The first lighthouses were built long before the time of Christ. The earliest known reference to a lighthouse dates back to 1200 BC. This reference appeared in the Iliad, Homer¹s Greek epic poem. The first onshore beacons that were used to guide ships were bonfires. Eventually, bonfires were replaced with iron baskets filled with burning wood or coal and suspended on long poles. It was not until the 18th century that these baskets were replaced by oil or gas lanterns. In the early to mid-20th century, electric beacons replaced oil and gas lanterns. One of the most important developments in the technology of lighthouse beacons was the invention of the Fresnel lens. Developed in 1822 by French physicist Augustin Fresnel, the Fresnel lens is a collection of multiple glass prisms (transparent bodies consisting of two or more usually flat surfaces and used to bend a beam of light that bends nearly all the available light into a powerful central magnifying lens. The magnifying lens projects an intense beam of light that is visible up to 28 miles away.
Modern lighthouse beacons vary in power from about 10,000 candelas to about 1 million candelas, depending on the prevailing weather conditions and the visibility requirements of shipping traffic in the particular area. (The candela is the unit used to measure the amount of light that a source radiates in a given direction-one candela is equivalent to about one two-hundredth of the brightness of a 50-watt light bulb.) Every lighthouse emits a distinctive series of flashes known as its characteristic. These flash sequences allow ship captains to time intervals between flashes so that they can identify lighthouses, using a publication called the Light List. This publication assigns a number to each lighted beacon and describes its identifying characteristics. Flash sequences include the following: fixed, occulting, flashing, isophase, group flashing, alternating, and Morse code. A fixed light is a light that shines continuously. An occulting light has longer periods of light than of darkness, while a flashing light has longer periods of darkness than of light. An isophase light has equal durations of light and darkness. A group flashing light has a specified number of regularly repeating flashes of light, while an alternating light shows light of different colors. Morse code shows short and long flashes that represent dots and dashes, respectively. These dots and dashes are grouped to represent code characters.