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A microlens is a small lens, generally with a diameter 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 refraction 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 asphere 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 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-, focus light by refraction in a set of concentric curved surfaces. Such lenses can be made very thin and lightweight. Binary optics 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 (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 , 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.
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 electroforming 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.
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.
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.
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 and mobile phone .
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.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 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's idea for integral photography 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) that eliminates the need for initial focusing prior to capturing images. Instead, focus is achieved in software during post-processing.
For example, because it is not practical to locate the 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.ISO 14880-1:2001. Optics and photonics - Microlens arrays - Part 1: VocabularyISO 14880-2:2006. Optics and photonics - Microlens arrays - Part 2: Test methods for wavefront aberrationsISO 14880-3:2006. Optics and photonics - Microlens arrays - Part 3: Test methods for optical properties other than wavefront aberrationsISO 14880-4:2006. Optics and photonics - Microlens arrays - Part 4: Test methods for geometrical properties.
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