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 the light sideways under the building. They declined in popularity with the advent of cheap electric lighting.
Some cities are systematically removing historic sidewalk lights; others are restoring them. Pavement lights have been used in a few new architectural designs. (See § Gallery , below.)
Sidewalk prisms are a method of daylighting basements, and are able to serve as a source of illumination during the day. At night, lighting in the beneath beneath produces a glowing sidewalk.  Vault lights can be used to make subterranean space useful.  They are more common in city centers, where space is valuable.  In dense , high-rent areas, improving not only the floor area ratio , but the amount of space that was naturally bed, could be profitable to landlords.  Historically, occupiers with daylight Could we save not only artificial lighting costs, drank aussi on the associated breakdown costs (if using gas lightingrather than arc lamps or early incandescent lights ), and could still be cooler in summer. 
Historically, pavement lights and related products are marketed as a way of saving on artificial lighting costs and making space more usable and pleasant. Modern studies of similar daylighting technology provide evidence for those claims. 
Vault lights are used under the roofs of glass roofs, for example in Budapest’s historic Párizsi udvar  and New York’s mostly-demolished old Pennsylvania Station .  (See § Current state and trends , below.) Vault lights could also be set on the floor, creating a double-deck arrangement, which would light the subbasement .  Manhole covers and coalhole covers with lighting elements were also made.  Some steps have gone up in the vertical stair risers . 
Older cities and smaller centers around the world have had pavement lights.  Most such lights are approximately a century old,  although they are being installed in some new construction. 
Basically, this is called an areaway,  a sidewalk vault,  , or a hollow sidewalk.  In some cities, these areas were created by the rise of the level of combat floods, and in some cases they form an underground tunnel network(often now abandoned) .    To light these spaces, sidewalks incorporated gratings, which were a trip hazard and let water and street dirt as well as light on the basement. Replacing the open gratings with glass was an obvious improvement. 
Sidewalk prisms developed from deck prisms , which were used to light through the decks of ships. The earliest light pavement (Rockwell, 1834)  used a single large round glass lens set in an iron frame. The broad lens was directly exposed to traffic, and if the lens broke, a wide hole was left in the pavement, which was potentially unsafe for pedestrians.  
Thaddeus Hyatt corrected these faults with his “Hyatt light” of 1854.  Many small lenses (“bull’s-eyes”) were set in a wrought-iron frame,   (later cast iron  ) , and the frame included to improve the quality of the skin. Even if the lenses were broken out, the panel would still be safe to walk on. 
In the 1930s, London authorities ruled that glass sections could not be larger than 100 mm by 100 mm.  Modern glass floors are made of laminated and toughened glass pavers, which can be larger. They have an upper protective layer that can be replaced if it becomes chipped or cracked.  The top surface of the pavers may also be chosen and treated to improve traction. 
Wrought iron,   cast iron,  and stainless steel  frames have all been used. Reinforced concrete slabs began to replace iron frames in the 1890s in New York. Benefits claimed included less condensation (due to the lower thermal conductivity )  and a less slippery surface when wet.  Concrete panels can be pre-cast or cast in-situ.   (for process details, see External links , below.)
Late concrete panels were often made with metal-framed “armored prisms”, which were intended to prevent breakage and make-up. The glass is not cast into the concrete but caulked into the frame. Rather than chiseling out the old glass, the glass can be popped out of the frame. 
Translucent concrete has also been proposed as a floor material.  This would essentially make it a very light ( fiberoptic ) lighting elements. It also involves the light from the angle of incidenceto an angle ~ parallel to the optical fibers (usually perpendicular to the surface of the concrete).
The transparent elements may be referred to as prisms or lenses (depending on shape), or as jewels. 
The glass in many old pavement lights is now purple gold straw-colored. This is a side-effect of the manufacturing process. Pure silica glass is transparent, but older glass manufactures often used silica sand, which contains iron and other impurities.  Iron produces a greenish tint in the finished glass. To remove this effect, a “decolorizer” such as manganese dioxide (“glassmakers’ soap”) was added during the manufacture of the glass.
When exposed to ultraviolet light , the manganese slowly “solarizes”, turning purple,  which is why many existing sidewalk prisms are now purple.  WWI has increased demand for manganese in the US and has been cut off from the supply of high-grade ore from Germany, so selenium dioxide was used as a decolorizer instead. Selenium also solarizes, but to a straw color. 
Replacement glass that has been used in the past, in order to match the current color, has been used in some historic restoration projects. 
In 1871 London, Hayward Brothers patented their “semi- prism “: changing the shape of the glass by adding prisms to the underside of the light sideways, allowing it to light the area under the main building. The shapes were right-angle (“half”) prisms , which reflected all incoming light sideways.  The horizontal ridges protruding from the top of the prism let it be in an opening in an iron or cement grating.
Some cast glass during prisms have the following parts of the building (see image). Some prisms were made with multiple during prisms, or with a Fresnel-lens- like sheet of identical prisms (“multi”) or a sheet of dissimilar prisms that could distribute the light (“three-way” etc.). 
The precise angles at which the prisms refracted or reflected light was important. An installation can be used for a number of different requirements of prism, or by a layman using standard algorithms.  This would also diffuse the light somewhat, as it would be the rough glass surfaces (the lenses are translucent , not transparent).
Larger castings are more expensive, not only Because They Use more glass, goal Because They take along to cool . Modern glass floors use laminate glass some centimeters (more than one inch) thick; it is often transparent. 
Non-glass translucent materials
Synthetic resin composites (such as fiberglass ), as well as plastics such as Lexan , have been proposed to replace missing prism lights.  Translucent decking panels made of fiberglass are often used for balconies which would otherwise be shade the windows below.  (See § gallery , below.) Peel-and-stick prism films recently have come up with the market, with which the micro-prisms that internally reflect light somewhat like glass during prisms.   
Two-stage refraction system for basement lighting; prism wall below center, shop above left. Note I-beam and masonry wall .
The same system used to light a salesroom inside a hollow sidewalk; prism wall is on the right
Basically daylit by sidewalk prisms (prisms out-of-shot to the left)
In some cases, a second vertical curtain of prisms was installed under the building sill .  These were analogous to the prism transoms used on above-ground windows and doors. The light could be bent in two stages and used to daylight the whole basement. 
The area under a street side usually has a wall separating it from the ground under the street. Support for the vault light frames Steel cross-beams supported by columns are common in older buildings; metal decks are common in newer ones. 
Current state and trends
Manufacture, maintenance, and repair
Some modern pavement lights are quite different from historic ones,   .
A few companies now manufacture and sell vault lights, or as glass-only, prefab panels, or installation.   Construction methods and prices vary widely.  Historically, glass lenses are standardized by each manufacturer; some modern manufacturers produce standardized prisms.    Some firms also supply replacement glass castings to order.  Cost varies greatly; complicated articulated molds are more expensive. 
Modern caulking materials are used for caulking in replacement glass. Broken and damaged frames can be patched, re-welded,  or re-cast.   Generally speaking, restoration requires only simple tools and technology. 
Promptly repairing sidewalk cracks, and avoiding de-icers that will corrode metal, helps keep the supporting structure dry and in good repair.  Keeping a sidewalk light watertight does not cost much in time or materials.  Vaults generally last many decades,  and many extant vaults are more than a century old. 
Reuse and preservation
Despite their reusability and repairability, old panels are often landfilled.    However, the city of Victoria, Canada is stockpiling removed pavement light panels for future restoration projects.   Often, these are not replaced, but instead, the opening is filled with concrete or other opaque materials,  such as metal, wood, and asphalt. 
When a building is renovated, vault lights can be removed or concreted over. For instance, the floor of New York’s mostly-demolished old Pennsylvania Station was made of vault lights, to let the floor through the platforms.  The undersides of the lights can still be seen, but the tops have been concretely over (see images). 
Whereas some cities have some things to do for you  Sometimes the outside appearance of the lights is replaced by the setting of the daylight and the day of the day.  Some areaways are “mothballed”; that is, filled will be gravel that could later be removed. 
Areaways are used in some cities as a convenient place to run utilities, which can make the cities reluctant to give legal protection.  In some cases, utility construction leads to areaways being filled. 
The load-bearing strength of vault lights varies widely with span, construction, and state of repair. Some damaged vaults May not be reliable to support has fire engine,  qui a sidewalk vault in sound conditions shoulds be reliable to do. 
Damp areaways may corrode the steel load-bearing elements supporting the pavement roof. Moisture may come from above or below. 
- Amsterdam , The Netherlands, Netherlands, for the Netherlands Department for Conservation. 
- Astoria , Oregon, has a community program for restoring vault lights, funded by the Astoria Downtown Historic District Association. A volunteer plan to replace broken glass with squares of Lexan , topped with resin embedded with glass teardrops, was prevented by legislation. 
- Budapest , Hungary, the historic párizsi udvar mall on Ferenciek tere (Square of the Franciscans). The mall has unusual, decorative pavement lights let into its polychrome tile floor, to allow light from the glass dome skylights into the basement level. There are also many other places, such as the old post office building. 
- Chicago , IL, has extremely extensive sidewalk vaults, but many of them do not have vault lights. There is no inventory of them. The city is filling in all the vaults, as some are structurally unsound.  See also the raising of Chicago .
- Deadwood , South Dakota, funded by major restoration and maintenance project for vault lights in approximately 2000. 
- Dublin , Ireland has many vault lights. 
- Dunedin , New Zealand has well-preserved Luxfer and Hayward Brothers vault lights. 
- London , England has many vault lights, many made by the Hayward Brothers.  Historic preservation legislation encourages a market in new pavement lights.
- New York City has large numbers of vault lights, mostly in the SoHo district. More than half of the subway stations originally had vault lights, but these had mostly been blocked off. Installing and restoring vault lights has become part of modern construction practices.  The city government has no policies or records about vault lights. 
- Philadelphia , Pennsylvania has many vault lights, some of them locally manufactured. 
- Portland , Oregon has prisms at multiple locations.  It has no preservation project for its prisms, however, and those that break with concrete.  There is some local opposition to the policy.  See also Portland Underground .
- Pretoria , South Africa has Hayward vault lights. 
- Sacramento , California has “hollow sidewalks”, which originated when the city rises to the street level; some of these spaces are lit by vault lights. There are many stories told about these areas. 
- Salem , Oregon has an extensive tunnel network with vault lights.  Historians-have found a wall-painted grocery drop,  a disco, a swimming pool, a firing range, opium dens, and bordellos in the tunnels.  There areabove-groundtours, but currently,  they are above-ground  or irregular and overbooked.  The Go Downtown Salem! Board welcomed the idea of regular underground tours.  Many of the tunnels have been filled during sewer construction. 
- San Diego , California has sixteen-sided pavement jewels of the “Searchlight” brand. 
- San Francisco does not evaluate the lights, but has a safety hazard for pedestrians. Most of the lights have been removed.  The City Lights Bookstore has vault lights. 
- Saskatoon , Saskatchewan has had sidewalk prisms. They have been used in music videos and a Facebook group fought to save them.  They were scheduled to be infilled in 2015. 
- Seattle , Washington Raised its street level, by up to 22 feet in some places, in the aftermath of the Great Seattle Fire of 1889.  Previously, the Pioneer Square area had flooded tidally. Seattle replaced some of its sidewalk vault lights in Pioneer Square with new pre-purpled ones in 2002. Seattle runs tourist trips through its underground . 
- Tijuana , Mexico City has armored unsolarized vault lights in the 1919 Casa de la Cultura . 
- Toronto , Ontario, 2869 Dundas St. West (near Keele). 
- Vancouver , British Columbia has  , but does not have a responsibility for a fire engine.  Some of the remaining areas have restaurants built into them.  A walking map of the sidewalk prisms has been produced.  There are ~ 130 remaining areaways, the records of which are not digitized, and no measures exist to promote their preservation. 
- Victoria , BC has more than eleven thousand sidewalk prisms in seven locations (as of 2006), including an underground gallery running around an entire block outside the Yarrow Building. Sidewalk prisms have been inherited-registered since 1990. Originally, there were hundreds of thousands of prisms. The city has some panels for storage, but is having difficulty finding a glass supplier. There are city plans to light the galleries, creating glowing purple sidewalks in the downtown core.  While they are protected, there is no funding for the preservation of sidewalk prisms. 
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