Lighthouses of Scotland

Fresnel's Merits and Legacy

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The first trials of using lenses in lighthouses took place in England around the end of the 17th century. These tests failed because of the large loss of light due to the thickness and the poor quality of the glass. These lenses also turned out to be unsuitable because they were too heavy.

To solve the weight and thickness problems, Fresnel designed a new type of lens (see 'Fresnel Lens-transformation' in the right column). After Fresnel's design was completed, he was told that Count Buffon and Condorcet, around 1750, had proposed similar designs.

(The earliest design of a stepped-surface lens was developed by Count Buffon in 1748. He proposed to reduce the material on the flat side of the lens along the spherical surface, so that it would become thinner. Condorcet improved Buffon's idea with lenses and a stepped surface. However, these earlier designs were never built because at the time it was technically not possible to grind the single piece of glass as required by Buffon or to make the one-piece rings proposed by Condorcet.)

Augustin Fresnel invented a lens in 1819 that would revolutionize lighthouse illumination worldwide. The first lens installed at the Cordouan lighthouse (Gironde - France) in 1823 consisted of eight panels of annular lenses placed around the lamp with a focal length of 920 mm. The table in the right column shows that this was a 1st order lens.

The first Fresnel lenses were cut from glass and then polished. It was an expensive method of lens production that significantly limited their mass production. This way of producing lenses did not change until the early 1950s with the advent of the glass-casting technique that made them relatively cheap. Over the next forty years, the emergence of optical-grade plastics, aided by injection molding and computer-controlled compression, enabled the mass production of premium lenses.

In 1822, Fresnel completed his design for a flashing lens using eight of his circular Bull's-eye flash panels. Each of the Bull's-eye panels reflected the light horizontally, forming a compact beam of light. The light from each Bull's eye, illuminated by a lamp with four concentric fuses, was equivalent to three and a quarter of the best reflectors (see: right column) then in use.

To capture the light that would escape above the lenses, Fresnel applied a series of flat silver-plated mirrors (M), placed at a 25-degree angle, above the Bull's-eye (L) panels. Additional mirrors (Z) were also added at the bottom to redirect the escaping light to the horizontal light beam.

Before electricity, these massive lenses were rotated by a clockwork mechanism similar to the concept of a grandfather clock. A weight would fall down the center shaft of the lighthouse, attached to a cable wrapped around a drum. The drum was coupled to a series of gears. The ratio of the gears is designed to drop the weight at a certain speed. A pinion from the clockwork would interact with a large ring gear attached to the pedestal and rotate the lens. (see the Clockwork Mechanism Fresnel lens in the right column).

Augustin created several other forms of lighthouse lenses after his design of the first revolving, first-order lens. He took the basic design of his bull’s-eye flash panel, revolved it around its vertical axis, and created the first fixed lens in 1824. This design was made from 12 vertical panels formed in a circle. He continued to use mirrors, both above and below the main lens belt, and small lenses set at an angle above and below the lamp to direct the light unto the upper and lower mirrors. The main panels in the fixed design collected the light from the lamp and refracted (bent) it vertically into a horizontal beam. The beam was aided by additional light directed in the horizontal direction from the mirrors. The fixed lens directed light in a flat disk shape, in a complete 360-degree circle. The mariner would see a constant light.

The fixed lens design was further modified in 1825, when Augustin added a set of revolving flash panels to create the first fixed/flashing lens. The fixed lens directed light in a flat disk shape, in a complete 360-degree circle. The revolving flash panels refracted the light from the fixed belt horizontally creating two to four horizontal beams of light. As the flash panels revolved around the fixed panels, the mariner would see a fixed light followed by a brilliant flash of light from the flash panel’s beam followed by a fixed light again.

Lamplight collected by the Fresnel Lens

In the right image there a Fresnel lens shows how light is collected from a single oil lamp placed in the center of the lens. The lamp radiates light in all directions. The prisms are positioned to collect all light directed toward them, not allowing any light to escape. The area at the top of the lens is reserved for exhausting the lamp. The area toward the floor is blocked by the shadow cast by the base of the oil lamp.

Bending of the Light

This standard cross section of a Fresnel lens shows how the light is refracted and reflected through the lens. All the light rays exit the lens horizontal, directing them seaward. The Fresnel lens contains two basic prism designs. The area near the center is referred to as the lens section. The technical term is "dioptric", which means the light is bent or refracted twice. Once as it enters the glass and again as it exits. Above and below this section, a different style prism was designed, to use refraction and reflection to direct the light. These prisms are called reflecting or "catadioptric" prisms. The light is refracted as it enters the prism, reflected off the second face, and refracted as it exits. These prisms were necessary because the angle of light from the lamp was too steep to direct the light seaward using simple refraction.

Types of Lenses

There are two basic types of Fresnel lenses. Fixed, producing a steady glow, and Rotating, producing a flash as the lens rotates in an observer's line of sight. Each have the same cross section. The Fixed lens have prisms generated around a vertical axis forming belt lenses. The Rotating lens have prisms generated around a horizontal axis forming Bull's eye lenses. The Fixed/Flashing lens is a combination of the two, producing a steady glow varied by flashes.

This animation demonstrates how light is refracted and reflected through a typical Fresnel lens section The red line represents a beam of light radiating out from the light source. The Fresnel lens collects the light from every angle and bends it to exit horizontal, sending it seaward.

Originally, the Fresnel lenses were divided into 6 classes (called "Orders"). Later at the end of 1800, this series was expanded with a number of types of lenses (Hyper-, Meso-Radial, 3,5th and 8th Order). The order is determined by the distance from the light source to the lens (the Focal length). A 1st Order lens consists of hundreds of glass prisms. The higher the "Order" number, the smaller the lenses become and the overall size of a compound lens becomes smaller (see table in the right column).

The glass prisms are shaped and positioned so that light from the source in the core of the lens is reflected horizontally outward by each prism. In this way, the light from the light source that radiates out in all different directions is maximized in large beams that shine out to sea. The number of beams depend on the configuration of the prisms. This can vary greatly per location. In addition, some lenses have red or green panels that reflect that light seaward.

The "First Order Fresnel Lens" is the largest lens commonly installed in the large "seacoast lights". Two larger sizes (hyper-radial and meso-radial) have been built in limited numbers and are intended for a number of special installations. Smaller Fresnel lenses, such as the sixth-order and higher-order lens, are installed in smaller lighthouses, such as inland waterway lighthouses. See the table in the right column.

Hyper-radial and Meso-radial Lenses

In the late 1860s, lighthouse development increasingly focused on lighting. The lamps get bigger and the fuel changes from animal and vegetable oil to kerosene and coal gas and bigger burners.

Thomas Stevenson found that with these larger burners, some of the light was lost because the light no longer fell into the focal length of the lenses. In addition, the lenses crack due to the high heat of these larger burners. He suggested to the NLB (Northern Lighthouse Board) to build larger lenses that would overcome these problems. However, this idea was not accepted by the NLB.

In Ireland, John Wigham also developed new gas burners. He went from 28 to 108 fuses. This system can be modulated based on lighthouse visibility. In 1877 he came to the same conclusions as Thomas Stevenson. Larger lenses were needed. These burners were also a concern for the lens manufacturers (Frédéric Barbier, Chance Brothers, Sautter and Henry-Lepaute). The heat emitted and the lost light were nevertheless a major problem.

Finally, around the 1880s, David and Charles Stevenson contacted the F. Barbier firm of Paris with a request to produce a larger lens. The company F. Barbier suggested using the lens developed by John Wigham with a focal length of 1330 mm. The tests with these new panels and the larger burner yielded good results. The other lens manufacturers were very impressed and the Hyper-radial lenses were born.

The hyperradial lenses are designed in three versions. The first version is identical to a Fresnel lens of the order of 1°, only with larger dimensions. The disadvantage is that the lantern is very large, both in diameter and in height. The lens is 4.6 meters high and weighs more than 8 tons.

The second version was developed by Charles Stevenson in 1891. The spherical design incorporates the Bull's eye lens located 1330mm from the focal point. Each successive ring is moved further from the focus. This device works, but when the light exceeds an angle of 40°, it is lost. It is placed at the Fair Isle North Lighthouse, Shetlands.

The third version, called equiangular prism lenses, also consists of a Bull's-eye placed 1330 mm from the focus. Up to an angle of 20°, the rings are placed closest to the focus. Then the prisms are placed as in the second version. This optic was installed in 1895 at Sule Skerry Lighthouse, Orkney, Scotland. This system gives better results but the design is more expensive.

These lenses were manufactured by three companies.

The first one is French. It was originally F. Barbier. It was founded in 1862 by Frédéric Barbier and Stanislas Fenestre under the name «Barbier et Fenestre». It changed its names several times, in 1887 «Barbier et Cie», «Barbier et Bénard» from 1889 and finally at the beginning of the 20th century it took its definitive name «Barbier, Bénard et Turenne». The company is better known as BBT. BBT specializes in the manufacture of lighthouses, optical appliances and lighting systems. It was dissolved in 1982.

Chance Brothers (Birmingham) was the most important English glassware. In 1848, under the direction of Georges Bontemps, a French glassmaker from Choisy-le-Roi, they made high precision lenses for optics. Hyper-radial lenses are part of this production. They are also known by the rotation mechanics of the optics.

The third is Henry-Lepautre. He partnered with Augustin Fresnel and specialized in the construction of clocks and rotating optical mechanisms. We only know 2 hyper-radial lenses made by Henry-Lepaute.

In Scotland, nine Hyper-radial lenses have been installed at the following locations:
Lighthouse	Year	Manufacturs	Comment
Bell Rock	1902	Henry-Lepautre	Replaced by a 1° order and now displayed at the Signal Tower Museum (Arbroath).
Buchan Ness	1910	Chance Brothers	Removed on 1978 and now exposed at Museum of Scottish Lights, Fraserburgh.
Fair Isle Nord	1892	Frédéric Barbier	Removed on april 23, 1980 and now exposed at Sumburgh Lighthouse, Shetlands.
Flannan Islands	1899	Henry-Lepautre	With automation in 1971 the magnificent lens was purposely destroyed by the keepers as ordered by HQ. The lens was perhaps too big to be taken off the rock, so the glass was smashed out of the weighty brass panels. The brass was taken ashore and allegedly sold for scrap. Now it has a fourth order lens on a Pelangi PRL 600 gearless pedestal which rotates at 1 rpm
Hyskeir	1904	Chance Brothers	Still has its original lens but the mercury bearing is due to be decommissioned in the next couple of years
Kinnard Head	1902	Chance Brothers	The lighthouse deactivated in 1992. The optics were in place in the museum, Fraserburgh.
Pladda	1901	Chance Brothers	Lost its original lens when automated in 1990. The mechanism and optic is dismantled and stored at Rosaburn House, Arran Heritage Museum, Rosaburn, Brodick.
Rubna Réidh	1909	Chance Brothers	Retired in 1985 and exhibited at the Gairloch Heritage Museum, Gairloch.
Sule Skerry	1928	Barbier & Bénard	Removed on april 23, 1977 and exhibites at the National Museume of Scotland, Edinburgh.

Year	Major Change or Improvements of Fresnel Lenses
1823	Fresnel's First 1st Order Flashing Lens Installed at Cordouan, France
1824	Fresnel's First 4th Order Fixed Lens
1825	Fresnel's First 4th Order Fixed-Flashing Lens
1826	Fresnel's First Catadioptric Lens
1835	Introduction of the Fresnel Lens into the Lighthouses in Scotland
1836	First 1st Order Cylindrical Fixed Lens with Upper and Lower Prisms By Cookson in Scotland for Isle of May Lighthouse
1840	First use of Lower Prisms (Lower Catadioptric Panels) in a Flashing lens by Alan Stevenson in Scotland - Skerryvore Lens
1841	First use of the Fresnel Lens in America at Navesink
1842	First lens design that equalized the power of the Red and White Panels by Alan Stevenson in Scotland
1848	Letourneau Proposes and Uses Split Flash Panels to Lengthen the Duration of the Flash in France
1849	First Holophotal Lens with Prisms above and below the Flash Panels by Thomas Stevenson in Scotland
1849	First Dioptric Spherical Mirror by Thomas Stevenson in Scotland
1850	Letourneau Designs Lenses with Alternating Flash and Fixed Panels in France
1851	First use of the Apparent Light Lens by Thomas Stevenson in Scotland
1855	Design and First use of Condensing Lenses by Thomas Stevenson in Scotland
1858	Proposal to develop Bi-Form Lenses - By J. W. D. Brown in Britain
1859	Design and First Use of the Bi-Valve Lens by Thomas Stevenson in Scotland
1869	Proposal for the Hyper-radial Lens by Thomas Stevenson
1872	Design and Use of Bi-Form Lenses in Ireland by John Wigham
1872	Proposal for Hyper-radial Lens By John Wigham in Ireland
1877	Design and Use of Tri-Form and Quadri-Form Lenses in Ireland by John Wigham
1877	First Hyper-radial Lens Drawings by Barbier & Fenestre in France
1884	Invention of the Group Flashing Lens By Dr. John Hopkinson of Chance Brothers in Britain
1885	First Hyper-radial Lens Panels Built by F. Barbier Tested at South Foreland Trials
1887	First Hyper-radial Lens Built by F. Barbier and installed at Tory Island, Ireland
1887	First Hyper-radial Lens Built by Chance Brothers Britain Installed at Bishop Rock, Britain
1892	Design and First Use of Equiangular Prisms by Charles Stevenson in Scotland
1892	Proposal and Construction of a Single "Giant Lens" Panel Proposed by John Wigham, Ireland Built by F. Barbier, France
1910	The Lighthouse Bureau in America requests that Macbeth-Evans Glass Company develop Fresnel Lenses of the 4th and 5th order made in America
1913	Macbeth-Evans produces the first 4th order American made Fresnel Lens