Prism lighting

Prism lighting uses prismatic glass to redirect daylight into interior spaces, including rooms below street level. Historic systems often incorporated glass blocks or pavement lights set into sidewalks.

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Refraction: The change in a wave’s direction as it enters a medium where its speed differs. Prismatic glass redirects sunlight by refracting it at angled surfaces.
Vault light: A glass or other transparent insert in a sidewalk or pavement that admits daylight to spaces below. Vault lights are a historic form of prism lighting installed over basement spaces.
Basement: A building level partly or wholly below the surrounding ground. Basements were a primary destination for sunlight admitted through prism-lit sidewalks.
Light pipe: A tube or reflective conduit that transports daylight from an exterior collector into an interior space. Light pipes guide daylight through a conduit, while prism lighting redirects it at glazing.
Prism (optics): A transparent optical element with nonparallel surfaces that redirects light through refraction. The prism’s facets provide the angled surfaces that steer daylight.
Luxfer prism: A prismatic glass tile patented by James Gray in the late nineteenth century to redirect daylight. Luxfer prisms became a widely recognized commercial system for channeling sidewalk light.
Pavement light: A glazed opening in a sidewalk or other paved surface that admits daylight below. Prismatic pavement lights combine a walking surface with a daylight-directing element.
Heliostat: A device that tracks the Sun and keeps reflected sunlight directed toward a fixed target. Heliostats actively track sunlight; prism lighting generally relies on fixed glass geometry.
Total internal reflection: Reflection of light back into a medium when it strikes a boundary beyond the critical angle. Some prism designs use internal reflection to redirect light efficiently.
James Gray (inventor): A nineteenth-century inventor associated with the patenting and promotion of Luxfer prismatic glass. Gray helped establish the commercial technology behind historic prism lighting.