The Fresnel Orders — a catalogue of eight lighthouse lenses

A metre of glass that could be lifted

In 1823 the new light at Cordouan, at the mouth of the Gironde, was seen from a ship thirty-three miles out. Augustin Fresnel had kept only the parts of a lens that do the work — the curved surfaces — and thrown away the thick glass between them. Cut into concentric rings and stepped back to a common thin profile, a lens a metre across could be cast, ground and hoisted into a lantern without breaking the tower beneath it, and it gathered light that a parabolic reflector simply let go.

Within a generation the design had been graded into orders by Focal length — the distance from the centre of the flame to the inner face of the glass. It fixes the lens's size, weight and light-gathering power, which is why the orders are named by it. — the distance from the flame to the glass. A first-order lens stands taller than its keeper and holds the flame 920 millimetres from the inner face; a sixth-order lens, at 150 millimetres, can be carried under one arm. Between them lie the sizes that lit nearly every coast in the world for a century.

This catalogue records the eight lenses that lit the Herne Coast between 1848 and 1931: one fixed and one revolving example of each of the four principal orders. Filter them by order or by whether the glass turned, open a keeper's note, enlarge a drawing. The table at the end sets the orders side by side so the arithmetic can be read directly.

How the glass works

Three ideas account for everything in the drawings that follow. Each is simple on its own; together they produced an instrument that stayed in service, essentially unchanged, from the age of the whale-oil wick to the age of the halogen bulb.

The stepped profile

A conventional lens of the size a lighthouse needs would be a hemisphere of glass the better part of a tonne, and would swallow a great share of the light passing through its thickness. Fresnel's insight was that only the surface curvature refracts. If the lens is divided into annular zones and each zone is slid back toward a common plane, the surfaces still bend the light to the same focus while the glass between them disappears. The result is the familiar stepped profile: a central bull's-eye surrounded by rings whose cross-section is a chain of small prisms, each ground to the angle its position demands.

Bull's-eye or drum

The two families of lens on this coast differ in what they do with the gathered light. A revolving lens is built as a set of panels, each a bull's-eye with its rings, arranged around the lamp like the faces of a polygon. Each panel collects the light that falls on it and sends it out as a single horizontal beam; as the whole assembly turns, the beams sweep the horizon and a distant observer sees a flash each time one passes.

A fixed lens is a drum: the same profile, but revolved about the vertical axis, so the rings become horizontal belts. It spreads light equally in every direction, which makes a steady light that is easier to build but far weaker in any one direction — the price of showing the same face everywhere.

The catadioptric belts

Light leaving the flame steeply upward or downward cannot be bent to the horizontal by refraction alone; the glass would need to be impossibly thick. Fresnel's answer, and the reason the later lenses are so tall, was to add belts of triangular prisms above and below the refracting panels. Each prism refracts the ray on entry, reflects it off its back face by total internal reflection, and refracts it again on exit, turning it through as much as 130 degrees with almost no loss. These Catadioptric — acting by both refraction (dioptric) and reflection (catoptric). The prisms bend a ray on the way in, bounce it off their back face, and bend it again on the way out. belts account for perhaps two-thirds of a first-order optic's height and a third of its light.

III

The catalogue

Eight lenses, one fixed and one revolving of each order, listed from the largest glass to the smallest. The drawings are schematic — the plan of a single bull's-eye panel for the revolving lenses, the elevation of the drum for the fixed ones — and are drawn to a common scale, so that a fourth-order drum appears at half the size of a first.

Showing of eight lenses.

Reading the light

At night a lighthouse is known by its Character — the pattern a light makes in time: fixed or flashing, how many flashes in a group, its colour, and the period in seconds before the pattern repeats. Printed on every chart beside the light.: the pattern it makes in time. Admiralty notation gives it in shorthand. F is a fixed light; Fl a flashing one, with the number of flashes in a group in brackets; W, R and G are the colours; and the figure in seconds is the period, the time from the start of one pattern to the start of the next.

Each pattern below is drawn as one period laid out left to right. The lamp beside it shows the same thing in time.

F — Fixed. A steady light with no period at all. Saltholm Point, Kinnaird Tor, Pilgrim Shoal and Inver Quay, in white, red, green and red.
Fl W 15s — One flash of about a second, then fourteen of dark. Garrow Rock: eight panels, one revolution in two minutes.
Fl W 5s — The same single flash at three times the rate. Tarrow Harbour: four panels, one revolution in twenty seconds.
Fl(2) W 10s — Group flashing, two. A pair of flashes close together, then a long eclipse. Blackwater Spit.
Fl(3) W 20s — Group flashing, three. Ness of Harl; the longest pattern on the coast, and the one mariners counted aloud.

The lamps are drawn in the amber of an oil flame; coloured characters took their colour from screens in the lantern, not from the lens.

The orders compared

The orders were standardised by the French Commission des Phares and adopted by the English and Scottish lighthouse boards with small variations. Focal length fixes everything else: the inside diameter of the drum is twice it, the height follows from the number of catadioptric belts, and the range follows from how much of the flame's light the optic gathers and how brightly the flame burns.

Nominal dimensions of the four principal orders, with the typical range of a nineteenth-century oil light in clear air.
Order	Focal length	Inside diameter	Height of optic	Typical range	On the Herne Coast
First	920 mm	1 840 mm	2.59 m	20–24 nmi	Garrow Rock, Saltholm Point
Second	700 mm	1 400 mm	2.07 m	17–20 nmi	Blackwater Spit, Kinnaird Tor
Third	500 mm	1 000 mm	1.58 m	14–17 nmi	Ness of Harl, Pilgrim Shoal
Fourth	250 mm	500 mm	0.72 m	10–13 nmi	Tarrow Harbour, Inver Quay

Three smaller sizes — the three-and-a-half order at 375 mm, the fifth at 187.5 mm and the sixth at 150 mm — were used for pier heads and river lights and are not represented on this coast.

About this catalogue

The Herne Coast and its eight stations are invented. The optical dimensions, the makers' names, the Admiralty notation and the mechanics of mercury floats, clockwork drives and mantle burners are not; they are drawn from the historical record of the lighthouse services and from Fresnel's own tables.

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