Crews spent the night testing the giant streams of light that symbolize the towers lost in the attack.

LOWER MANHATTAN, New York City (WABC) -- Preparations are underway for an annual Tribute in Light in Lower Manhattan honoring the victims of the 9/11 attacks, and crews spent Wednesday night testing the giant streams of light that symbolize the World Trade Center towers.

Assembled on the roof of the Battery Parking Garage south of the 9/11 Memorial, the twin beams reach up to four miles into the sky and are comprised of 88 7,000-watt xenon lightbulbs positioned into two 48-foot squares, echoing the shape and orientation of the Twin Towers.

"One of the first installations relied on automated light fixtures and the kind of things you might see at a rock and roll concert, just a bigger version," lighting designer Michael Hemmenway said. "And that ended up being the wrong answer because we couldn't get the type of precision control necessary to create what you see behind you."

It was initially located adjacent to Ground Zero, but designers soon realized that in order to get the best results, they'd have to move it -- and engineers and stagehands would need to tinker with it manually.

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"So we designed a custom installation and mounting enclosure that literally relies on the threads of a screw to tilt the lights from side to side," Hemmenway said.

The testing will continue this week ahead of the solemn reading of the victims' names Sunday morning, with the lights to then shine Sunday night into Monday.

"It is about memory and the loss of the buildings, and it's about the souls of people going up to the heavens," National September 11 Memorial & Museum CEO Alice Greenwald said. "But it also is about our resilience as a city, that we stand up tall."

Designers will check their results all over the 6-mile radius to make sure the lights do justice to what once stood there and all that happened on such an unfathomable day.

While the two beams are called the Tribute in Light, the citywide commemoration with additional buildings lit up in blue is named the Tribute in Lights.

The installation can also be viewed from a 60-mile radius around Lower Manhattan.

The lights, which are best viewed when it is completely dark, will be turned on beginning at sunset on September 11 and will fade away at dawn on September 12.

9/11 21 Years Later: Click for reflections, photos and stories

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