wafer will cover π/4 = 78.5% of the wafer. A higher fill factor is desirable, and achievable by hexagonal packing of lenslets as seen in Figure 1 – but these arrays are usually not applicable. The fill factor determines the light throughput of the material, apart from the transmissivity of the MLA.

Micro-Lens Array 1
Figure 1. Square/hexagonal arrangement pattern

MLAs are implemented in devices that require an increase in optical fill factor due to metallic shielding and non-photosensitive areas, without the use of additional optical components. For example, MLAs are used in charge-coupled devices (CCD) to concentrate the light onto its photodiode rather than metallic exposure gates and shield, where the imaging information is lost.

Micro-Lens Array 3
Figure 2. Parameters of a conventional lenslet
Micro-Lens Array 5
Figure 3. Ray diagram of single/dual-surface lenslet

Important Parameters

  • Lens Diameter
  • Effective Focal Length (EFL)
  • Radius of Curvature (ROC)
  • Array Size
  • Refractive Index, n
  • Lens Sag


MLAs can homogenize, collimate, and image light from various emitters, from excimer lasers to high powered LEDs. They are useful for applications that require high frequency and non-Gaussian uniformity.

Depending on the use case, specific types of micro-lenses are used such as in (a) & (b) examples.

a) Gradient-Index (GRIN) lenses

GRIN lenses are made of two flat and parallel surfaces, where only plane optical surfaces are used instead of conventional curved surfaces. The lens features a varied refractive index through the lens, which causes light rays to bend inside the lens (Figure 4). GRIN lenses are also commonly found in photocopiers and scanners.

Micro-Lens Array 7
Figure 4. GRIN lens causing rays of light to bend within

b) Micro-Fresnel Lenses (MFL)

Fresnel lenses are made up of a series of concentric grooves engraved into plastic or glass. Acting as individual refracting surfaces, the contours bend parallel light rays to a common focal length (Figure 5), or collimate the beam, depending on the direction. The direction of propagation of light does not change within a medium but is only deviated at the surface. This lens provides a better-focusing performance compared to conventional lenses. MFL is commonly utilized in TLR/SLR camera screens.

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Figure 5. MFL lens causing rays to focus

MLA systems are used in, but not limited to the following application scenarios:

  • Shack-Hartmann Wavefront Sensor
  • Medical/Aesthetic Laser Treatments
  • Laser Material Processing
  • CCD & CMOS Image Sensors

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