How Lighthouse Illumination Evolved
Open Fires and Candles
The earliest lighthouse lights were the simplest conceivable: an open coal fire at the top of a tower, or a cluster of tallow candles mounted in a lantern. Neither was adequate by any serious navigational standard. A coal fire of the kind maintained at Tynemouth Castle light in the seventeenth century had an effective range of perhaps five miles in good weather; in rain or sea mist, its visibility collapsed. Coal fires required a substantial fireman as well as a keeper, since the grate had to be tended almost continuously. The smoke stained the masonry, and in strong winds the fire could be extinguished or driven sideways, making the light directionally unreliable. In the coal-basket lights of the early Eddystone designs and at Dungeness on the Kent coast, coal fires were finally abandoned in the eighteenth century, but the transition was driven as much by the practical misery of maintaining them as by any systematic understanding of optics.
Candle lanterns represented an improvement in terms of control and consistency, but were feeble sources by modern standards. A typical eighteenth-century lighthouse lantern held perhaps twenty to sixty candles, burning together to produce a combined output that a single modern LED lantern would vastly exceed. Samuel Pepys recorded his concern, in his capacity as Secretary of the Navy, about the inadequacy of the lights maintained on the English south coast by private patentees whose financial incentive was to minimise fuel costs. The conflict between the revenue from light dues and the actual quality of the light provided was a recurring problem throughout the pre-government era of lighthouse management.
The Argand Lamp
The breakthrough that transformed lighthouse illumination was the Argand lamp, invented by the Swiss physicist Ami Argand around 1780 and refined for lighthouse use through the final decade of the eighteenth century. The Argand lamp burned whale or vegetable oil through a hollow cylindrical wick, through which air circulated both inside and outside the flame. This double air supply greatly increased combustion efficiency and produced a flame several times brighter than any equivalent candle. A central glass chimney encouraged an upward draught that stabilised the flame and made it largely windproof even in draughty lantern rooms.
The Argand lamp was adopted by Trinity House in England, by the Northern Lighthouse Board in Scotland, and eventually by lighthouse authorities across Europe and North America. Its adoption approximately doubled the range of existing lights overnight in favourable conditions, and the improvement was immediately visible to mariners. The lamp was used with reflectors in the catoptric system — curved polished metal mirrors, initially of copper or silver, that concentrated the flame's output into a directed beam — before the advent of the Fresnel lens.
The Fresnel Lens
Augustin-Jean Fresnel, a French physicist working at the Bureau des Longitudes, devised the compound refracting lens that bears his name in 1822, and it was first deployed at the Cordouan lighthouse at the mouth of the Gironde in 1823. Fresnel's system replaced the metal reflector with a precisely ground glass lens consisting of a central bull's-eye surrounded by concentric rings of prisms, each angled to refract light from the flame into a near-parallel horizontal beam. The lens gathered light from nearly the full sphere around the lamp and projected it outward, compared with the reflector system which only captured the light directed at the mirror's surface.
The efficiency improvement was dramatic. A first-order Fresnel lens, the largest class designed for major ocean lighthouses, could project a beam visible 20 to 28 nautical miles in clear conditions from a flame that no candle or reflector array could match. The lens was classified by order: first through sixth, with first order being the largest at roughly 1840 millimetres in diameter at the focal plane, and sixth order the smallest at 150 millimetres. The order was matched to the role: major headland lights received first- or second-order lenses, while harbour entrance lights used fourth, fifth, or sixth order.
The production of Fresnel lenses required the highest standards of optical grinding available in the nineteenth century. The leading French manufacturers Barbier, Bénard et Turenne and Henry-Lepaute produced lenses that are still regarded as objects of exceptional precision. Chance Brothers of Smethwick, England, became the dominant British supplier. The lenses rotated on clockwork mechanisms, producing the characteristic flashing patterns that gave each light its individual identity — its light character.
Kerosene and the Vapour Burner
The Fresnel lens system was initially used with colza oil (rapeseed oil) and later with paraffin, or kerosene. The development of the incandescent oil vapour burner at the end of the nineteenth century, in which kerosene was vaporised under pressure and the vapour burned through an incandescent mantle (similar in principle to a camping gas mantle), substantially increased the brightness of the flame for a given lens. Lighthouse authorities adopted the vapour burner widely from around 1900. Trinity House, the Northern Lighthouse Board, and the Commissioners of Irish Lights all converted major stations to vapour burners in the first decade of the twentieth century. The increased brightness allowed some lights to use smaller-order lenses where first-order had previously been specified, reducing equipment weight and cost.
Electric Light
The first electrically lit lighthouse was the South Foreland lighthouse in Kent, which adopted electric arc lighting on an experimental basis in 1858. The experiment was not immediately adopted universally because electric power supply in coastal locations was unreliable and the generating equipment was bulky and expensive. The Dungeness lighthouse followed in 1862, but the wider transition to electricity for major lighthouses took place mainly between 1890 and 1940, as mains electricity or reliable diesel generation became available at coastal stations.
Electric filament lamps, and later electric arc sources, were used within existing Fresnel lenses, replacing the oil or vapour burner without requiring optical changes. The consistency and brightness of electric illumination, and the elimination of the daily oil management tasks that had occupied keepers for a century, made electrical operation strongly preferable from an operational standpoint. Automation of lighthouse operation — removing the keeper altogether — became practicable once a reliable electrical supply was established, since the electrical components required far less frequent attention than oil-burning equipment.
LED and Solar
The final stage of illumination technology, still ongoing, is the transition to light-emitting diode sources. An LED lighthouse lantern produces the required beam pattern and light character with a fraction of the energy consumption of filament or discharge lamps, requires no warm-up period, can be self-monitoring via sensor circuits, and has a rated service life of tens of thousands of hours. Combined with solar charging and battery storage, LED technology has made fully automated lighthouse operation practical even in the most remote locations. Open the map to explore the lighthouses of the world: the buildings may look similar across the centuries, but the light sources inside them have undergone a complete technological revolution, from tallow to LED, in less than 250 years.