Editing Fresnel lens (section)

=== After Fresnel ===

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.<ref>Levitt, 2013, pp.&nbsp;28, 72, 99.<!-- Elton, (2009, p.190) says Leonor became Secretary in "1837", which is either a mistyping of "1827" or a misreading of the primary source cited. --></ref>

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.<ref>Elton, 2009, pp.&nbsp;199, 200, 202; Levitt, 2013, pp.&nbsp;104–105.</ref> 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&nbsp;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''.<ref>Levitt, 2013, pp.&nbsp;108–110, 113–116, 122–123. Elton (2009, p.{{nbsp}}208) notes that although the Skerryvore lens was lit on 1&nbsp;February 1844, the catadioptric portion was yet to be added.</ref>

[[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.<ref>Elton, 2009, pp.&nbsp;209–210, 238.</ref> 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&nbsp;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.<ref>Elton, 2009, pp.&nbsp;210–213.</ref>

[[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.<ref>Elton, 2009, pp.&nbsp;221–223.</ref>

[[File:Fresnel Lens at Point Arena Lighthouse Museum.jpg|thumb|First-order group-flashing Fresnel lens, on display at the Point Arena Lighthouse Museum, [[Point Arena Light]]house, [[Mendocino County, California]]. The three dioptric panels (inside the brass rings) and three catadioptric panels (outside) are partly split in two, giving three double-flashes per rotation.]]

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 Reef Lighthouse|Little Basses]] (Sri&nbsp;Lanka) in 1875, and a triple-flashing lens at [[Casquets lighthouses|Casquets Lighthouse]] ([[Channel Islands]]) in 1876.<ref>Elton, 2009, pp.&nbsp;227–230; Levitt, 2013, p.{{nbsp}}219.</ref> The example shown (right) is the double-flashing lens of the [[Point Arena Light]], which was in service from 1908 to 1977.<ref>Point Arena Lighthouse Keepers, Inc., [https://www.pointarenalighthouse.com/about/lighthouse-history/ "Lighthouse History"] {{Webarchive|url=https://web.archive.org/web/20210119171021/https://www.pointarenalighthouse.com/about/lighthouse-history/ |date=19 January 2021 }}, accessed 1&nbsp;March 2021.</ref>

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 [[Barbier, Benard, et Turenne|F.&nbsp;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, Isles of Scilly|Bishop Rock]] Lighthouse in 1887.<ref>Elton, 2009, p.{{nbsp}}233; Levitt, 2013, pp.&nbsp;222–224.</ref> In the same year, Barbier installed a hyper-radial at [[Tory&nbsp;Island]]. But only about 30 hyper-radials went into service{{r|tag-hyper}} before the development of more compact bright lamps rendered such large optics unnecessary (see ''[[Hyperradiant Fresnel lens]]'').

Production of one-piece stepped dioptric lenses—roughly as envisaged by Buffon—became feasible in 1852, when John L. Gilliland of the [[Corning Inc.|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;{{r|tag-us}} such lenses were common in the United States by the 1870s.{{r|ripley-dana-1879|p=488}} In 1858 the company produced "a&nbsp;very small number of pressed flint-glass sixth-order lenses" for use in lighthouses—the first Fresnel lighthouse lenses made in America.{{r|tag-us}} By the 1950s, the substitution of plastic for glass made it economic to use Fresnel lenses as condensers in overhead projectors.<ref>A. Finstad, "New developments in audio-visual materials", ''Higher Education'', vol.&nbsp;8, no.{{nbsp}}15 (1&nbsp;April 1952), pp.&nbsp;176–178, at p.{{nbsp}}176.</ref>