Microlens

In the 17th century, Robert Hooke and Antonie van Leeuwenhoek both developed techniques to make small glass lenses for use with their microscopes. Hooke melted small filaments of Venetian glass and allowed the surface tension in the molten glass to form the smooth spherical surfaces required for lenses, then mounting and grinding the lenses using conventional methods.^[1] The principle has been repeated by performing photolithography into materials such as photoresist or UV curable epoxy and melting the polymer to form arrays of multiple lenses.^[2][3] More recently microlens arrays have been fabricated using convective assembly of colloidal particles from suspension.^[4]

Advances in technology have enabled micro-lenses to be designed and fabricated to close tolerances by a variety of methods. In most cases multiple copies are required and these can be formed by moulding or embossing from a master lens array. The master lens array may also be replicated through the generation of an electroform using the master lens array as a mandrel. The ability to fabricate arrays containing thousands or millions of precisely spaced lenses has led to an increased number of applications.^[5]

The optical efficiency of diffracting lenses depends on the shape of the groove structure and, if the ideal shape can be approximated by a series of steps or multilevels, the structures may be fabricated using technology developed for the integrated circuit industry, such as wafer-level optics. The study of such diffracting lenses is known as binary optics.^[6]

Micro-lenses in recent imaging chips have attained smaller and smaller sizes. The Samsung NX1 mirrorless system camera packs 28.2 million micro-lenses onto its CMOS imaging chip, one per photo-site, each with a side length of just 3.63 micrometer. For smartphones this process is miniaturized even further: The Samsung Galaxy S6 has a CMOS sensor with pixels only 1.12 micrometer each. These pixels are covered with micro-lenses of an equally small pitch.

Micro-lenses can be also made from liquids.^[7] Recently, a glass-like resilient free-form micro-lenses were realized via ultra-fast laser 3D nanolithography technique. The sustained ~2 GW/cm² intensity for femtosecond pulsed irradiation shows its potential in high power and/or harsh environment applications.^[8]

Bio-microlenses have been developed to image biological specimens without causing damage.^[9][10] These can be made from a single cell attached to a fiber probe.

Wafer-level optics (WLO) enables the design and manufacture of miniaturized optics at the wafer level using advanced semiconductor-like techniques. The end product is cost effective, miniaturized optics that enable the reduced form factor of camera modules for mobile devices.^[11]

The technology is scalable from a single-element CIF/VGA lens to a multi-element mega pixel lens structure, where the lens wafers are precision aligned, bonded together and diced to form multi-element lens stacks. As of 2009 the technology was used in about 10 percent of the mobile phone camera lens market.^[12]

Semiconductor stacking methodology can now be used to fabricate wafer-level optical elements in a chip scale package. The result is a wafer-level camera module that measures .575 mm x 0.575 mm. The module can be integrated into a catheter or endoscope with a diameter as small as 1.0 mm.^[13]

Single micro-lenses are used to couple light to optical fibres; microlens arrays are often used to increase the light collection efficiency of CCD arrays and CMOS sensors, to collect and focus light that would have otherwise fallen onto the non-sensitive areas of the sensor. Micro-lens arrays are also used in some digital projectors, to focus light to the active areas of the LCD used to generate the image to be projected. Current research also relies on micro-lenses of various types to act as concentrators for high efficiency photovoltaics for electricity production.^[14]

Combinations of microlens arrays have been designed that have novel imaging properties, such as the ability to form an image at unit magnification and not inverted as is the case with conventional lenses. Micro-lens arrays have been developed to form compact imaging devices for applications such as photocopiers and mobile-phone cameras.

Another application is in 3D imaging and displays. In 1902, Frederic E. Ives proposed the use of an array of alternately transmitting and opaque strips to define the viewing directions for a pair of interlaced images and hence enable the observer to see a 3D stereoscopic image.^[15] The strips were later replaced by Hess with an array of cylindrical lenses known as a lenticular screen, to make more efficient use of the illumination.^[16]

Hitachi have 3D displays free of 3D glasses using arrays of microlens to create the stereoscopic effect. ^{[citation needed]}

More recently, the availability of arrays of spherical micro-lenses has enabled Gabriel Lippmann's idea for integral photography to be explored and demonstrated.^[17][18] Colloidal micro-lenses have also enabled single molecule detection when used in conjunction with a long working distance, low light collection efficiency objective lens.^[19]

Micro-lens arrays are also used by Lytro to achieve light field photography (plenoptic camera) that eliminates the need for initial focusing prior to capturing images. Instead, focus is achieved in software during post-processing.^[20]

In order to characterize micro-lenses it is necessary to measure parameters such as the focal length and quality of transmitted wavefront.^[21] Special techniques and new definitions have been developed for this.

For example, because it is not practical to locate the principal planes of such small lenses, measurements are often made with respect to the lens or substrate surface. Where a lens is used to couple light into an optical fibre the focused wavefront may exhibit spherical aberration and light from different regions of the microlens aperture may be focused to different points on the optical axis. It is useful to know the distance at which the maximum amount of light is concentrated in the fibre aperture and these factors have led to new definitions for focal length. To enable measurements on micro-lenses to be compared and parts to be interchanged, a series of international standards has been developed to assist users and manufacturers by defining microlens properties and describing appropriate measurement methods.^{[22][23][24][25]}

Examples of micro-optics are to be found in nature ranging from simple structures to gather light for photosynthesis in leaves to compound eyes in insects. As methods of forming micro-lenses and detector arrays are further developed, the ability to mimic optical designs found in nature will lead to new compact optical systems.^[26][27]

• Lenticular lens
• Integral imaging
• Plenoptic camera