The coloured wristbands that turn stadiums into a spectacle of flashing lights are the real stars of Coldplay's shows, according to some.
But fans have been spooked after the glow-in-the-dark devices suddenly began flashing of their own accord, days after the event. Producers of Xylobands, given free to two million gig-goers on Coldplay's tour, have moved to assure fans, who took wristbands home, that they have no sinister powers.
The Xylobands use ultra-low-power microcontrollers to receive and process wireless signals that trigger the wristband's LEDs to light up in sync with the music and stage lights show. They cannot be reactivated outside of the show – in theory. But a concert-goer named Denise wrote on the band's web forum:"My xyloband woke me up in the middle of the night and it's still glowing."
Another wrote: "Really weird, my dad's white xyloband just started flashing again." A Belgian fan added: "Mine is alive! It started flickering about an hour ago." A respondent said that the only way to stop the glowing was by jamming a pencil clip into the unit.
The band has invested in RB Concepts, the Devon firm which developed the devices. Although Martin said the cost of producing two million bands for the tour was excessive, the company has received hundreds of requests to use them, from weddings to US sports franchises. Clive Banks, who is the co-founder of RB Concepts, said that the "afterglow" is no cause for concern. "There's no mind control or tracking, they are just for fun," he said.
The devices may be activated by their proximity to a Coldplay concert. "Our record transmission is 2km," Mr Banks said. "It depends on buildings but they have quite a range in a straight line. There might be some faulty units going off."
Fans speculated that the bands had become activated after locking into a particular radio frequency. "They operate on different frequencies in different jurisdictions so we have to get permission to transmit," Mr Banks said. "We have to meet the health and safety requirements. We don't want to interfere with any emergency services."
The Xylobands were designed by Jason Regler of RB Concepts, who approached the band after seeing them play at Glastonbury. There is some hope for fans disturbed by their glowing bands. "They have a battery and a finite life span," Mr Banks said.
Bankable stars: rock's investors
A $90m (£58m) investment in Facebook established an initial value of $1.9bn for the U2 singer's Elevation fund when the social network floated. But Bono played down reports that he was richer than Sir Paul McCartney.
An investor in Turntable.fm, a music-sharing network, Gaga has used her earnings to raise $4m for Backplane, a platform that connects music and sports stars with their fans across various social networks.
Shawn Carter's empire now includes a multimillion-dollar investment in Powermat Technologies, an Israeli wireless recharging company. Jay-Z will be the face of the Duracell Powermat JV, "the company to bring on the revolution".
An early digital pioneer, Gabriel co-founded On Demand Distribution, a precursor to iTunes and made $10m from its 2004 sale. Then he invested in We7, an ad-supported music streaming service.
After playing Facebook investor Sean Parker in The Social Network, Timberlake took a stake in a $35m deal to buy the ailing MySpace from News Corp in 2011 and promised to play a "major role" in its new direction.
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