Glossary - Part F

•  Fixed Light: A light showing continuously and steadily, as opposed to a rhythmic light. A steady, non-flashing beam.

•  Flash Panel: A vertical section of a lens assembly that always includes a bull’s-eye lens and usually also includes dioptric ring prisms and upper and lower catadioptric prisms.

•  Flash Panel, External: A panel which is not an integral part of the main lens assembly but is fastened to that assembly in such a way as to produce a flash or flashes in an ordinarily fixed lens as either the whole assembly or only the external panel(s) rotates.

•  Flash Tube: An electronically controlled high-intensity discharge lamp with a very brief flash duration.

•  Flashing Light: A light in which the total duration of light in each period is clearly shorter than the total duration of darkness and in which the flashes of light are all of equal duration. (Commonly used for a single-flashing light, which exhibits only single flashes, which are repeated at regular intervals.)

•  Focal Length (or Focal Distance): The Focal Length is the distance, in millimeters, from the focal point of the lens (the center of the flame or the filament of the light bulb) to the inside surface of the nearest (dioptric) element of the lens.

•  Focal Plane: The narrow beam of light emitted from a Fresnel lens or modern optic. The distance from the water surface to the center of the beam is known as the height of the focal plane.

•  Focal Plane Height: A measurement taken in feet or meters from the horizontal center of a lens to Mean High Water Mark (MHWM) at lighthouses, and to the water line of light vessels.

•  Focal Point: The point in a Fresnel lens where the vertical centerline of the lens assembly intersects the focal plane (the center of the flame or the filament of the light bulb).

•  Fog Detector: An electronic device used to automatically determine conditions of visibility, which warrant the activation of a sound signal or additional light signals.

•  Fog Signal: Any type of audible device that could warn mariners from obstacles during period of heavy fog when the light could not be seen. Bells, whistles and horns, either manually or power operated were all used with varying degrees of success.

•  Fresnel Lens (Fray-nel): An optic array manufactured using the design principles of Augustin Fresnel, the French physicist who first established the design, and after whom the Fresnel Lens was named. A type of optic consisting of a convex lens and many prisms of glass, which focus and intensify the light through reflection and refraction.

•  Fuel: A material that is burned to produce light (fuels used for lighthouses included wood, lard, whale oil, tallow, kerosene). Today, besides electricity and acetylene gas, solar power is also used.