A Fresnel lens is a type of compact lens 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. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances; the idea of creating a thinner, lighter lens in the form of a series of annular steps is attributed to Georges-Louis Leclerc, Comte de Buffon. Whereas Buffon proposed grinding such a lens from a single piece of glass, the Marquis de Condorcet proposed making it with separate sections mounted in a frame. French physicist and engineer Augustin-Jean Fresnel is most given credit for the development of the multi-part lens for use in lighthouses.
According to Smithsonian magazine, the first Fresnel lens was used in 1823 in the Cordouan lighthouse at the mouth of the Gironde estuary. Scottish physicist Sir David Brewster is credited with convincing the United Kingdom to adopt these lenses in their lighthouses; the Fresnel lens reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections. An ideal Fresnel lens would have an infinite number of sections. In each section, the overall thickness is decreased compared to an equivalent simple lens; this divides the continuous surface of a standard lens into a set of surfaces of the same curvature, with stepwise discontinuities between them. In some lenses, the curved surfaces are replaced with flat surfaces, with a different angle in each section; such a lens can be regarded as an array of prisms arranged in a circular fashion, with steeper prisms on the edges, a flat or convex center. In the first Fresnel lenses, each section was a separate prism.'Single-piece' Fresnel lenses were produced, being used for automobile headlamps, brake and turn signal lenses, so on.
In modern times, computer-controlled milling equipment might be used to manufacture more complex lenses. Fresnel lens design allows a substantial reduction in thickness, at the expense of reducing the imaging quality of the lens, why precise imaging applications such as photography still use larger conventional lenses. Fresnel lenses are made of glass or plastic. In many cases they are thin and flat flexible, with thicknesses in the 1 to 5 mm range. Modern Fresnel lenses consist of all refractive elements; however many of the lighthouses have both refracting and reflecting elements, as shown in the photographs and diagram. That is, the outer elements are sections of reflectors while the inner elements are sections of refractive lenses. Total internal reflection was used to avoid the light loss in reflection from a silvered mirror. Fresnel produced six sizes of lighthouse lenses, divided into four orders based on their size and focal length. In modern use, these are classified as first through sixth order.
An intermediate size between third and fourth order was added as well as sizes above first order and below sixth. A first-order lens has a focal length of a maximum diameter 2590 mm high; the complete assembly is 1.8 m wide. The smallest has a focal length of an optical diameter 433 mm high; the largest Fresnel lenses are called hyperradiant Fresnel lenses. One such lens was on hand when it was decided to outfit the Makapuu Point Light in Hawaii. Rather than order a new lens, the huge optic construction, 3.7 metres tall and with over a thousand prisms, was used there. There are two main types of Fresnel lens: non-imaging. Imaging Fresnel lenses use segments with curved cross-sections and produce sharp images, while non-imaging lenses have segments with flat cross-sections, do not produce sharp images; as the number of segments increases, the two types of lens become more similar to each other. In the abstract case of an infinite number of segments, the difference between curved and flat segments disappears.
Spherical A spherical Fresnel lens is equivalent to a simple spherical lens, using ring-shaped segments that are each a portion of a sphere, that all focus light on a single point. This type of lens produces a sharp image, although not quite as clear as the equivalent simple spherical lens due to diffraction at the edges of the ridges. Cylindrical A cylindrical Fresnel lens is equivalent to a simple cylindrical lens, using straight segments with circular cross-section, focusing light on a single line; this type produces a sharp image, although not quite as clear as the equivalent simple cylindrical lens due to diffraction at the edges of the ridges. Spot A non-imaging spot Fresnel lens uses ring-shaped segments with cross sections that are straight lines rather than circular arcs; such a lens does not produce a sharp image. These lenses have application such as focusing sunlight on a solar panel. Fresnel lenses may be used as components of Köhler illumination optics resulting in effective nonimaging optics Fresnel-Köhler solar concentrators.
Linear A non-imagin
In electromagnetics and communications engineering, the term waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used to carry radio waves; this type of waveguide is used as a transmission line at microwave frequencies, for such purposes as connecting microwave transmitters and receivers to their antennas, in equipment such as microwave ovens, radar sets, satellite communications, microwave radio links. A dielectric waveguide employs a solid dielectric rod rather than a hollow pipe. An optical fibre is a dielectric guide designed to work at optical frequencies. Transmission lines such as microstrip, coplanar waveguide, stripline or coaxial cable may be considered to be waveguides; the electromagnetic waves in a waveguide may be imagined as travelling down the guide in a zig-zag path, being reflected between opposite walls of the guide. For the particular case of rectangular waveguide, it is possible to base an exact analysis on this view.
Propagation in a dielectric waveguide may be viewed in the same way, with the waves confined to the dielectric by total internal reflection at its surface. Some structures, such as non-radiative dielectric waveguides and the Goubau line, use both metal walls and dielectric surfaces to confine the wave. Depending on the frequency, waveguides can be constructed from either conductive or dielectric materials; the lower the frequency to be passed the larger the waveguide is. For example, the natural waveguide the earth forms given by the dimensions between the conductive ionosphere and the ground as well as the circumference at the median altitude of the Earth is resonant at 7.83 Hz. This is known as Schumann resonance. On the other hand, waveguides used in high frequency communications can be less than a millimeter in width. During the 1890s theorists did the first analyses of electromagnetic waves in ducts. Around 1893 J. J. Thomson derived the electromagnetic modes inside a cylindrical metal cavity.
In 1897 Lord Rayleigh did a definitive analysis of waveguides. He showed that the waves could travel without attenuation only in specific normal modes with either the electric field or magnetic field, or both, perpendicular to the direction of propagation, he showed each mode had a cutoff frequency below which waves would not propagate. Since the cutoff wavelength for a given tube was of the same order as its width, it was clear that a hollow conducting tube could not carry radio wavelengths much larger than its diameter. In 1902 R. H. Weber observed that electromagnetic waves travel at a slower speed in tubes than in free space, deduced the reason. Prior to the 1920s, practical work on radio waves concentrated on the low frequency end of the radio spectrum, as these frequencies were better for long-range communication; these were far below the frequencies that could propagate in large waveguides, so there was little experimental work on waveguides during this period, although a few experiments were done.
In a June 1, 1894 lecture, "The work of Hertz", before the Royal Society, Oliver Lodge demonstrated the transmission of 3 inch radio waves from a spark gap through a short cylindrical copper duct. In his pioneering 1894-1900 research on microwaves, Jagadish Chandra Bose used short lengths of pipe to conduct the waves, so some sources credit him with inventing the waveguide. However, after this, the concept of radio waves being carried by a tube or duct passed out of engineering knowledge. During the 1920s the first continuous sources of high frequency radio waves were developed: the Barkhausen-Kurz tube, the first oscillator which could produce power at UHF frequencies; these made possible the first systematic research on microwaves in the 1930s. It was discovered that transmission lines used to carry lower frequency radio waves, parallel line and coaxial cable, had excessive power losses at microwave frequencies, creating a need for a new transmission method; the waveguide was developed independently between 1932 and 1936 by George C.
Southworth at Bell Telephone Laboratories and Wilmer L. Barrow at the Massachusetts Institute of Technology, who worked without knowledge of one another. Southworth's interest was sparked during his 1920s doctoral work in which he measured the dielectric constant of water with a radio frequency Lecher line in a long tank of water, he found that if he removed the Lecher line, the tank of water still showed resonance peaks, indicating it was acting as a dielectric waveguide. At Bell Labs in 1931 he resumed work in dielectric waveguides. By March 1932 he observed waves in water-filled copper pipes. Rayleigh's previous work had been forgotten, Sergei A. Schelkunoff, a Bell Labs mathematician, did theoretical analyses of waveguides and rediscovered waveguide modes. In December 1933 it was realized that with a metal sheath the dielectric is superfluous and attention shifted to metal waveguides. Barrow had become interested in high frequencies in 1930 studying under Arnold Sommerfeld in Germany. At MIT beginning in 1932 he worked on high frequency antennas to generate narrow beams of radio waves to locate aircraft in fog.
He invented a horn antenna and hit on the idea of using a hollow pipe as a feedline to feed radio waves to the antenna. By March 1936 he had derived the propagation modes and cutoff frequency in a rectangular waveguide; the source he was using had a large wavelength of
A microwave antenna is a physical transmission device used to broadcast microwave transmissions between two or more locations. In addition to broadcasting, antennas are used in radar, radio astronomy and electronic warfare. One-way and two-way telecommunication using communications satellites Terrestrial microwave relay links in telecommunications networks including backbone or backhaul carriers in cellular networks linking BTS-BSC and BSC-MSC. Radar Radio astronomy Communications intelligence Electronic warfare A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves; these devices range anywhere from 6" to more than 12' diameter depending on use. A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are used as antennas at UHF and microwave frequencies, above 300 MHz. A lens antenna uses a lens to collect microwave radiation.
An array antenna is a high gain antenna consisting of an array of smaller antenna elements. A leaky wave antenna uses a leaking transmission line to obtain radation. Antenna
A patch antenna is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a flat rectangular sheet or "patch" of metal, mounted over a larger sheet of metal called a ground plane, they are the original type of microstrip antenna described by Howell in 1972. The radiation mechanism arises from discontinuities at each truncated edge of the microstrip transmission line; the radiation at the edges causes the antenna to act larger electrically than its physical dimensions, so in order for the antenna to be resonant, a length of microstrip transmission line shorter than one-half the wavelength at the frequency is used. The patch antenna is practical at microwave frequencies, at which wavelengths are short enough that the patches are conveniently small, it is used in portable wireless devices because of the ease of fabricating it on printed circuit boards. Multiple patch antennas on the same substrate called microstrip antennas, can be used to make high gain array antennas, phased arrays in which the beam can be electronically steered.
A variant of the patch antenna used in mobile phones is the shorted patch antenna, or planar inverted-F antenna. In this antenna, one corner of the patch is grounded with a ground pin; this variant has better matching than the standard patch. Another variant of patch antenna with the etched ground plane known as printed monopole antenna, is a versatile antenna for dual-band operations. Microwave Radiometer Patch Antenna Tutorial EM Talk Patch Antenna Calculator The basics of patch antennas A Dual-Band Rectangular CPW Folded Slot Antenna for GNSS Applications
A Luneburg lens is a spherically symmetric gradient-index lens. A typical Luneburg lens's refractive index n decreases radially from the center to the outer surface, they can be made for use with electromagnetic radiation from visible light to radio waves. For certain index profiles, the lens will form perfect geometrical images of two given concentric spheres onto each other. There are an infinite number of refractive-index profiles; the simplest such solution was proposed by Rudolf Luneburg in 1944. Luneburg's solution for the refractive index creates two conjugate foci outside of the lens; the solution takes a simple and explicit form if one focal point lies at infinity, the other on the opposite surface of the lens. J. Brown and A. S. Gutman subsequently proposed solutions which generate one internal focal point and one external focal point; these solutions are not unique. Each point on the surface of an ideal Luneburg lens is the focal point for parallel radiation incident on the opposite side.
Ideally, the dielectric constant ϵ r of the material composing the lens falls from 2 at its center to 1 at its surface, according to n = ϵ r = 2 − 2, where R is the radius of the lens. Because the refractive index at the surface is the same as that of the surrounding medium, no reflection occurs at the surface. Within the lens, the paths of the rays are arcs of ellipses. Maxwell's fish-eye lens is an example of the generalized Luneburg lens; the fish-eye, first described by Maxwell in 1854, has a refractive index varying according to n = ϵ r = n 0 1 + 2. It focuses each point on the spherical surface of radius R to the opposite point on the same surface. Within the lens, the paths of the rays are arcs of circles; the properties of this lens are described in one of a number of set problems or puzzles in the 1853 Cambridge and Dublin Mathematical Journal. The challenge is to find the refractive index as a function of radius, given that a ray describes a circular path, further to prove the focusing properties of the lens.
The solution is given in the 1854 edition of the same journal. The problems and solutions were published anonymously, but the solution of this problem were included in Niven's The Scientific Papers of James Clerk Maxwell, published 11 years after Maxwell's death. In practice, Luneburg lenses are layered structures of discrete concentric shells, each of a different refractive index; these shells form a stepped refractive index profile that differs from Luneburg's solution. This kind of lens is employed for microwave frequencies to construct efficient microwave antennas and radar calibration standards. Cylindrical analogues of the Luneburg lens are used for collimating light from laser diodes. A radar reflector can be made from a Luneburg lens by metallizing parts of its surface. Radiation from a distant radar transmitter is focussed onto the underside of the metallization on the opposite side of the lens. A difficulty with this scheme is that metallized regions block the entry or exit of radiation on that part of the lens, but the non-metallized regions result in a blind-spot on the opposite side.
A Luneburg lens can be used as the basis of a high-gain radio antenna. This antenna is comparable to a dish antenna, but uses the lens rather than a parabolic reflector as the main focusing element; as with the dish antenna, a feed to the receiver or from the transmitter is placed at the focus, the feed consisting of a horn antenna. The phase centre of the feed horn must coincide with the point of focus, but since the phase centre is invariably somewhat inside the mouth of the horn, it cannot be brought right up against the surface of the lens, it is necessary to use a variety of Luneburg lens that focusses somewhat beyond its surface, rather than the classic lens with the focus lying on the surface. A Luneburg lens antenna offers a number of advantages over a parabolic dish; because the lens is spherically symmetric, the antenna can be steered by moving the feed around the lens, without having to bodily rotate the whole antenna. Again, because the lens is spherically symmetric, a single lens can be used with several feeds looking in different directions.
In contrast, if multiple feeds are used with a parabolic reflector, all must be within a small angle of the optical axis to avoid suffering coma. Apart from offset systems, dish antennas suffer from the feed and its supporting structure obscuring the main element. A variation on the Luneburg lens antenna is the hemispherical Lu
In physics, physical optics, or wave optics, is the branch of optics that studies interference, diffraction and other phenomena for which the ray approximation of geometric optics is not valid. This usage tends not to include effects such as quantum noise in optical communication, studied in the sub-branch of coherence theory. Physical optics is the name of an approximation used in optics, electrical engineering and applied physics. In this context, it is an intermediate method between geometric optics, which ignores wave effects, full wave electromagnetism, a precise theory; the word "physical" means that it is more physical than geometric or ray optics and not that it is an exact physical theory. This approximation consists of using ray optics to estimate the field on a surface and integrating that field over the surface to calculate the transmitted or scattered field; this resembles the Born approximation, in that the details of the problem are treated as a perturbation. In optics, it is a standard way of estimating diffraction effects.
In radio, this approximation is used to estimate some effects. It models several interference and polarization effects but not the dependence of diffraction on polarization. Since it is a high-frequency approximation, it is more accurate in optics than for radio. In optics, it consists of integrating ray-estimated field over a lens, mirror or aperture to calculate the transmitted or scattered field. In radar scattering it means taking the current that would be found on a tangent plane of similar material as the current at each point on the front, i. e. the geometrically illuminated part, of a scatterer. Current on the shadowed parts is taken as zero; the approximate scattered field is obtained by an integral over these approximate currents. This is useful for bodies for lossy surfaces; the ray-optics field or current is not accurate near edges or shadow boundaries, unless supplemented by diffraction and creeping wave calculations. The standard theory of physical optics has some defects in the evaluation of scattered fields, leading to decreased accuracy away from the specular direction.
An improved theory introduced in 2004 gives exact solutions to problems involving wave diffraction by conducting scatterers. Optical physics Electromagnetic modeling History of optics Negative-index metamaterials Serway, Raymond A.. Physics for Scientists and Engineers. Brooks/Cole. ISBN 0-534-40842-7. Akhmanov, A. Physical Optics. Oxford University Press. ISBN 0-19-851795-5. Hay, S. G.. "A double-edge-diffraction Gaussian-series method for efficient physical optics analysis of dual-shaped-reflector antennas". IEEE Transactions on Antennas and Propagation. 53: 2597. Bibcode:2005ITAP...53.2597H. Doi:10.1109/tap.2005.851855. Asvestas, J. S.. "The physical optics method in electromagnetic scattering". Journal of Mathematical Physics. 21: 290–299. Bibcode:1980JMP....21..290A. Doi:10.1063/1.524413. Media related to Physical optics at Wikimedia Commons
A lens is a transmissive optical device that 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 arranged along a common axis. Lenses are made from materials such as glass or plastic, are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism. Devices that focus or disperse waves and radiation other than visible light are called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses; the word lens comes from lēns, the Latin name of the lentil, because a double-convex lens is lentil-shaped. The lentil plant gives its name to a geometric figure; some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. The so-called Nimrud lens is a rock crystal artifact dated to the 7th century BC which may or may not have been used as a magnifying glass, or a burning glass.
Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses". The oldest certain reference to the use of lenses is from Aristophanes' play The Clouds mentioning a burning-glass. Pliny the Elder confirms. Pliny has the earliest known reference to the use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald. Both Pliny and Seneca the Younger described the magnifying effect of a glass globe filled with water. Ptolemy wrote a book on Optics, which however survives only in the Latin translation of an incomplete and poor Arabic translation; the book was, received, by medieval scholars in the Islamic world, commented upon by Ibn Sahl, in turn improved upon by Alhazen. The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century. Between the 11th and 13th century "reading stones" were invented; these were primitive plano-convex lenses made by cutting a glass sphere in half. The medieval rock cystal Visby lenses may not have been intended for use as burning glasses.
Spectacles were invented as an improvement of the "reading stones" of the high medieval period in Northern Italy in the second half of the 13th century. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the late 13th century, in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses; the practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands. With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries by those trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces.
Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention claimed by fellow Englishman John Dollond in a 1758 patent. Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres; each surface can be concave, or planar. The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens; the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may ground after manufacturing to give them a different shape or size; the lens axis may not pass through the physical centre of the lens. Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes, they have a different focal power in different meridians. This forms an astigmatic lens. An example is eyeglass lenses. More complex are aspheric lenses.
These are lenses where one or both surfaces have a shape, neither spherical nor cylindrical. The more complicated shapes allow such lenses to form images with less aberration than standard simple lenses, but they are more difficult and expensive to produce. Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave. If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is meniscus, it is this type of lens, most used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot behind the lens. In this case, the lens is called a