Electric-filament lamps came into general use in the 1920s and are standard today. They may be augmented by modern, more efficient arc lamps. In some lighthouses the xenon flash tube is used. It is similar to the flashing light mounted on some skyscrapers to ward off aircraft and birds. To increase light intensity and focus it into a beam, mirrors and other reflectors came into use in the 18th century.
Modern lighthouses have reflectors, as well as lenses and prisms, to carry the light farther.
So that the light can be seen from all directions, the lantern rotates at a controlled speed. In bad weather , such as dense fog , even a modern light with 10 billion candlepower is barely visible from half a mile. To offset the problem of poor visibility, lighthouses also produce sound to give position and warning information. For centuries bells and explosive charges have helped guide mariners , but today the most common device is the foghorn, the sound of which is familiar to anyone who has lived near an area of water navigation. The largest of these, the diaphone, makes its sounds by a rapid release of compressed air.