Photo: Mikal Schlosser

Translucent concrete

Tuesday 19 Jul 16

How optical fibres are produced

Mikal Sclosser  

A polycarbonate block surrounded by acrylic is heated in a cylinder and then drawn just like sweets from a sugar mass. The thickness of the optical fibre is regulated by the speed of the two bottom wheels pulling it, and temperature and speed must be finely balanced. The process is carried out in a so-called drawing tower at DTU Fotonik.

Concrete wall as TV screen

Mikal Schlosser 

It may sound odd that a solid concrete wall can be used as a TV screen. The Digistone project, however—headed by the Danish Technological Institute and completed in 2015—showed that the idea was feasible. If you place an LED screen behind a concrete wall with a 1 mm optical fibre grid, it is possible to display live images on the concrete façade.

The size and closeness of the optical fibres are not insignificant factors. The concrete has to be cast and obtain the right strength despite perforation by the optical fibres, and the image has to be clear and have the right colours—that it, the exact right pixel resolution must be set. This formed an integral part of the development work headed by the Danish Technological Institute’s Concrete Centre and DTU Fotonik.

The result was good, and the company Dupont Lightstone is currently marketing concrete screens around the world as a robust advertising sign with the ability to withstand vandalism and brighten up dull concrete façades.

Is it possible to combine the construction benefits of concrete with the desire to get more daylight into buildings? The answer is yes. An innovative building material with specially designed optical fibres has opened up completely new architectural possibilities.

A concrete wall offers many advantages in terms of construction, but is generally not the most exciting thing to look at. The dull façade can, however, be given a more lively impression by embedding optical fibres into the concrete during construction. A load-bearing wall can thus both function as a screen displaying live images and increase light incidence in a building.

 The story begins in 2012 when DTU—together with the Danish Technological Institute and the small innovative company Dupont Lightstone, among others—was involved in the EU-funded project Brightwall. The aim was to produce a concrete element which light could penetrate by casting optical fibres made of plastic into the concrete.

They quickly realized, however, that standard optical fibres could not be used for this purpose. The optical fibres were available in the right scale, but only in a material that did not meet the fire safety requirements of the construction, and they were also curved, making them unsuitable to be led through a concrete wall.

So researchers at DTU Fotonik started experimenting and developed a useful optical fibre made of polycarbonate. The reason this was possible is that DTU Fotonik has a so-called drawing tower capable of drawing plastic materials into long fibres.

Multi-colour polycarbonate

The new optical fibre consists of a polycarbonate core surrounded by a thin layer of acrylate, also known as Plexiglas. The two materials have—like air and water—different refractive indexes. Therefore, the light is reflected when penetrating the fibre instead of being dispersed, which is what normally happens when light falls on concrete. You catch the light, so to speak, and most of it is transported through the concrete.

Producing such a fibre is fairly simple, but it was, nevertheless, a bit of a hassle to make the fibre sufficiently translucent, says Kristian Nielsen, Development Engineer from DTU Fotonik:

“Polycarbonate can be composed in many ways, and when buying the material, you don’t know what you’re actually getting. It comes in different colours—from red to purple. A thin sheet could be quite suitable, but when stretched, it changed colour and the material became opaque. At the same time, we had to make sure that the two layers of material stuck together, so they wouldn’t delaminate when stretched”.

Optical fibres differ in terms of thicknesses and length, and can therefore be used for different purposes. The thinnest fibres (0.5-1.0 mm) can be arranged into a grid and transmit images in the same way as a TV screen. The slightly thicker optical fibres (e.g. 3 mm) can be used to transport daylight through both load-bearing and isolated concrete walls.

Letting in daylight

Until recently, you could see a solid, grey wall with illuminating dots shaped like a flower and the name Brightwall though the windows at the Danish Technological Institute. However, at nightfall when the lights in the building were switched off, it looked like a regular concrete wall—the pattern only appeared when the lights were on.

 In the same way, you could not tell the difference between Brightwall and a regular concrete wall from the inside in the dark, but during the day, light would penetrate the 5,000 tiny dots and bring daylight into the office. If it was too bright, it was possible to turn down the light by means of an electric filter made of liquid crystal.

Brightwall is the result of a project that involves casting optical fibres into concrete, which initially led to the production of concrete walls used as TV screens. These optical fibres are slightly thicker than the ones used for concrete screens—approx. 3 mm in diameter. And the project has demonstrated that it is possible to place them exactly where you want—also in industrial production.

“You take a digital file and transfer it to the concrete production equipment. It’s a new technique which we have developed in the project, and which the business partners currently have patent pending,” says Johannes Portielje Rauff Greisen, Project Manager and Architect from the Danish Technological Institute.

“We can place each optical fibre randomly and adjust the location of rebars and binders, so they don’t collide with the optical fibres. It doesn’t cost much to install, so for a couple of thousand kroner, you can have your name or house number written into the house. Or you can experiment with patterns and logos. The greatest advantage of Brightwall, however, is that you can get daylight into a building in places where it would be too bright or warm to use windows, or where you’re not interested in people looking in through the windows. And, of course, we also add thermal mass to the façade, thereby stabilizing the indoor climate and reducing the energy consumption of the house.”

The wall is designed as a sandwich construction made of concrete with foam insulation in the middle. It can be up to 30 cm thick, but still appear wafer-thin if you put your hand on one side. You only see the shadow of your hand, because the optical fibres only serve as carriers of light, not as windows—they are too small for that.

The Brightwall panel has also proved successful in fire tests thanks to the newly developed optical fibres. This means that the panel is able to meet the Danish Building Code and be used in practice. The next step is to streamline the production of optical fibres with a view to reducing the price.

Learn more

About the Brightwall project: