Microlens
Encyclopedia
A microlens is a small lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

, generally with a diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

less than a millimetre
Millimetre
The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....

(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
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

at the small features. A typical microlens may be a single element with one plane surface and one spherical convex surface to refract
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

the light. Because microlenses 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
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

across the lens. These are known as gradient-index (GRIN) lenses. Some microlenses 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
Fresnel lens
A Fresnel lens is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design...

es, focus light by refraction in a set of concentric curved surfaces. Such lenses can be made very thin and lightweight. Binary-optic microlenses focus light by diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

. 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
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...

and RIE.

Microlens 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 microlenses with non-circular apertures. With optical sensor arrays tiny lens systems serve to focus and concentrate the light onto the photodiode 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, ie that area which directs light to the photosensor, to the total contiguous area occupied by the microlens array.

## Fabrication

In the 17th century, Robert Hooke
Robert Hooke
Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

and Antonie van Leeuwenhoek both developed techniques to make small glass lenses for use with their microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

s. Hooke melted small filaments of Venetian glass
Venetian glass
Venetian glass is a type of glass object made in Venice, Italy, primarily on the island of Murano. It is world-renowned for being colourful, elaborate, and skillfully made....

and allowed the surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

in the molten glass to form the smooth spherical surfaces required for lenses, then mounting and grinding the lenses using conventional methods. The principle has been repeated by performing photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...

into materials such as photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....

or UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

curable epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

and melting the polymer to form arrays of multiple lenses. More recently microlens arrays have been fabricated using convective assembly of colloidal particles from suspension.

Advances in technology have enabled microlenses 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
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....

or embossing from a master lens array. The master lens array may also be replicated through the generation of an electroform
Electroforming
Electroforming is a metal forming process that forms thin parts through the electroplating process. The part is produced by plating a metal skin onto a base form, known as a mandrel, which is removed after plating...

using the master lens array as a mandrel
Mandrel
A mandrel is one of the following:* an object used to shape machined work.* a tool component that grips or clamps materials to be machined.* a tool component that can be used to grip other moving tool components.- Variants :...

. The ability to fabricate arrays containing thousands or millions of precisely spaced lenses has led to an increased number of applications.

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
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

industry, such as wafer-level optics
Wafer-level optics
Wafer-level optics enables the design and manufacture of miniaturized optics at the wafer level using advanced semiconductor-like techniques...

. This area is known as binary optics.

Microlenses in recent imaging chips have attained smaller and smaller sizes. The Canon EOS-1Ds Mark III packs 21.1 million microlenses onto its CMOS imaging chip, one per photosite, each just 6.4 micrometer across. An announced Sony DSLR 24.6MP image sensor will have even smaller microlenses.

Microlenses can be also made from liquids.

## Applications

Single microlenses are used to couple light to optical fibres while microlens arrays are often used to increase the light collection efficiency of CCD arrays. They collect and focus light that would have otherwise fallen on to the non-sensitive areas of the CCD. Microlens 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 microlenses 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
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

and not inverted as is the case with conventional lenses. Microlens arrays have been developed to form compact imaging devices for applications such as photocopier
Photocopier
A photocopier is a machine that makes paper copies of documents and other visual images quickly and cheaply. Most current photocopiers use a technology called xerography, a dry process using heat...

s and mobile-phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

camera
Camera
A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

s.

Another application is in 3D imaging and displays
3D display
A 3D display is any display device capable of conveying a stereoscopic perception of 3-D depth to the viewer. The basic requirement is to present offset images that are displayed separately to the left and right eye. Both of these 2-D offset images are then combined in the brain to give 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. The strips were later replaced by Hess with an array of cylindrical lens
Cylindrical lens
A cylindrical lens is a lens which focuses light which passes through on to a line instead of on to 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 onto a line parallel to the intersection of the...

es known as a lenticular screen, to make more efficient use of the illumination.

Hitachi have 3D displays free of 3D glasses using arrays of microlens to create the stereoscopic pictures felling.

More recently, the availability of arrays of spherical microlenses has enabled Gabriel Lippmann
Gabriel Lippmann
Jonas Ferdinand Gabriel Lippmann 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 to be explored and demonstrated. Colloidal microlenses have also enabled single molecule detection when used in conjunction with a long working distance, low light collection efficiency objective lens.

Microlens Arrays are also used by Lytro
Lytro
Lytro is a startup company founded in 2006 by Ren Ng, Ph.D, a light-field photography researcher at Stanford University that aims to deliver a light field camera for consumers by the end of 2011....

to achieve Light Field Photography (Plenoptic Camera
Plenoptic camera
A light-field camera, also called a plenoptic camera, is a camera that uses a microlens array to capture 4D light field information about a scene...

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

## Characterisation

In order to characterise microlenses it is necessary to measure parameters such as the focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

and quality of transmitted wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...

. 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
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...

and light from different regions of the microlens aperture may be focused to different points on the optical axis
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 or microscope.The optical axis is an imaginary line that defines the path along which light propagates through the system...

. It is useful to know the distance at which the maximum amount of light is concentrated in the fibre aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

and these factors have led to new definitions for focal length. To enable measurements on microlenses 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.

## Microoptics in nature

Examples of microoptics are to be found in nature ranging from simple structures to gather light for photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

in leaves to compound eyes in insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s. As methods of forming microlenses and detector arrays are further developed then the ability to mimic optical designs found in nature will lead to new compact optical systems.