A deck prism, or bullseye, is a

prism PRISM is a code name for a program under which the United States National Security Agency (NSA) collects internet communications from various U.S. internet companies. The program is also known by the SIGAD . PRISM collects stored internet ...

inserted into the deck of a ship to provide light down below. For centuries,

sailing ship A sailing ship is a sea-going vessel that uses sails mounted on Mast (sailing), masts to harness the power of wind and propel the vessel. There is a variety of sail plans that propel sailing ships, employing Square rig, square-rigged or Fore-an ...

s used deck prisms to provide a safe source of natural

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

to illuminate areas below decks. Before

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

, light below a vessel's deck was provided by

candle A candle is an ignitable candle wick, wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a Aroma compound, fragrance. A candle can also provide heat or a method of keeping time. ...

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) and lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...

and

kerosene lamp A kerosene lamp (also known as a paraffin lamp in some countries) is a type of lighting device that uses kerosene as a fuel. Kerosene lamps have a wick or mantle as light source, protected by a glass chimney or globe; lamps may be used on a t ...

s—all dangerous aboard a wooden ship. The deck prism laid flush into the deck, the glass prism refracted and dispersed natural light into the space below from a small deck opening without weakening the planks or becoming a

fire hazard Fire safety is the set of practices intended to reduce destruction caused by fire. Fire safety measures include those that are intended to prevent the ignition of an uncontrolled fire and those that are used to limit the spread and impact of a ...

. In normal usage, the prism hangs below the overhead and disperses the light sideways; the top is flat and installed flush with the deck, becoming part of the deck. The lens shapes were naturally derived from the process of handmaking the glass on an 'iron' and would have predated the ability to manufacture flat glass. (A plain flat glass window would just form a single bright spot below—not very useful for general illumination—hence the prismatic shape.) To maximize light output, the glass used was originally made colorless with the addition of manganese dioxide; the purple hue of some specimens is caused by decades of exposure to UV. Aboard colliers (coal ships), prisms were also used to keep check on the cargo hold: light from a fire would be collected by the prism and be made visible on the deck even in daylight. The names "deck light", "dead light" or "deadlight" are sometimes used, though the latter is uncommon as a reference to prisms, as more often refers to non-opening plain-glass panels. Deadlights were commonplace for lighting underground vaults in the 19th century, in which application they were also called "

pavement light Pavement lights (UK), vault lights (US), floor lights, or sidewalk prisms are flat-topped walk-on skylights, usually set into pavement (sidewalks) or floors to let sunlight into the space below. They often use anidolic lighting prisms to throw t ...

s" (UK) or "vault lights"Streetscapes/Subway Platforms; Letting the Sun Shine In
By CHRISTOPHER GRAY, New York Times, May 19, 2002 (US).

References

{{reflist

Prism PRISM is a code name for a program under which the United States National Security Agency (NSA) collects internet communications from various U.S. internet companies. The program is also known by the SIGAD . PRISM collects stored internet ...

Prisms (optics) Energy-saving lighting

See also

References