A microlens is a small

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

, generally with a

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...

less than a millimetre (mm) and often as small as 10 micrometres (µm). The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects arise due to optical diffraction at the small features. A typical microlens may be a single element with one plane surface and one spherical convex surface to

refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...

the light. Because micro-lenses are so small, the substrate that supports them is usually thicker than the lens and this has to be taken into account in the design. More sophisticated lenses may use aspherical surfaces and others may use several layers of optical material to achieve their design performance. A different type of microlens has two flat and parallel surfaces and the focusing action is obtained by a variation of

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

across the lens. These are known as gradient-index (GRIN) lenses. Some micro-lenses achieve their focusing action by both a variation in refractive index and by the surface shape. Another class of microlens, sometimes known as micro-

Fresnel lens A Fresnel lens ( ; ; or ) is a type of composite compact lens developed by the French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses. It has been called "the invention that saved a million ships." The design allows the c ...

es, focus light by refraction in a set of concentric curved surfaces. Such lenses can be made very thin and lightweight. Binary-optic micro-lenses focus light by diffraction. They have grooves with stepped edges or multilevels that approximate the ideal shape. They have advantages in fabrication and replication by using standard semiconductor processes such as photolithography and

reactive-ion etching Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The pl ...

(RIE). Micro-lens arrays contain multiple lenses formed in a one-dimensional or two-dimensional array on a supporting substrate. If the individual lenses have circular apertures and are not allowed to overlap, they may be placed in a hexagonal array to obtain maximum coverage of the substrate. However, there will still be gaps between the lenses which can only be reduced by making the micro-lenses with non-circular apertures. With optical sensor arrays, tiny lens systems serve to focus and concentrate the light onto the photo-diode surface, instead of allowing it to fall on non-photosensitive areas of the pixel device. Fill-factor is the ratio of the active refracting area, i.e. that area which directs light to the photo-sensor, to the total contiguous area occupied by the microlens array.

Fabrication

In the 17th century, Robert Hooke and Antonie van Leeuwenhoek both developed techniques to make small glass lenses for use with their

microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...

s. Hooke melted small filaments of

Venetian glass Venetian glass () is glassware made in Venice, typically on the island of Murano near the city. Traditionally it is made with a soda–lime "metal" and is typically elaborately decorated, with various "hot" glass-forming techniques, as well as ...

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.Hooke R, Preface to ''Micrographia''. The Royal Society of London. (1665). The principle has been repeated by performing photolithography into materials such as

photoresist A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry. ...

or UV curable epoxy and melting the polymer to form arrays of multiple lenses.Daly D, Stevens R F, Hutley M C, Davies N, "The manufacture of microlenses by melting photoresist". ''Proceedings of seminar "Microlens Arrays"'', May 1991. IOP Short Meeting Series No 30, 23–34. More recently microlens arrays have been fabricated using convective assembly of colloidal particles from suspension. 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.Borrelli, N F. ''Microoptics technology: fabrication and applications of lens arrays and devices''. Marcel Dekker, New York (1999). 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.Veldkamp W B, McHugh T J. "Binary optics", ''Scientific American'', Vol. 266 No. 5 pp 50–55, (May 1992). 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. 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. Bio-microlenses have been developed to image biological specimens without causing damage. These can be made from a single cell attached to a fiber probe.

Wafer-level optics

Wafer-level optics (WLO) enables the design and manufacture of miniaturized optics at the wafer level using advanced

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

-like techniques. The end product is cost effective, miniaturized optics that enable the reduced form factor of camera modules for

mobile devices A mobile device (or handheld computer) is a computer small enough to hold and operate in the hand. Mobile devices typically have a flat LCD or OLED screen, a touchscreen interface, and digital or physical buttons. They may also have a physical ...

."Wafer-Level Camera Technologies Shrink Camera Phone Handsets", ''Photonics.com'', August 2007
The technology is scalable from a single-element CIF/VGA lens to a multi-element

mega pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...

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. 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.

Applications

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 sensor An active-pixel sensor (APS) is an image sensor where each pixel sensor unit cell has a photodetector (typically a pinned photodiode) and one or more active transistors. In a metal–oxide–semiconductor (MOS) active-pixel sensor, MOS field-effec ...

s, 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 projector A video projector is an image projector that receives a video signal and projects the corresponding image on a projection screen using a lens system. Video projectors use a very bright ultra-high-performance lamp (a special mercury arc lamp), X ...

s, 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. 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

photocopier A photocopier (also called copier or copy machine, and formerly Xerox machine, the generic trademark) is a machine that makes copies of documents and other visual images onto paper or plastic film quickly and cheaply. Most modern photocopiers ...

s and mobile-phone

camera A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...

s. Another application is in

3D imaging In computer vision and computer graphics, 3D reconstruction is the process of capturing the shape and appearance of real objects. This process can be accomplished either by active or passive methods. If the model is allowed to change its shape ...

and

displays A display device is an output device for presentation of information in visual or tactile form (the latter used for example in tactile electronic displays for blind people). When the input information that is supplied has an electrical signal the ...

. 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.Ives FE. ''Parallax stereogram and process of making same''. US Patent 725,567 (1903). The strips were later replaced by Hess with an array of

cylindrical lens A cylindrical lens is a lens which focuses light into a line instead of a point, as a spherical lens would. The curved face or faces of a cylindrical lens are sections of a cylinder, and focus the image passing through it into a line parallel to ...

es known as a lenticular screen, to make more efficient use of the illumination.Hess W. ''Improved manufacture of stereoscopic pictures''. UK Patent 13,034 (1912). Hitachi have 3D displays free of 3D glasses using arrays of microlens to create the stereoscopic effect. More recently, the availability of arrays of spherical micro-lenses has enabled

Gabriel Lippmann Jonas Ferdinand Gabriel Lippmann (16 August 1845 – 13 July 1921) was a Franco-Luxembourgish physicist and inventor, and Nobel laureate in physics for his method of reproducing colours photographically based on the phenomenon of interference. ...

's idea for

integral photography Integral imaging is a three-dimensional imaging technique that captures and reproduces a light field by using a two-dimensional array of microlenses (or lenslets), sometimes called a fly's-eye lens, normally without the aid of a larger overall objec ...

to be explored and demonstrated.Stevens R F, Davies N. "Lens arrays and photography". ''The Journal of Photographic Science''. Vol 39 pp 199–208, (1991). Colloidal micro-lenses have also enabled single molecule detection when used in conjunction with a long working distance, low light collection efficiency objective lens. Micro-lens arrays are also used by Lytro to achieve light field photography (

plenoptic camera A light field camera, also known as a plenoptic camera, is a camera that captures information about the '' light field'' emanating from a scene; that is, the intensity of light in a scene, and also the precise direction that the light rays are ...

) that eliminates the need for initial focusing prior to capturing images. Instead, focus is achieved in software during post-processing.

Characterization

In order to characterize micro-lenses it is necessary to measure parameters such as the focal length and quality of transmitted

wavefront In physics, the wavefront of a time-varying '' wave field'' is the set ( locus) of all points having the same '' phase''. The term is generally meaningful only for fields that, at each point, vary sinusoidally in time with a single temporal fr ...

.Iga K, Kokburn Y, Oikawa M. ''Fundamentals of microoptics''. Academic Press, London (1984). Special techniques and new definitions have been developed for this. For example, because it is not practical to locate the

principal plane In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the '' focal points'', the principal points, and the nodal points. For ''ideal'' s ...

s 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 An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propaga ...

. It is useful to know the distance at which the maximum amount of light is concentrated in the fibre

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

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.ISO 14880-1:2001. ''Optics and photonics - Microlens arrays - Part 1: Vocabulary''ISO 14880-2:2006. ''Optics and photonics - Microlens arrays - Part 2: Test methods for wavefront aberrations''ISO 14880-3:2006. ''Optics and photonics - Microlens arrays - Part 3: Test methods for optical properties other than wavefront aberrations''ISO 14880-4:2006. ''Optics and photonics - Microlens arrays - Part 4: Test methods for geometrical properties''.

Microoptics in nature

Examples of micro-optics are to be found in nature ranging from simple structures to gather light for

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

in leaves to

compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...

s in

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...

s. 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.Land M. "The optics of animal eyes". ''Proc Royal Institution'', vol 57, pp. 167–189, (1985)Duparré J. et al., "Microoptical telescope compound eye". ''Optics Express'', Vol. 13, Issue 3, pp. 889–903 (2005).

References