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 the light sideways under the building. They were developed in the 19th century, but declined in popularity with the advent of cheap electric lighting in the early 20th. Older cities and smaller centers around the world have, or once had, pavement lights. In the early 21st century, such lights are over a century old, although lights are being installed in some new construction.

Uses

Sidewalk prisms are a method of daylighting basements, and are able to serve as a sole source of illumination during the day. At night, lighting in the basements beneath produces a glowing sidewalk. Vault lights may be used to make subterranean space useful. They are more common in city centers, dense, high-rent areas where space is valuable. Historically, landlords took an interest in improving not only the floor area ratio, but the amount of space that was naturally lit, on the grounds that this was profitable. Occupiers valued daylight not only as a way of saving on artificial lighting costs, which were higher historically, but also as a way to let premises remain cooler in summer, and a way to save on ventilation costs, if using gas lighting rather than arc lamps or early incandescent lights. Pavement lights and related products were historically 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 also are used in floors under glass roofs, for example in Budapest's historic Párizsi udvar and New York's mostly-demolished old Pennsylvania Station (see § Current state and trends). Vault lights also could be set into the basement floor, underneath other vault lights, creating a double-deck arrangement, which would light the subbasement. Manhole covers and coalhole covers with lighting elements were also made. Some steps have vault lights set into the vertical stair risers.

History A basement that extends below a sidewalk or pavement is called an areaway, a vaulted sidewalk, or a hollow sidewalk. In some cities, these areaways were created by the raising of the street level to combat floods, and in some cases they form, an often now-abandoned, tunnel network. To light these spaces, sidewalks incorporated gratings, which were a trip hazard and let water and street dirt as well as light into the basement. Replacing the open gratings with glass was an obvious improvement.

Frames

Sidewalk prisms developed from deck prisms, which were used to let light through the decks of ships. The earliest pavement light (Rockwell, 1834) used a single large round glass lens set in an iron frame. The large lens was directly exposed to traffic, and if the lens broke, a large 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 raised nubs around each lens to improve traction in wet weather and to protect them from damage and wear. Even if all 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 substantially 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 may be pre-cast or cast in-situ. (For process details, see § External links, below.) Late concrete panels often were made with metal-framed "armored prisms", which were intended to prevent breakage and make replacing individual prisms easier. The glass is not cast into the concrete but caulked into the frame. Rather than chiselling 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 vault light with very small (fiberoptic) lighting elements. It also innately redirects the light from the angle of incidence to an angle ~parallel to the optical fibers (usually, perpendicular to the surface of the concrete).

Transparent elements The transparent elements may be referred to as prisms or lenses, depending on shape, or as jewels.

Glass color

The glass in many old pavement lights is now either purple or straw-colored. This is a side-effect of the manufacturing process. Pure silica glass is transparent, but older glass manufacture often used silica from 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 increased demand for manganese in the US and cut off 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 tinted purple deliberately, in order to match the current colour, has been used in some historic restoration projects.

Glass shape

… excerpt ends here. Continue reading the full article.