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Penn Station, 1938 Prism glass is architectural glass used to redirect sunlight to interior spaces through refraction and reflection-- "daylighting". Before electric lighting became common around 1900, light was provided free of charge by the sun, and at night by candle, lamp, or other flame. Penn Station in 1938 was a glorious example-- the glass roof lets in sunlight which falls through the glass-embedded floor and illuminates the tunnels below. Since sunlight is obviously the superior light source, while flames are dim, smelly, smoky, expensive and dangerous, anything which could extend the reach of the sun's free and safe light to interior spaces would make that space more useful and valuable. Group of original deck lights, bottoms shown (tops are flat) Deck lights were the first form of prism glass-- the earliest known patent is Wyndus' of 1684: GREAT AND DURABLE INCREASE OF LIGHT BY EXTRAORDINARY GLASSES AND LAMPS; sadly, the details are not specified. Fire at sea is disaster, more so on a wooden ship or with a flammable cargo. Safely lighting a ship's interior with daylight and prisms instead of flames was a practice widely adopted. Colliers and lime cargos were especially dangerous (slaking lime becomes very hot). The glasses work both directions: they daylight the hold, but also show on deck any light from a cargo fire. Before: open grate Eventually the idea was adapted for land use as vault lights (UK pavement lights). Light used to be provided to vaults and basements by open grates, which were difficult to walk on (especially with women's shoes) and obviously let in water. By placing glass lenses in an iron (or later concrete) frame, daylight could be introduced below while excluding the weather. Iron nubs protruding above the glass surface protect the glass and aid traction. After: vault lights Private collection (Amityville) Patent drawing X8,058 In situ (Brooklyn, New York) In 1834, E. Rockwell patented a round iron plate with a single mammoth bulls-eye lens, but Hyatt later complained in his own patent application, "These glasses are extremely liable to fracture, and when broken leave large and dangerous openings within their rims...". Rockwell's plate apparently did not not see widespread use; only three examples of the iron are known to exist today, and none of the giant glass jewels. Thaddeus Hyatt (Library of Congress) Thaddeus Hyatt's patent basement extension Brown Brothers #4 21" Hyatt Light As an improvement on Cornell, Thaddeus Hyatt in 1845 patented his Vault Cover, an illuminated iron plate set with numerous small, tougher bulls-eye lenses, protected with protruding iron knobs. His Hyatt Lights worked well and were a hit; they made him a rich man. From the 1850s on, Hyatt spent much of his time and fortune fighting for abolition of slavery. The next big advance was Hayward Brothers' 1871 Improvements in Pavement Lights. Pendant prisms had been tried before but did not work well. Edward Hayward's idea was to use a semi-prism shape instead: "...the object of my Invention is so to construct them [prisms] that they may not simply allow the light to pass through, but that they may also direct the light in an inclined direction into the rooms it is desired to light." Using a combination of internal reflection and refraction, the prisms could re-direct and disperse light from the vertical to the horizontal, throwing light from the one bright area deep into the backs of otherwise useless spaces. Vault lights construction New York City, 1907 Display of vault lights, deck prism, coal plates, etc. Illuminated sidewalk lift Etna, California, 2010 Am. 3-Way Prism Co Trademark tile The most familiar type of prism glass is prism tiles, which take the reflection/refraction idea of semi-prism vault lights and applies it to vertical windows. They were based on Pennycuick's 1885 patent for window glass with horizontal prisms on the inner surface, and introduced commercially in 1897 by the Luxfer Prism Company. They were an instant success and the tiles were very widely used. Prism tile installations can still be seen in many small towns where they were especially popular as transom lights above storefront windows. Like vault lights, prism tiles take concentrated light from the sky which falls at the front of a room and redirects and diffuses it to the darker parts of a space. Sometimes called "Luxfer tiles", they were also made by the American 3-Way Prism Company (with whom they later merged), Solar Prism Company, Condie-Neale, Jupiter Prism Company and others. Fresnel 2nd order Lighthouse lens The Fresnel lens was developed by Augustin-Jean Fresnel in 1822 for magnifying lighthouse lanterns. It is essentially the outer surface of a large double-convex lens, without the core glass. It functions as the whole solid larger lens would, and makes it possible to build compound lenses much larger than a single lens could ever be.