Trains typically aren't the best place to use mobile phones, with some cars offering reception similar to that of an elevator shaft. To help remedy this, a team of researchers at the École Polytechnique Fédérale de Lausanne (EPFL) have developed train windows made of a laser-treated glass that offer the same insulating properties as regular passenger cars, but don't interfere with mobile phone reception.

When it comes to mobile phones and passenger trains, there's something of an engineering paradox at work. According to EPFL, a third of the energy used by a train is to heat or cool passenger cars with some three percent of this escaping through the windows. To help insulate the cars, the double-glazed windows are often equipped with an ultra-thin metal coating that lets light through, but reflects heat away, so the car interior maintains the desired temperature using minimal energy. These coated, doubled-glazed windows have four times the energy efficiency of regular, untreated windows.

Unfortunately, the coating not only reflects heat, it also blocks telecommunication signals. Since railway cars are made out of metal, the metal coating in the windows effectively turns them into Faraday cages, earthed metal screens that enclose a space to cut it off from electromagnetic radiation. The upshot is that passenger cars have lousy phone reception.

The standard solution to this problem is to equip the cars with electronic boosters and repeaters, but this means using more energy as well as more money on installation and maintenance. EPFL's alternative is to alter the metal coating to make the windows more selective as to what exactly can pass through.

The design makes use of the fact that electromagnetic waves of mobile phones, which are centimetric in size, are different to heat waves (which are micrometric in size), which are different again to light waves (nanometric in size). The team used a high-precision laser to remove about 2.5 percent of the metal coating and inscribe a special pattern into the surface that is invisible to the naked eye, but lets the electromagnetic waves and light waves through, but still blocks the heat waves and therefore doesn't affect insulation.

According to EPFL, the prototypes have proven successful and Bern–Lötschberg–Simplon railway (BLS) in Switzerland have installed the windows on a NINA-type self-propelled regional train for field testing. Based on the results, the new windows are as transparent to phone reception as plain glass and BLS plans to expand the installation to most of 36 NINA trains beginning in September.

In addition, EPFL sees the new windows having applications beyond trains.

"Some glass buildings also act like Faraday cages," says Andreas Schüler, from EPFL's Nanotechnology for Solar Energy Conversion Group. "And as the internet of things continues to grow, there is a real interest in improving the properties of building materials that allow mobile signals through. More broadly, by making materials more frequency-selective, we could, for example, imagine a building that lets electromagnetic waves through but blocks Wi-Fi waves, thus enhancing corporate security."

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