Prism Lighting – The Art Of Steering Daylight

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Prism Lighting – The Art Of Steering Daylight

The incandescent light bulb was one of the first early applications of electricity, and it’s hard to underestimate its importance. But before the electric light, people didn’t live in darkness — they thought of ways to redirect sunlight to brighten up interior spaces. This was made possible through the understanding of the basic principles of optics and the work of skilled glassmakers who constructed prism tiles, deck prisms, and vault lights. These century-old techniques are still being applied today for the diffusion of LEDs or for increasing the brightness of LCD displays.

Semantics First!

People in optics are a bit sloppy when it comes to the definition of a prism. While many of them are certainly not geometric prisms, Wikipedia defines it as a transparent optical element with flat, polished surfaces of which at least one is angled. As can be seen in the pictures below some of the prisms here do not even stick to this definition. Browsing the catalog of your favorite optics supplier you will find a large variety of prisms used to reflect, invert, rotate, disperse, steer, and collimate light. It is important to point out that we are not so much interested in dispersive prisms that split a beam of white light into its spectrum of colors, although they make great album covers. The important property of prisms in this article is their ability to redirect light through refraction and reflection.

A Safe Way to Bring Light Under Deck

A collection of deck lights used to direct sunlight below deck in ships. Credit: glassian.org

One of the most important uses of prism lighting was on board ships. Open flames could have disastrous consequences aboard a wooden ship, so deck prisms were installed as a means to direct sunlight into the areas below decks. One of the first patents for deck lights “THE GREAT AND DURABLE INCREASE OF LIGHT BY EXTRAORDINARY GLASSES AND LAMPS” was filed by Edward Wyndus as early as 1684. Deck prisms had typical sizes of 10 to 15 centimeters. The flat top was installed flush with the deck and the sunlight was refracted and directed downward from the prism point. Because of the reversibility of light paths (“If I can see you, you can see me”) deck prisms also helped to spot fires under deck.

Making Shopping Easier for the Ladies

The shopping experience in the 19th century was much improved after the invention of vault lights. Credit: glassian.org

In the 19th century, the idea of prism lighting was adapted to vault lights that directed sunlight into sidewalk vaults and basements. Compared to open grates, these not only provided protection from rain but were also easier to walk on. The latter was apparently considered a great advantage for shop owners helping them to bring women closer to their store windows despite their impractical footwear.

Prism shaped pavement light redirecting sunlight through total internal reflection. Credit: glassian.org

In the beginning, vault covers used round plano-convex lenses or even just flat glasses. The prism shape that was used in ship decks was adapted for vault lights only later by Edward Hayward in 1871. His glass prisms not only let light pass through but also redirected it sideways into the room. Hayward’s prisms were based on total internal reflection which occurs when light tries to exit the glass above a critical angle.

From Prism Tiles to Gas Cylinders

Eventually, prism glass was also applied to vertical windows with much commercial success. The biggest player in the game was the Luxfer Prism Company which started to sell prism tiles in 1897 based on an earlier patent by James G. Pennycuick. The 4-inch wide square Luxfer tiles were typically installed above storefront windows.

Nice tiles on a storefront. Credit: Luxfer Prism Glass Tile Collector.

The inner surface of the tile was covered with horizontal prisms that redirect light deeper into the room than the sunlight would otherwise reach on its own.

Although their commercial success was brought to a halt with the availability of cheap electric lighting, the tiles can still be seen in many small towns in the US. The Luxfer company survived by changing its business to metal products and is now the world’s biggest manufacturer of high-pressure aluminum cylinders for gas storage. Luxfer tiles that often contain ornamental patterns designed by lead architects of the time are now a collector’s item. Frank Lloyd Wright was a high-profile booster.

LED Diffusers and LCD Screens

Although nowadays electricity is cheap and LEDs are super-efficient compared to incandescent lamps, the more efficient use of sunlight for interior lighting is undoubtedly a worthy goal, if not just for the superior color rendition. The modern version of prism tiles are daylight redirecting window films. The thin plastic films include a microstructured saw-tooth pattern that refracts light upwards to the ceiling from where it is reflected and reaches deeper into the building — the thin plastic equivalent of Luxfer tiles. According to 3M the film can save up to 52% on lighting energy costs.

Typical configuration of an LCD screen and working principle of the prism film: ray I is totally reflected and recycled by diffuse reflection, ray II is converged by the prism refraction, and ray III is recycled by other prisms. Credit: Zong Qin

You might already be familiar with prism films in case you were ever looking for ways to diffuse LEDs. Micro prism sheets made from polystyrene or polycarbonate are a common solution to homogenize the light of an LED array. In addition, if you ever tore down an LCD screen you will have noticed several plastic sheets sandwiched underneath the glass. These also contain prism films to enhance the brightness of the backlight screen.

As shown in the picture the prism film will converge light towards the viewer thereby increasing the on-axis brightness while at the same time limiting the viewing angle. Often two of these films are stacked with 90 degrees rotation to converge light in both the horizontal and vertical planes.

These films are also starting to show up in high-end LED lighting applications, and it’s probably only a matter of time, and price, before they become ubiquitous. The incandescent bulb killed the prism, the LED is killing the incandescent, and the prism is getting rediscovered. What’s old is new again!

Prisms are somehow the swiss army knife of optics, a multipurpose tool when it comes to steering light. Even if manufacturing techniques, materials, shapes, and dimensions have changed over the centuries the ability to redirect light through simply shaped transparent bodies still finds a lot of useful applications.