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Pharology - How Lighthouses Work - H28

H28: Occulting Systems

It is not always necessary to use rotation of an optic to achieve a flashing light, although it has generally been the most common method in lighthouses. It is perhaps a more obvious method to periodically show and obscure the light source. This is called an occulting light. Nothing is rotated, but a cylindrical screen is periodically raised and lowered around the light source.

It is often common in these cases that the period of time that the light is showing is longer than the period of time that it is obscured. This is in contrast to a flashing light where it is almost always the case that the period of light is exceeded by the period of dark.

The fact that the apparatus does not rotate gives rise to quite different designs of optics. Many of these resemble beehives and the designs are thus called by that name.