Hotel group Marriott is set to project ads into the night sky using a new technology — but the public won’t be directly looking at them.

Using a technique called persistence of vision, the ads will be seen for between a tenth and a quarter of a second, but only when the viewer moves his or her head.

Marriott’s ads will show the Eiffel Tower and the Colosseum in Rome projected across London’s River Thames for three days from Thursday afternoon, as part of its Travel Brilliantly campaign, it said in an emailed statement.

“While viewers are given the impression that the dramatic images are projected into the night sky, they are in fact ‘printed’ temporarily and safely onto the viewer’s eye using the persistence of vision effect,” the statement added.

Outdoor ad agency Kinetic worked with technology company Lightvert on the campaign and Marriott said it’s the first time the tech — called ECHO — has been used commercially.

The ads will be projected by a thin vertical strip of light around 30mm wide, with imagery able to appear up to 35 meters into the sky, according to Lightvert founder Daniel Siden who spoke to CNBC by phone. The technology uses the concept of “low attention processing,” a model identified by academic Robert Heath in 2000 showing that advertising messages can work even if someone is not paying active attention to them.

“Lightvert’s ECHO technology is a real glimpse into the future, bringing something very special to audiences on the move,” Rosh Singh, Director of Digital Innovation at Kinetic, said in the statement.

Lightvert has raised £982,000 ($1.3 million) in seed funding and Siden told CNBC that it plans to launch in China and other parts of Western Europe. It is also working with U.K.-based art college Central St Martin’s on creative applications of the technology.