Lighting the Darkness Below
Long before electricity reached the sea, ships faced a dangerous problem: how to light the decks below without risking fire. Wooden ships relied on candles or oil lamps, but one clever solution changed that — the deck prism. These glass prisms, set flush into the deck, refracted sunlight below, safely illuminating dark spaces without open flames.
How the Prism Worked
Deck prisms weren’t just plain glass. Handmade by glassworkers, they took on a prismatic shape that scattered light sideways instead of creating a single bright spot. A flat pane of glass would have offered little illumination, but the angled surfaces of a deck prism allowed sunlight to spread into the lower decks. Often made colorless with manganese dioxide, some prisms turned purple over decades from UV exposure.
More Than Just Light
On coal ships, or colliers, deck prisms served another unexpected purpose: safety. If a fire broke out in the hold, light from the flames would shine up through the prism, alerting the crew on deck — even during daylight. While names like “deck light” or “deadlight” were sometimes used, “deadlight” more often referred to fixed plain-glass panels, not prisms.
From Ship to Shore
The same idea crossed onto land in the 19th century, where similar prisms, called pavement lights or vault lights, were embedded in sidewalks to brighten underground vaults. Today, deck prisms survive as nautical antiques, their edges worn smooth by time, their once-clear surfaces now holding the faint violet tint left by decades of sun — reminders of how sailors once harnessed daylight to make life safer below deck.
Before electricity, ships faced a challenge: how to light dark decks safely.
Open flames from candles or oil lamps were dangerous on wooden ships.
The solution?
Deck prisms—glass prisms set into the deck, refracting sunlight below without risk of fire…🧵👇 pic.twitter.com/tILyK12UBR
— Detective Tiger's Stories (@TigerDetective) July 16, 2025