Flame first
For most of history a lighthouse burned whatever it could: wood, coal, tallow candles, whale or vegetable oil. Open coal fires were bright but smoky and wildly variable in wind and rain. When Smeaton's Eddystone tower was lit in 1759, its lantern held twenty-four candles. A naked flame sends light in every direction, including up into the sky and down into the rock, so most of its output was wasted.
The Argand lamp and the mirror
In the 1780s the Swiss inventor Aimé Argand devised an oil lamp with a hollow circular wick and a glass chimney. Air rose through the centre, the flame burned hotter and cleaner, and the light was markedly steadier. Lighthouse engineers placed Argand lamps at the focus of silvered parabolic reflectors, gathering the light into a beam. This mirror-based arrangement is called catoptric, and a single lantern might carry a dozen or more lamp-and-reflector pairs.
Fresnel's lens
The decisive change came from France. Augustin-Jean Fresnel, a physicist working for the French lighthouse commission, presented in 1822 a lens built not as one thick piece of glass but as a set of concentric rings of prisms. The central rings refract the light into a horizontal beam; rings above and below use total internal reflection to bend stray light back into line. Lenses using refraction are called dioptric, and those combining refraction and reflection catadioptric. The first Fresnel lens was installed at Cordouan in the Gironde estuary in 1823. It caught far more of the flame's light than any mirror, and it was thinner and lighter than a solid lens of the same power.
Orders and makers
Fresnel lenses were graded by focal distance into orders. A first-order lens, with a focal distance of about 920 millimetres, served major coastal lights; the smaller orders, down to sixth, marked harbours, piers and rivers. Later in the nineteenth century the still larger hyperradiant lens, around 1,330 millimetres, was introduced for a few exposed headlands. The principal makers included Henry-Lepaute and Barbier, Bénard et Turenne in Paris, and Chance Brothers of Smethwick, near Birmingham.
Turning the light
To make a light flash, the lens itself was rotated so its beams swept past the observer. The mechanism was clockwork, driven by a falling weight that keepers wound up by hand at regular intervals through the night. Heavy lenses were often floated on a bath of mercury, which allowed several tonnes of glass and brass to turn smoothly with little force. The toxicity of mercury has led many authorities to remove these baths during modernisation.
Reading a light's character
Every light has a character: a pattern of light and darkness, and sometimes colour, that identifies it. Charts and official lists write it in a short code. National lists such as the Admiralty List of Lights and Fog Signals, published by the UK Hydrographic Office, and the United States Coast Guard Light List give every detail.
- F — Fixed
A continuous, steady light. - Fl — Flashing
Periods of light are shorter than periods of darkness. - Oc — Occulting
Light is longer than darkness; the light is briefly eclipsed. - Iso — Isophase
Light and darkness last exactly as long as each other. - Q — Quick
Rapid flashes, usually 50 to 60 per minute. - Fl(2) W 10s
An example: two white flashes, the whole pattern repeating every 10 seconds.
Coloured sectors, usually red, white and green, show a vessel whether it stands in safe water or over danger. The nominal range given on a chart describes how far the light carries in clear weather.
Gas, electricity and LED
Paraffin vapour burners with incandescent mantles replaced plain oil wicks. The Swedish engineer Gustaf Dalén developed the sun valve, which switched acetylene lights on at dusk and off at dawn, and he received the Nobel Prize in Physics in 1912. Electric light arrived at Souter in 1871, among the earliest stations designed for it. Today most lights are compact LED lanterns, often solar-powered, while the great glass lenses survive in museums and in a number of towers where they are still turned.