Pavement light

They were developed in the 19th century, but declined in popularity with the advent of cheap electric lighting in the early 20th.

[3]Sidewalk prisms are a method of daylighting basements, and are able to serve as a sole source of illumination during the day.

[2] 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.

[6] Vault lights also are used in floors under glass roofs, for example in Budapest's historic Párizsi udvar [hu][7] and New York's mostly-demolished old Pennsylvania Station (see § Current state and trends).

[12] 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.

The earliest pavement light (Rockwell, 1834)[16] used a single large round glass lens set in an iron frame.

Benefits claimed included less condensation (due to the lower thermal conductivity)[24] and a less slippery surface when wet.

To remove this effect, a "decolorizer" such as manganese dioxide ("glassmakers' soap") was added during the manufacture of the glass.

[4] WWI increased demand for manganese in the US and cut off the supply of high-grade ore from Germany,[30] so selenium dioxide was used as a decolorizer instead.

[29] Replacement glass that has been tinted purple deliberately, in order to match the current colour, has been used in some historic restoration projects.

The pendant shapes were right-angle ("half") prisms, which reflected all incoming light sideways.

[15] The horizontal ridges protruding from the top of the prism let it be set into an opening in an iron or cement grating.

Some cast glass pendant prisms have flat portions to shed light directly below, as well as throwing it sideways under the main body of the building (see image).

An installation would generally consist of multiple different prescriptions of prism, chosen either by an on-site expert contractor or by a layman using standard algorithms.

[21] Synthetic resin composites (such as fiberglass), as well as plastics such as Lexan, have been proposed to replace missing prism lights.

A few companies now manufacture and sell vault lights, either as glass-only, prefab panels, or installation.

[40][4][2] However, the city of Victoria, Canada is stockpiling removed pavement light panels for future restoration projects.

[4][2] Often, individual broken sidewalk prisms are not replaced, but instead, the opening is filled with concrete or other opaque materials,[2] such as metal, wood, and asphalt.

[41] While some cities have preservation measures for vault lights, others actively remove them and fill areaways.

[13] The load-bearing strength of vault lights varies widely with span, construction, and state of repair.

view down onto a sidewalk with vault lights; a slab of concrete with a regular grid of inset squares of slightly domed purple glass. Within each square, faint vertical lines can be seen through the glass, and points of light shining up through the glass are duplicated in horizontal lines of three points of light. Some yellow leaves have fallen on the damp concrete.
Sidewalk prisms from above, with lights shining up from inside the hollow sidewalk. The purple shade has developed over a century. These are multi prisms; the ridges can be seen running vertically ( see § Transparent elements ).
Cross-section of a road and adjacent building, showing the basement extending under the sidewalk and part of the road. Sunlight is shining through the sidewalk into the basement.
A sidewalk vault , which is daylit through vault lights, 1880
A large circular glass lens with a flat top and a stepped domed bottom. The glass is purple, with the shade darkening towards the top. The glass is set in a wide and ornate circular frame engraved with a semi-geometric ring-of-roundels design that looks a bit Art Deco, although from 1834.
Rockwell vault light, with one large piece of glass [ 16 ]
A shallow square of glass, surrounded by a metal rim, set in rebar-reinforced concrete. Cutaway view.
Armoured glass in concrete
Shallow purple glass squares with deep right-angled prisms hanging from them. They are coloured purple, with the shade intensifying towards the top.
Solarized pendant prisms, showing a gradient in the solarization.
Two people in work gloves carefully transferring a heavy panel of pavement lights from a small flat-bed truck to a pile of similar panels, separated by wooden spacers. The pile is on the outside edge of the sidewalk, and pedestrians pass them on the inside. The panel has a black metal frame and round white lenses. It is about 1.5x0.5m (possibly 2x4 feet)
Restored vault lights being re-installed on Broadway in New York City
A dingy panel of pale purple squares set deeply into a shallow yellow brick arch. People with a light by an arched doorway leading to a further gallery in the background. Pipes run through the picture.
Disused sidewalk prisms lighting the abandoned Seattle Underground