A deck prism, also known as a dead light or deck light, is a piece of molded glass that reflects light into the lower reaches of a ship. It was used on wooden ships before the availability of electricity as a safer alternative for lighting than kerosene and oil lamps or candles. The prisms have a flat base, which rises to a smaller pointed or rounded top. They were typically hung from the ceiling, under a small hole on deck, with the pointed side hanging down so that light could come in from above. Deck prisms have several sides so that light that comes through the base can reflect on multiple surfaces below deck.
Though the earliest versions of deck prism have been traced back to the 1840s, it is believed they were used many years — and possibly centuries — previous to that date. Original deck prisms are rare and difficult to find, though there are some that can be viewed in nautical museum collections. It can also be challenging to determine their age, as glass is not easily dated. There has also been no solid information as to how deck prisms were developed or who, if anyone in particular, was responsible for their invention.
All deck prisms have multiple sides in order to allow for the even distribution of light below deck. Otherwise, they can come in a variety of shapes and sizes. A typical deck prism will have a circular base with a domed or pointed top. Some have a rectangular base that rises up to a three-dimensional triangle.
From the top side of the deck, the prism will appear as a flat piece of round or rectangular glass. Under the deck, it has a look similar to a light fixture. It can be secured into the deck or hang from a holder. There can be one or multiple deck prisms on a ship, usually depending upon the size of the craft.
The deck prism continues to be used in some sailing ships, but primarily as a nostalgic decorative element. Though they are not meant to replace electricity, the prisms can still help to light the areas below deck and potentially save on energy costs. Reproductions of traditional deck prisms are also sold as decorative items for nautical enthusiasts. They are usually either mounted so that they can hang from a special holder, or sit flat side down on a pedestal. The prisms are also marketed in a variety of colors and even incorporated into novelty items such as Christmas ornaments.