The largest stadium in the United States, filled with tens of thousands of rowdy American football fans, was bathed in blue light. People in the stands held up their phones, creating a sea of ​​star-like spots in the crowd.

“This is our team! This is Michigan!” A video played on the giant screen as cheers rang out.

The atmosphere was enhanced by a new video entertainment system, which on September 16 made his debut at Michigan Stadium. Colorful sequences of flashing, sweeping lights celebrate touch or accompany music.

The University of Michigan’s team colors are yellow or “maize” and blue. The light show was designed to match.

“It 100% affects the in-stadium experience,” says Jake Stocker, director of game presentation and fan experience at the University of Michigan.

“Another fun element of coming to a football game that you don’t have sitting at home on the sofa.”

Like most stadiums, Michigan Stadium’s light show features light-emitting diodes (LEDs).

But not so long ago, blue LEDs powerful enough to light a stadium this large – the third largest in the world – seemed incredibly advanced. Bright LEDs that emit blue light weren’t invented until the 1990s. Later, the scientists who invented this technology received the Nobel Prize.

Scientists say that LEDs could be even cheaper and more efficient than today. They can replace everything from outdoor lighting to virtual reality headsets.

At Michigan Stadium, the various colors displayed are produced by entertainment lighting systems with red (R), green (G) and blue (B) LED arrays, or fixtures, says Brad Schlesselman, senior science engineer at supplier Musco Lighting. technology. RGB systems can actually produce a huge range of colors simply by mixing red, green and blue in different intensities.

“We’re getting even down to the high school level where there’s a demand for the color change and theater stuff that we see in Michigan,” adds Mr. Schlesselman.

In addition, cities across the U.S. are installing LED lighting at local landmarks, including water towers, to illuminate the structures in special colors for certain events or occasions. For example, pink is breast cancer awareness month, which is this month, October.

Perhaps the most impressive use of LEDs is in the Las Vegas Sphere, which opened last month. Millions of LEDs can transform the outside into just about any pattern or image you can imagine, while the inside illuminates massive displays.

But in the 1970s and 1980s, LEDs were routinely dismissed as ineffective. “There’s no way this little dingy toy light bulb is going to do anything useful, that was the attitude at the time,” says Paul Scheidt, senior manager of product marketing at Cree LED, a major fixture maker. These expensive and weakly emitting light sources were good for a tiny red indicator light or an infrared TV remote control, but for little else.

That changed when engineers were able to make LEDs that emitted many more photons, or light, than before. LEDs emit light when electrons—negatively charged particles—drop from a higher energy state to a lower one in the device. This process releases energy in the form of light. By using different materials, you can adjust the size of the drop (known as band gap) and the wavelength or color of the emitted light.

Blue was particularly difficult because the main material required for this shade, gallium nitride, was difficult to produce without defects. However, blue is a powerful, very energetic color (with a high bandwidth), so blue LEDs can be used as a base for all other colors in some RGB OLED TV screens, for example red and green would simply be illuminated. , blue LEDs.

And yet, a brand new LED technology is waiting in the wings, as researchers say the technology could be even more efficient.

Dan Congreve and colleagues at Stanford University are working on light-emitting diodes made from perovskite crystals, a material often used in solar cells. Perovskites are cheap and easy to make. Dr. Congreve says they can be “tuned” to the color you want and can even be mixed with a solution and then painted onto surfaces as light-emitting layers.

However, it is difficult to get perovskite LEDs to remain stable. They keep breaking.

“We turn them over and measure them and they die quite quickly,” says Mr Congreve. He adds that he hopes that this problem can be overcome. He and his colleagues have already improved the stability since the first experiments.

If they can solve such problems, perovskite LEDs could be used in a variety of devices, says John Buckeridge, a materials physicist at University College London.

Separately, researchers in Japan recently came up with a blue LED that can be powered by a single AA battery that supplies just 1.47 volts. You usually need at least 4 volts. “It’s cool, like an engineering feat,” says Dr. Congreve, who himself did not take part in the work.

The system uses clever physics to increase photon production. In traditional LEDs, when power is applied, the internal materials reach excited states that do not actually emit light three-quarters of the time. The Japanese team was able to induce these excited states to combine and produce light, while initially requiring less energy. They published their works September. in the newspaper.