The simpler dioptric (purely refractive) form of the lens was first proposed by Georges-Louis Leclerc, Comte de Buffon[2], and independently reinvented by the French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses.[3][4] The catadioptric (combining refraction and reflection) form of the lens, entirely invented by Fresnel, has outer prismatic elements that use total internal reflection as well as refraction to capture more oblique light from the light source and add it to the beam, making it visible at greater distances.

Glass Design - The quality of glass formulations has been paramount in the evolution of modern microscope optics, and there are currently several hundred of optical glasses available for the design of microscope objectives. The suitability of glass for the demanding optical performance of a microscope objective is a function of its physical properties such as refractive index, dispersion, light transmission, contaminant concentrations, residual autofluorescence, and overall homogeneity throughout the mixture. Care must be taken by optical designers to ensure that glass utilized in high-performance objectives has a high transmission in the near-ultraviolet region and also produces high extinction factors for applications such as polarized light or differential interference contrast.

Multilayer Antireflection Coatings - One of the most significant advances in objective design during recent years is the improvement in antireflection coating technology, which helps to reduce unwanted reflections (flare and ghosts) that occur when light passes through a lens system, and ensure high-contrast images. Each uncoated air-glass interface can reflect between four and five percent of an incident light beam normal to the surface, resulting in a transmission value of 95-96 percent at normal incidence. Application of a quarter-wavelength thick antireflection coating having the appropriate refractive index can increase this value by three to four percent. As objectives become more sophisticated with an ever-increasing number of lens elements, the need to eliminate internal reflections grows correspondingly. Some modern objective lenses with a high degree of correction can contain as many as 15 lens elements having many air-glass interfaces. If the lenses were uncoated, the reflection losses of axial rays alone would drop transmittance values to around 50 percent. The single-layer lens coatings once utilized to reduce glare and improve transmission have now been supplanted by multilayer coatings that produce transmission values exceeding 99.9 percent in the visible spectral range. These specialized coatings are also used on the phase plates in phase contrast objectives to maximize contrast.

Most modern Fresnel lenses consist only of refractive elements. Lighthouse lenses, however, tend to include both refracting and reflecting elements, the latter being outside the metal rings seen in the photographs. While the inner elements are sections of refractive lenses, the outer elements are reflecting prisms, each of which performs two refractions and one total internal reflection, avoiding the light loss that occurs in reflection from a silvered mirror.

The first person to focus a lighthouse beam using a lens was apparently the London glass-cutter Thomas Rogers, who proposed the idea to Trinity House in 1788.[6] The first Rogers lenses, 53 cm in diameter and 14 cm thick at the center, were installed at the Old Lower Lighthouse at Portland Bill in 1789. Behind each lamp was a back-coated spherical glass mirror, which reflected rear radiation back through the lamp and into the lens. Further samples were installed at Howth Baily, North Foreland, and at least four other locations by 1804. But much of the light was wasted by absorption in the glass.[6][7]

Some objectives specifically designed for transmitted light fluorescence and darkfield imaging are equipped with an internal iris diaphragm that allows for adjustment of the effective numerical aperture. Abbreviations inscribed on the barrel for these objectives include I, Iris, and W/Iris. The 60x apochromat objective illustrated above has a numerical aperture of 1.4, one of the highest attainable in modern microscopes using immersion oil as an imaging medium.

Fresnellight

Canon and Nikon have used Fresnel lenses to reduce the size of telephoto lenses. Photographic lenses that include Fresnel elements can be much shorter than corresponding conventional lens design. Nikon calls the technology Phase Fresnel.[75][76] The Polaroid SX-70 camera used a Fresnel reflector as part of its viewing system. View and large format cameras can utilize a Fresnel lens in conjunction with the ground glass, to increase the perceived brightness of the image projected by a lens onto the ground glass, thus aiding in adjusting focus and composition.

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.

Mechanical Tube Length - This is the length of the microscope body tube between the nosepiece opening, where the objective is mounted, and the top edge of the observation tubes where the oculars (eyepieces) are inserted. This aspect of microscope design is discussed in more thoroughly in our mechanical tube length section of the primer. Tube length is usually inscribed on the objective as the size in number of millimeters (160, 170, 210, etc.) for fixed lengths, or the infinity symbol (¥) for infinity-corrected tube lengths. The objective illustrated in Figure 1 is corrected for a tube length of infinity, although many older objectives will be corrected for tube lengths of either 160 (Nikon, Olympus, Zeiss) or 170 (Leica) millimeters.

Specialized Optical Properties - Microscope objectives often have design parameters that optimize performance under certain conditions. For example, there are special objectives designed for polarized illumination signified by the abbreviations P, Po, POL, or SF (strain-free and/or having all barrel engravings painted red), phase contrast (PH, and/or green barrel engravings), differential interference contrast (DIC), and many other abbreviations for additional applications. A list of several abbreviations, often manufacturer specific, is presented in Table 1. The apochromat objective illustrated in Figure 1 is optimized for DIC photomicrography and this is indicated on the barrel. The capital H beside the DIC marking indicates that the objective must be used with a specific DIC Wollaston prism optimized for high-magnification applications.

Multi-focal Fresnel lenses are also used as a part of retina identification cameras, where they provide multiple in- and out-of-focus images of a fixation target inside the camera. For virtually all users, at least one of the images will be in focus, thus allowing correct eye alignment.

The use of Fresnel lenses for image projection reduces image quality, so they tend to occur only where quality is not critical or where the bulk of a solid lens would be prohibitive. Cheap Fresnel lenses can be stamped or molded of transparent plastic and are used in overhead projectors and projection televisions.

In the same year he designed the first fixed lens—for spreading light evenly around the horizon while minimizing waste above or below.[11] Ideally the curved refracting surfaces would be segments of toroids about a common vertical axis, so that the dioptric panel would look like a cylindrical drum. If this was supplemented by reflecting (catoptric) rings above and below the refracting (dioptric) parts, the entire apparatus would look like a beehive.[38] The second Fresnel lens to enter service was indeed a fixed lens, of third order, installed at Dunkirk by 1 February 1825.[39] However, due to the difficulty of fabricating large toroidal prisms, this apparatus had a 16-sided polygonal plan.[40]

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 and a flat or slightly convex center. In the first (and largest) Fresnel lenses, each section was actually a separate prism. 'Single-piece' Fresnel lenses were later produced, being used for automobile headlamps, brake, parking, and turn signal lenses, and so on. In modern times, computer-controlled milling equipment (CNC) or 3-D printers might be used to manufacture more complex lenses.[citation needed]

Fresnel lenses are usually made of glass or plastic; their size varies from large (old historical lighthouses, meter size) to medium (book-reading aids, OHP viewgraph projectors) to small (TLR/SLR camera screens, micro-optics). In many cases they are very thin and flat, almost flexible, with thicknesses in the 1 to 5 mm (1⁄32 to 3⁄16 in) range.[citation needed]

High-quality glass Fresnel lenses were used in lighthouses, where they were considered state of the art in the late 19th and through the middle of the 20th centuries; most lighthouses have now retired glass Fresnel lenses from service and replaced them with much less expensive and more durable aerobeacons, which themselves often contain plastic Fresnel lenses.[citation needed] Lighthouse Fresnel lens systems typically include extra annular prismatic elements, arrayed in faceted domes above and below the central planar Fresnel, in order to catch all light emitted from the light source. The light path through these elements can include an internal reflection, rather than the simple refraction in the planar Fresnel element. These lenses conferred many practical benefits upon the designers, builders, and users of lighthouses and their illumination. Among other things, smaller lenses could fit into more compact spaces. Greater light transmission over longer distances, and varied patterns, made it possible to triangulate a position.[citation needed]

The interactive tutorial linked above allows the visitor to adjust the correction collar on a microscope objective. There are some applications that do not require objectives to be corrected for cover glass thickness. These include objectives designed for reflected light metallurgical specimens, tissue culture, integrated circuit inspection, and many other applications that require observation with no compensation for a cover glass.

James Timmins Chance modified Thomas Stevenson's all-glass holophotal design by arranging the double-reflecting prisms about a vertical axis. The prototype was shown at the 1862 International Exhibition in London. Later, to ease manufacturing, Chance divided the prisms into segments, and arranged them in a cylindrical form while retaining the property of reflecting light from a single point back to that point. Reflectors of this form, paradoxically called "dioptric mirrors", proved particularly useful for returning light from the landward side of the lamp to the seaward side.[51]

Immersion Medium - Most objectives are designed to image specimens with air as the medium between the objective and the cover glass.

Parfocal Distance - This is another specification that can often vary by manufacturer. Most companies produce objectives that have a 45 millimeter parfocal distance, which is designed to minimize refocusing when magnifications are changed.

Illustrated in Figure 4 is a schematic drawing of light waves reflecting and/or passing through a lens element coated with two antireflection layers. The incident wave strikes the first layer (Layer A in Figure 4) at an angle, resulting in part of the light being reflected (R(o)) and part being transmitted through the first layer. Upon encountering the second antireflection layer (Layer B), another portion of the light is reflected at the same angle and interferes with light reflected from the first layer. Some of the remaining light waves continue on to the glass surface where they are again both reflected and transmitted. Light reflected from the glass surface interferes (both constructively and destructively) with light reflected from the antireflection layers. The refractive indices of the antireflection layers vary from that of the glass and the surrounding medium (air). As the light waves pass through the antireflection layers and glass surface, a majority of the light (depending upon the incident angle--usual normal to the lens in optical microscopy) is ultimately transmitted through the glass and focused to form an image.

Fresnel acknowledged the British lenses and Buffon's invention in a memoir read on 29 July 1822 and printed in the same year.[25] The date of that memoir may be the source of the claim that Fresnel's lighthouse advocacy began two years later than Brewster's;[14] but the text makes it clear that Fresnel's involvement began no later than 1819.[26]

The French Commission des Phares [FR] (Commission of Lighthouses) was established by Napoleon in 1811, and placed under the authority of French physicist Augustin-Jean Fresnel's employer, the Corps of Bridges and Roads. As the members of the commission were otherwise occupied, it achieved little in its early years.[15] However, on 21 June 1819—three months after winning the physics Grand Prix of the Academy of Sciences for his celebrated memoir on diffraction—Fresnel was "temporarily" seconded to the commission on the recommendation of François Arago (a member since 1813), to review possible improvements in lighthouse illumination.[11][16]

Fresnel's next lens was a rotating apparatus with eight "bull's-eye" panels, made in annular arcs by Saint-Gobain,[12] giving eight rotating beams—to be seen by mariners as a periodic flash. Above and behind each main panel was a smaller, sloping bull's-eye panel of trapezoidal outline with trapezoidal elements.[27] This refracted the light to a sloping plane mirror, which then reflected it horizontally, 7 degrees ahead of the main beam, increasing the duration of the flash.[28] Below the main panels were 128 small mirrors arranged in four rings, stacked like the slats of a louver or Venetian blind. Each ring, shaped like a frustum of a cone, reflected the light to the horizon, giving a fainter steady light between the flashes. The official test, conducted on the unfinished Arc de Triomphe on 20 August 1822, was witnessed by the Commission—and by Louis XVIII and his entourage—from 32 kilometres (20 mi) away. The apparatus was stored at Bordeaux for the winter, and then reassembled at Cordouan Lighthouse under Fresnel's supervision—in part by Fresnel's own hands. On 25 July 1823, the world's first lighthouse Fresnel lens was lit.[29] As expected, the light was visible to the horizon, more than 32 kilometres (20 mi) out.[30]

High Power Magnifying Glass

As lighthouses proliferated, they became harder to distinguish from each other, leading to the use of colored filters, which wasted light. In 1884, John Hopkinson eliminated the need for filters by inventing the "group-flashing" lens, in which the dioptric and/or the catadioptric panels were split so as to give multiple flashes—allowing lighthouses to be identified not only by frequency of flashes, but also by multiplicity of flashes. Double-flashing lenses were installed at Tampico (Mexico) and Little Basses (Sri Lanka) in 1875, and a triple-flashing lens at Casquets Lighthouse (Channel Islands) in 1876.[52] The example shown (right) is the double-flashing lens of the Point Arena Light, which was in service from 1908 to 1977.[53]

With an official budget of 500 francs, Fresnel approached three manufacturers. The third, François Soleil, found a way to remove defects by reheating and remolding the glass. Arago assisted Fresnel with the design of a modified Argand lamp with concentric wicks (a concept that Fresnel attributed to Count Rumford[22]), and accidentally discovered that fish glue was heat-resistant, making it suitable for use in the lens. The prototype, finished in March 1820, had a square lens panel 55 cm on a side, containing 97 polygonal (not annular) prisms—and so impressed the Commission that Fresnel was asked for a full eight-panel version. This model, completed a year later in spite of insufficient funding, had panels 76 cm square. In a public spectacle on the evening of 13 April 1821, it was demonstrated by comparison with the most recent reflectors, which it suddenly rendered obsolete.[23]

Most manufacturers have now transitioned to infinity-corrected objectives that project emerging rays in parallel bundles from every azimuth to infinity. These objectives require a tube lens in the light path to bring the image into focus at the intermediate image plane. Infinity-corrected and finite-tube length microscope objectives are not interchangeable and must be matched not only to a specific type of microscope, but often to a particular microscope from a single manufacturer. For example, Nikon infinity-corrected objectives are not interchangeable with Olympus infinity-corrected objectives, not only because of tube length differences, but also because the mounting threads are not the same pitch or diameter. Objectives usually contain an inscription denoting the tube focal length as will be discussed.

Thomas Stevenson (younger brother of Alan) went a step beyond Fresnel with his "holophotal" lens, which focused the light radiated by the lamp in nearly all directions, forward or backward, into a single beam.[49] The first version, described in 1849, consisted of a standard Fresnel bull's-eye lens, a paraboloidal reflector, and a rear hemispherical reflector (functionally equivalent to the Rogers mirror of 60 years earlier, except that it subtended a whole hemisphere). Light radiated into the forward hemisphere but missing the bull's-eye lens was deflected by the paraboloid into a parallel beam surrounding the bull's-eye lens, while light radiated into the backward hemisphere was reflected back through the lamp by the spherical reflector (as in Rogers' arrangement), to be collected by the forward components. The first unit was installed at North Harbour, Peterhead, in August 1849. Stevenson called this version a "catadioptric holophote", although each of its elements was either purely reflective or purely refractive. In the second version of the holophote concept, the bull's-eye lens and paraboloidal reflector were replaced by a catadioptric Fresnel lens—as conceived by Fresnel, but expanded to cover the whole forward hemisphere. The third version, which Stevenson confusingly called a "dioptric holophote", was more innovative: it retained the catadioptric Fresnel lens for the front hemisphere, but replaced the rear hemispherical reflector with a hemispherical array of annular prisms, each of which used two total internal reflections to turn light diverging from the center of the hemisphere back toward the center. The result was an all-glass holophote, with no losses from metallic reflections.[50]

Special Features - Objectives often have additional special features that are specific to a particular manufacturer and type of objective. The plan apochromat objective illustrated in Figure 1 has a spring-loaded front lens to prevent damage when the objective is accidentally driven onto the surface of a microscope slide.

Fresnel designed six sizes of lighthouse lenses, divided into four orders based on their size and focal length.[58] The 3rd and 4th orders were sub-divided into "large" and "small". In modern use, the orders are classified as first through sixth order. An intermediate size between third and fourth order was added later, as well as sizes above first order and below sixth.

The first fixed lens to be constructed with toroidal prisms was a first-order apparatus designed by the Scottish engineer Alan Stevenson under the guidance of Léonor Fresnel, and fabricated by Isaac Cookson & Co. using French glass; it entered service at the Isle of May, Scotland, on 22 September 1836.[47] The first large catadioptric lenses were made in 1842 for the lighthouses at Gravelines and Île Vierge, France; these were fixed third-order lenses whose catadioptric rings (made in segments) were one metre in diameter. Stevenson's first-order Skerryvore lens, lit in 1844, was only partly catadioptric; it was similar to the Cordouan lens except that the lower slats were replaced by French-made catadioptric prisms, while mirrors were retained at the top. The first fully catadioptric first-order lens, installed at Pointe d'Ailly in 1852, also gave eight rotating beams plus a fixed light at the bottom; but its top section had eight catadioptric panels focusing the light about 4 degrees ahead of the main beams, in order to lengthen the flashes. The first fully catadioptric lens with purely revolving beams—also of first order—was installed at Saint-Clément-des-Baleines in 1854, and marked the completion of Augustin Fresnel's original Carte des Phares.[48]

In 1748, Georges-Louis Leclerc, Comte de Buffon was the first to replace a convex lens with a series of concentric annular prisms, ground as steps in a single piece of glass,[2]to reduce weight and absorption. In 1790[8] (although secondary sources give the date as 1773[9]: 609  or 1788[10]), the Marquis de Condorcet suggested that it would be easier to make the annular sections separately and assemble them on a frame; but even that was impractical at the time.[11][12] These designs were intended not for lighthouses,[2] but for burning glasses.[9]: 609  David Brewster, however, proposed a system similar to Condorcet's in 1811,[2][10][13] and by 1820 was advocating its use in British lighthouses.[14]

For many years, natural fluorite was commonly used in the manufacture of fluorite (semi-apochromat) and apochromat objectives. Unfortunately, many newly developed fluorescence techniques often rely on ultraviolet excitation at wavelengths significantly below 400 nanometers, which is severely compromised by autofluorescence that occurs from natural organic constituents present in this mineral. Also, the tendency of natural fluorite to exhibit widespread localized regions of crystallinity can seriously degrade performance in polarized light microscopy. Many of these problems are circumvented with new, more advanced materials, such as fluorocrown glass.

Color Codes - Microscope manufacturers label their objectives with color codes to help in rapid identification of the magnification and any specialized immersion media requirements. The dark blue color code on the objective illustrated in Figure 1 indicates the linear magnification is 60x. This is very helpful when you have a nosepiece turret containing 5 or 6 objectives and you must quickly select a specific magnification. Some specialized objectives have an additional color code that indicates the type of immersion medium necessary to achieve the optimum numerical aperture. Immersion lenses intended for use with oil have a black color ring, and those intended for use with glycerin have an orange ring, as illustrated with the objective on the left in Figure 2. Objectives designed to image living organisms in aqueous media are designated water immersion objectives with a white ring, and highly specialized objectives for unusual immersion media are often engraved with a red ring. Table 3 lists current magnification and imaging media color codes in use by most manufacturers.

Highest power magnifying glass

Fresnel lenses have also been used in the field of popular entertainment. The British rock artist Peter Gabriel made use of them in his early solo live performances to magnify the size of his head, in contrast to the rest of his body, for dramatic and comic effect. In the Terry Gilliam film Brazil, plastic Fresnel screens appear ostensibly as magnifiers for the small CRT monitors used throughout the offices of the Ministry of Information. However, they occasionally appear between the actors and the camera, distorting the scale and composition of the scene to humorous effect. The Pixar movie Wall-E features a Fresnel lens in the scenes where the protagonist watches the musical Hello, Dolly! magnified on an iPod.

In late 1825,[43] to reduce the loss of light in the reflecting elements, Fresnel proposed to replace each mirror with a catadioptric prism, through which the light would travel by refraction through the first surface, then total internal reflection off the second surface, then refraction through the third surface.[44] The result was the lighthouse lens as we now know it. In 1826 he assembled a small model for use on the Canal Saint-Martin,[45] but he did not live to see a full-sized version: he died on 14 July 1827, at the age of 39.

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 effectively divides the continuous surface of a standard lens into a set of surfaces of the same curvature, with stepwise discontinuities between them.

Fresnel lens

Other features found on specialized objectives are variable working distance (LWD) and numerical aperture settings that are adjustable by turning the correction collar on the body of the objective as illustrated in Figure 2. The plan fluor objective on the left has a variable immersion medium/numerical aperture setting that allows the objective to be used with both air and an alternative liquid immersion medium, glycerin. The plan apo objective on the right has an adjustable working distance control (termed a "correction collar") that allows the objective to image specimens through glass coverslips of variable thickness. This is especially important in dry objectives with high numerical aperture that are particularly susceptible to spherical and other aberrations that can impair resolution and contrast when used with a cover glass whose thickness differs from the specified design value.

Perhaps the most widespread use of Fresnel lenses, for a time, occurred in automobile headlamps, where they can shape the roughly parallel beam from the parabolic reflector to meet requirements for dipped and main-beam patterns, often both in the same headlamp unit (such as the European H4 design). For reasons of economy, weight, and impact resistance, newer cars have dispensed with glass Fresnel lenses, using multifaceted reflectors with plain polycarbonate lenses. However, Fresnel lenses continue in wide use in automobile tail, marker, and reversing lights.

Fresnel lens design allows a substantial reduction in thickness (and thus mass and volume of material) at the expense of reducing the imaging quality of the lens, which is why precise imaging applications such as photography usually still use larger conventional lenses.

Optical lenses

Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

Objective Screw Threads - The mounting threads on almost all objectives are sized to standards of the Royal Microscopical Society (RMS) for universal compatibility. The objective in Figure 1 has mounting threads that are 20.32 mm in diameter with a pitch of 0.706, conforming to the RMS standard. This standard is currently used in the production of infinity-corrected objectives by manufacturers Olympus and Zeiss. Nikon and Leica have broken from the standard with the introduction of new infinity-corrected objectives that have a wider mounting thread size, making Leica and Nikon objectives usable only on their own microscopes. Nikon's reasoning is explained in our section describing the Nikon CFI60 200/60/25 Specification for biomedical microscopes. Abbreviations commonly used to denote thread size are: RMS (Royal Microscopical Society objective thread), M25 (metric 25-millimeter objective thread), and M32 (metric 32-millimeter objective thread).

By the end of August 1819, unaware of the Buffon-Condorcet-Brewster proposal,[11][13] Fresnel made his first presentation to the commission,[17] recommending what he called lentilles à échelons ('lenses by steps') to replace the reflectors then in use, which reflected only about half of the incident light.[18] Another report by Fresnel, dated 29 August 1819 (Fresnel, 1866–70, vol. 3, pp. 15–21), concerns tests on reflectors, and does not mention stepped lenses except in an unrelated sketch on the last page of the manuscript. The minutes of the meetings of the Commission go back only to 1824, when Fresnel himself took over as Secretary.[19] Thus the exact date on which Fresnel formally recommended lentilles à échelons is unknown.[citation needed] Much to Fresnel's embarrassment, one of the assembled commissioners, Jacques Charles, recalled Buffon's suggestion.[20] However, whereas Buffon's version was biconvex and in one piece,[21] Fresnel's was plano-convex and made of multiple prisms for easier construction.

Cements employed in building multiple lens elements usually have a thickness around 5-10 microns, which can be a source of artifacts in groups that have three or more lens elements cemented together. Doublets, triplets, and other multiple lens arrangements can display spurious absorption, transmission, and fluorescence characteristics that will disqualify the lenses for certain applications.

Microscope manufacturers offer a wide range of objective designs to meet the performance needs of specialized imaging methods, to compensate for cover glass thickness variations, and to increase the effective working distance of the objective. Often, the function of a particular objective is not obvious simply by looking at the construction of the objective. Finite microscope objectives are designed to project a diffraction-limited image at a fixed plane (the intermediate image plane), which is dictated by the microscope tube length and located at a pre-specified distance from the rear focal plane of the objective. Microscope objectives are usually designed to be used with a specific group of oculars and/or tube lenses strategically placed to assist in the removal of residual optical errors. As an example, older Nikon and Olympus compensating eyepieces were used with high numerical aperture fluorite and apochromatic objectives to eliminate lateral chromatic aberration and improve flatness of field. Newer microscopes (from Nikon and Olympus) have objectives that are fully corrected and do not require additional corrections from the eyepieces or tube lenses.

Soon after this demonstration, Fresnel published the idea that light, including apparently unpolarized light, consists exclusively of transverse waves, and went on to consider the implications for double refraction and partial reflection.[24]

Virtual reality headsets, such as the Meta Quest 2 and the HTC Vive Pro use Fresnel lenses,[71] as they allow a thinner and lighter form factor than regular lenses.[72] Newer devices, such as the Meta Quest Pro, have switched to a pancake lens design[73] due to its smaller form factor and less chromatic aberration than Fresnel lenses.[74]

From the discussion above it is apparent that objectives are the most important optical element of a compound microscope. It is for this reason that so much effort is invested in making sure that they are well-labeled and suited for the task at hand. We will explore other properties and aspects of microscope objectives in other sections of this tutorial.

Optical Corrections - These are usually listed as Achro and Achromat (achromatic), as Fl, Fluar, Fluor, Neofluar, or Fluotar (fluorite) for better spherical and chromatic corrections, and as Apo (apochromatic) for the highest degree of correction for spherical and chromatic aberrations. Field curvature corrections are abbreviated Plan, Pl, EF, Achroplan, Plan Apo, or Plano. Other common abbreviations are ICS (infinity corrected system) and UIS (universal infinity system), N and NPL (normal field of view plan), Ultrafluar (fluorite objective with glass that is transparent down to 250 nanometers), and CF and CFI (chrome-free; chrome-free infinity). The objective in the illustration (Figure 1) is a plan apochromat that enjoys the highest degree of optical correction. See Table 1 for a complete list of abbreviations often found inscribed on objective barrels.

Magnifying lenses

The development of hyper-radial lenses was driven in part by the need for larger light sources, such as gas lights with multiple jets, which required a longer focal length for a given beam-width, hence a larger lens to collect a given fraction of the generated light. The first hyper-radial lens was built for the Stevensons in 1885 by F. Barbier & Cie of France, and tested at South Foreland Lighthouse with various light sources. Chance Brothers (Hopkinson's employers) then began constructing hyper-radials, installing their first at Bishop Rock Lighthouse in 1887.[54] In the same year, Barbier installed a hyper-radial at Tory Island. But only about 30 hyper-radials went into service[55] before the development of more compact bright lamps rendered such large optics unnecessary (see Hyperradiant Fresnel lens).

Fresnel lenses of different focal lengths (one collimator, and one collector) are used in commercial and DIY projection. The collimator lens has the lower focal length and is placed closer to the light source, and the collector lens, which focuses the light into the triplet lens, is placed after the projection image (an active matrix LCD panel in LCD projectors). Fresnel lenses are also used as collimators in overhead projectors.

Linear Magnification - In the case of the apochromatic objective in Figure 1, the linear magnification is 60x, although the manufacturers produce objectives ranging in linear magnification from 0.5x to 250x with many sizes in between.

Aircraft carriers and naval air stations typically use Fresnel lenses in their optical landing systems. The "meatball" light aids the pilot in maintaining proper glide slope for the landing. In the center are amber and red lights composed of Fresnel lenses. Although the lights are always on, the angle of the lens from the pilot's point of view determines the color and position of the visible light. If the lights appear above the green horizontal bar, the pilot is too high. If it is below, the pilot is too low, and if the lights are red, the pilot is very low.[67]

Cover Glass Thickness - Most transmitted light objectives are designed to image specimens that are covered by a cover glass (or cover slip). The thickness of these small glass plates is now standardized at 0.17 mm for most applications, although there is often some variation in thickness within a batch of coverslips. For this reason, some of the more advanced objectives have a correction collar adjustment of the internal lens elements to compensate for this variation. Abbreviations for the correction collar adjustment include Corr, w/Corr, and CR, although the presence of a movable, knurled collar and graduated scale is also an indicator of this feature.

Fresnel lenses can concentrate sunlight onto solar cells with a ratio of almost 500:1.[77] This allows the active solar-cell surface to be reduced, lowering cost and allowing the use of more efficient cells that would otherwise be too expensive.[78] In the early 21st century, Fresnel reflectors began to be used in concentrating solar power (CSP) plants to concentrate solar energy. One application was to preheat water at the coal-fired Liddell Power Station, in Hunter Valley Australia.

Manufacturer - The name of the objective manufacturer is almost always included on the objective. The objective illustrated in Figure 1 was made by a fictitious company named Nippon from Japan, but comparable objectives are manufactured by Nikon, Olympus, Zeiss, and Leica, companies who are some of the most respected manufacturers in the microscope business.

Since plastic Fresnel lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used to concentrate sunlight for heating in solar cookers, in solar forges, and in solar collectors used to heat water for domestic use. They can also be used to generate steam or to power a Stirling engine.

Another automobile application of a Fresnel lens is a rear view enhancer, as the wide view angle of a lens attached to the rear window permits examining the scene behind a vehicle, particularly a tall or bluff-tailed one, more effectively than a rear-view mirror alone. Fresnel lenses have been used on rangefinding equipment and projected map display screens.[70]

The first stage of the development of lighthouse lenses after the death of Augustin Fresnel consisted in the implementation of his designs. This was driven in part by his younger brother Léonor—who, like Augustin, was trained as a civil engineer but, unlike Augustin, had a strong aptitude for management. Léonor entered the service of the Lighthouse Commission in 1825, and went on to succeed Augustin as Secretary.[46]

Although not common today, other types of adjustable objectives have been manufactured in the past. Perhaps the most interesting example is the compound "zoom" objective that has a variable magnification, usually from about 4x to 15x. These objectives have a short barrel with poorly designed optics that have significant aberration problems and are not very practical for photomicrography or serious quantitative microscopy.

There are two main types of Fresnel lens: imaging and 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, and do not produce sharp images.[63] 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.

Glass Fresnel lenses also are used in lighting instruments for theatre and motion pictures (see Fresnel lantern); such instruments are often called simply Fresnels. The entire instrument consists of a metal housing, a reflector, a lamp assembly, and a Fresnel lens. Many Fresnel instruments allow the lamp to be moved relative to the lens' focal point, to increase or decrease the size of the light beam. As a result, they are very flexible, and can often produce a beam as narrow as 7° or as wide as 70°.[66] The Fresnel lens produces a very soft-edged beam, so is often used as a wash light. A holder in front of the lens can hold a colored plastic film (gel) to tint the light or wire screens or frosted plastic to diffuse it. The Fresnel lens is useful in the making of motion pictures not only because of its ability to focus the beam brighter than a typical lens, but also because the light is a relatively consistent intensity across the entire width of the beam of light.

Identification of the properties of individual objectives is usually very easy because important parameters are often inscribed on the outer housing (or barrel) of the objective itself as illustrated in Figure 1. This figure depicts a typical 60x plan apochromat objective, including common engravings that contain all of the specifications necessary to determine what the objective is designed for and the conditions necessary for proper use.

Production of one-piece stepped dioptric lenses—roughly as envisaged by Buffon—became feasible in 1852, when John L. Gilliland of the Brooklyn Flint-Glass Company patented a method of making lenses from pressed and molded glass. The company made small bull's-eye lenses for use on railroads, steamboats, and docks;[56] such lenses were common in the United States by the 1870s.[13]: 488  In 1858 the company produced "a very small number of pressed flint-glass sixth-order lenses" for use in lighthouses—the first Fresnel lighthouse lenses made in America.[56] By the 1950s, the substitution of plastic for glass made it economic to use Fresnel lenses as condensers in overhead projectors.[57]

Extra Low Dispersion (ED) glass was introduced as a major advancement in lens design with optical qualities similar to the mineral fluorite but without its mechanical and optical demerits. This glass has allowed manufacturers to create higher quality objectives with lens elements that have superior optical corrections and performance. Because the chemical and optical properties of many glasses are of a proprietary nature, documentation is difficult or impossible to obtain. For this reason the literature is often vague about the specific properties of glasses utilized in the construction of microscope objectives.

The objective depicted on the left in Figure 3 has a parfocal distance of 45mm and is labeled with an immersion medium color code in addition to the magnification color code. Parfocal distance is measured from the nosepiece objective mounting hole to the point of focus on the specimen as illustrated in the figure. The objective on the right in Figure 3 has a longer parfocal distance of 60 millimeters, which is the result of its being produced to the Nikon CFI60 200/60/25 Specification, again deviating from the practice of other manufacturers such as Olympus and Zeiss, who still produce objectives with a 45mm parfocal distance. Most manufacturers also make their objective nosepieces parcentric, meaning that when a specimen is centered in the field of view for one objective, it remains centered when the nosepiece is rotated to bring another objective into use.

To attain higher working numerical apertures, many objectives are designed to image the specimen through another medium that reduces refractive index differences between glass and the imaging medium. High-resolution plan apochromat objectives can achieve numerical apertures up to 1.40 when the immersion medium is special oil with a refractive index of 1.51. Other common immersion media are water and glycerin. Objectives designed for special immersion media usually have a color-coded ring inscribed around the circumference of the objective barrel as listed in Table 3 and described below. Common abbreviations are: Oil, Oel (oil immersion), HI (homogeneous immersion), W, Water, Wasser (water immersion), and Gly (glycerol immersion).

Magnesium fluoride is one of many materials utilized in thin-layer optical antireflection coatings, but most microscope manufacturers now produce their own proprietary formulations. The general result is a dramatic improvement in contrast and transmission of visible wavelengths with a concurrent destructive interference in harmonically-related frequencies lying outside the transmission band. These specialized coatings can be easily damaged by mis-handling and the microscopist should be aware of this vulnerability. Multilayer antireflection coatings have a slightly greenish tint, as opposed to the purplish tint of single-layer coatings, an observation that can be employed to distinguish between coatings. The surface layer of antireflection coatings used on internal lenses is often much softer than corresponding coatings designed to protect external lens surfaces. Great care should be taken when cleaning optical surfaces that have been coated with thin films, especially if the microscope has been disassembled and the internal lens elements are subject to scrutiny.

Real Glass Magnifying Glass

In 1825 Fresnel extended his fixed-lens design by adding a rotating array outside the fixed array. Each panel of the rotating array was to refract part of the fixed light from a horizontal fan into a narrow beam.[11][41]

A first-order lens has a focal length of 920 mm (36+1⁄4 in) and stands about 2.59 m (8 ft 6 in) high, and 1.8 m (6 ft) wide. The smallest (sixth) order has a focal length of 150 mm (6 in) and a height of 433 mm (17+1⁄16 in).[58][59][60]

The largest Fresnel lenses are called hyperradiant (or hyper-radial). One such lens was on hand when it was decided to build and outfit the Makapuu Point Light in Hawaii. Rather than order a new lens, the huge optic construction, 3.7 metres (12 ft) tall and with over a thousand prisms, was used there.[61]

Working Distance - This is the distance between the objective front lens and the top of the cover glass when the specimen is in focus. In most instances, the working distance of an objective decreases as magnification increases. Working distance values are not included on all objectives and their presence varies depending upon the manufacturer. Common abbreviations are: L, LL, LD, and LWD (long working distance), ELWD (extra-long working distance), SLWD (super-long working distance), and ULWD (ultra-long working distance). Newer objectives often contain the size of working distance (in millimeters) inscribed on the barrel. The objective illustrated in Figure 1 has a very short working distance of 0.21 millimeters.

Asphericallens

Annealing of optical glass for the manufacture of objectives is critical in order to remove stress, improve transmission, and reduce the level of other internal imperfections. Some of the glass formulations intended for apochromat lens construction are slow-cooled and annealed for extended periods, often exceeding six months. True apochromat objectives are manufactured with a combination of natural fluorite and other glasses that have reduced transmission in the near-ultraviolet region.

In May 1824,[13] Fresnel was promoted to Secretary of the Commission des Phares, becoming the first member of that body to draw a salary,[34] albeit in the concurrent role of Engineer-in-Chief.[35] Late that year, being increasingly ill, he curtailed his fundamental research and resigned his seasonal job as an examiner at the École Polytechnique, in order to save his remaining time and energy for his lighthouse work.[36][37]

There is a wealth of information inscribed on the barrel of each objective, which can be broken down into several categories. These include the linear magnification, numerical aperture value, optical corrections, microscope body tube length, the type of medium the objective is designed for, and other critical factors in deciding if the objective will perform as needed. A more detailed discussion of these properties is provided below and in links to other pages dealing with specific issues.

Numerical Aperture - This is a critical value that indicates the light acceptance angle, which in turn determines the light gathering power, the resolving power, and depth of field of the objective.

The day before the test of the Cordouan lens in Paris, a committee of the Academy of Sciences reported on Fresnel's memoir and supplements on double refraction—which, although less well known to modern readers than his earlier work on diffraction, struck a more decisive blow for the wave theory of light.[31] Between the test and the reassembly at Cordouan, Fresnel submitted his papers on photoelasticity (16 September 1822), elliptical and circular polarization and optical rotation (9 December), and partial reflection and total internal reflection (7 January 1823),[32] essentially completing his reconstruction of physical optics on the transverse wave hypothesis. Shortly after the Cordouan lens was lit, Fresnel started coughing up blood.[33]

Also in 1825, Fresnel unveiled the Carte des Phares ('lighthouse map'), calling for a system of 51 lighthouses plus smaller harbor lights, in a hierarchy of lens sizes called "orders" (the first being the largest), with different characteristics to facilitate recognition: a constant light (from a fixed lens), one flash per minute (from a rotating lens with eight panels), and two per minute (16 panels).[42]

Fresnel lenses are used as simple hand-held magnifiers. They are also used to correct several visual disorders, including ocular-motility disorders such as strabismus.[68] Fresnel lenses have been used to increase the visual size of CRT displays in pocket televisions, notably the Sinclair TV80. They are also used in traffic lights.

Fresnel lenses are used in left-hand-drive European lorries entering the UK and Republic of Ireland (and vice versa, right-hand-drive Irish and British trucks entering mainland Europe) to overcome the blind spots caused by the driver operating the lorry while sitting on the wrong side of the cab relative to the side of the road the car is on. They attach to the passenger-side window.[69]

A Fresnel lens (/ˈfreɪnɛl, -nəl/ FRAY-nel, -⁠nəl; /ˈfrɛnɛl, -əl/ FREN-el, -⁠əl; or /freɪˈnɛl/ fray-NEL[1]) is a type of composite compact lens which reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections.