Glossary - Part D

•  Davit: A small crane used to hoist boats and cargo.

•  Daymark: The unique color scheme, pattern, or architectural configuration of design used to identify a specific lighthouse during daylight hours.

•  Decommissioned: A lighthouse that no longer functions as a navigational aid.

•  De-staffed: An automated lighthouse without a light-keeper.

•  Diaphone: A sound signal, which produces sound by means of a slotted piston moved back and forth by compressed air. A “two-tone” diaphone produces two sequential tones with the second tone of lower pitch.

•  Dioptric: A system directing the rays from a light source into a beam by means of refraction.

•  Dioptric Belt: The central dioptric elements of a fixed lens.

•  Directional Light: A light illuminating a sector or very narrow angle and intended to mark a direction to be followed.

•  Divergence: The vertical and/or horizontal spread of light emitting from a navigation light apparatus.

•  Drum Lens: A common variation of the Fresnel lens but lacking a central bullseye and having the form of an open cylinder or "drum".