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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 refract 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 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 lenses, 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. 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 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, i.e. 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 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.〔Hooke R, Preface to ''Micrographia''. The Royal Society of London. (1665).〕 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.〔Popovic CD, Sprague RA, Neville Connell GA, "Techniques for monolithic fabrication of microlens arrays", ''Appl. Opt.'' 27 1281–1284, (1988).〕〔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.〔Kumnorkaew P, Ee Y, Tansu N, and Gilchrist J F, "Investigation of the Deposition of Microsphere Monolayers for Fabrication of Microlens Arrays", ''Langmuir'', 24, 12150-12157, (2008)〕 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 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. This area is known as binary optics.〔Veldkamp W B, McHugh T J. "Binary optics", ''Scientific American'', Vol. 266 No. 5 pp 50–55, (May 1992).〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「microlens」の詳細全文を読む スポンサード リンク
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