How lighthouses work

The engineering of a lighthouse is a compromise between three things: how bright the light needs to be, how far it needs to be seen, and how reliably it needs to work alone in a cyclone. Every lighthouse on the Australian coast is a solution to some version of that equation.

The optical principle: Fresnel's invention

Before 1822, lighthouses used open-flame lamps (typically burning whale or sperm oil) backed by polished silver reflectors. The light was weak — visible perhaps 10-12 nautical miles on a clear night — and the fuel was expensive.

The transformative invention was the Fresnel lens, developed by the French physicist Augustin-Jean Fresnel in 1822. A Fresnel lens is a specially-ground piece of glass that captures almost all of a lamp's output and concentrates it into a horizontal beam, roughly one degree wide. Fitted to the same oil lamp, a Fresnel lens can triple or quadruple the effective range of a light.

By the 1850s, Fresnel lenses had been standardised into orders — numbered 1 through 7 based on focal length. First-order lenses (the biggest, with a focal length of 920mm) were reserved for major coastal headlands; seventh-order lenses (focal length 150mm) were used on minor harbour lights. Most Australian mainland lighthouses used second-order lenses (focal length 700mm) — strong enough to be seen 20+ nautical miles, compact enough to be shipped to the colonies and installed in towers of reasonable height.

The flash: identifying one light from another

To a mariner approaching the coast, every light on the coastline needs to be unambiguously identifiable. A single steady light can be confused with a house light, a ship's light, or a flare. The solution is a characteristic — a unique pattern of flashes that identifies each individual lighthouse.

Traditional characteristics are created by rotating the Fresnel lens around a stationary lamp. As the lens rotates, its focal beam sweeps past a given observer, producing a flash. A single flash every 7.5 seconds, a double flash every 10 seconds, four flashes in 20 seconds — each is assigned to only one lighthouse in a given region. The characteristic of Macquarie Lighthouse is "Flashing (1) every 7.5s"; the characteristic of Cape Otway is "Flashing (4) every 18s".

The rotation of large first-order lenses — weighing several tonnes — originally relied on clockwork mechanisms driven by falling weights that keepers had to wind every four hours. From the 1890s, electric motors replaced the clockwork, and from the 1960s, modern lights abandoned rotating optics entirely in favour of fixed LEDs that flash on and off electronically.

Power: whale oil to solar

Australian lighthouses have burned, in order:

The transition to acetylene in the 1920s — using Sweden's Dalén valve, which could produce a flashing light without a rotating lens or human intervention — was the first step toward unattended lighthouses. By the 1950s, some remote Australian lights were already operating for weeks at a time without a keeper present. Full automation arrived with reliable solar power in the 1980s.

Height: how far can you see a light?

The maximum range of a lighthouse is determined by two things: the brightness of the lamp (and its optics), and the geographic range — how far the horizon extends at the observer's height.

Geographic range is a function of the Earth's curvature. From a ship's bridge 15 metres above the water, the horizon is 8 nautical miles away. A lighthouse built on a 50-metre cliff with its focal plane 70 metres above sea level can be seen by that same ship from 25 nautical miles away. The cliff does the work; the tower itself only needs to be tall enough to lift the lamp clear of any obstructions.

This is why Australian lighthouses vary so much in tower height. Cape Byron Lighthouse is 22 metres tall but stands on a 95-metre headland — its focal plane is 118 metres above the sea, and its nominal range is 26 nautical miles. Point Hicks Lighthouse is 37 metres tall but built on a low headland — same focal plane, similar range.

Construction

Australian lighthouse towers were built from whichever material was available nearby. Sandstone (Macquarie, Bustard Head), granite (Cape Wickham, Cape Bruny), concrete (Cape Naturaliste, Cape Leeuwin), pre-fabricated iron plates shipped from Britain (Troubridge Island, Gabo Island), and in Queensland a distinctively local form: cast-iron panels riveted over a timber frame and sheathed in corrugated iron (Low Isles, Dent Island).

Most towers are narrower at the top than the base — both for structural reasons (a wider base sheds wind loads better) and for aesthetic reasons (the taper reads as height to an observer). The iconic red-and-white horizontal banding of many Australian lights is a daymark — designed to make the tower distinguishable in daylight when the light isn't lit. Different daymark patterns identify different towers.

Today

A typical modern aid to navigation on the Australian coast is a small solar array, a battery bank, and a fixed LED lamp inside a compact optic — maybe half a metre tall, unmanned, monitored via satellite telemetry. It can operate for weeks without a site visit. The classic first-order Fresnel lens in a 30-metre stone tower is now a heritage feature, still lit, but assisted by and often superseded by these smaller units.

The engineering has changed entirely. The purpose hasn't.