How Window Washers Almost Sunk Salesforce Tower’s Interactive Light Sculpture

Electrical engineer and artist Jim Campbell explains the technology behind the highest public art installation in the world, and the challenge of avoiding window washers

4 min read

Photograph of Jim Campbell's LED art installation on top of Salesforce tower.
Photo: Jim Campbell/Boston Properties

On the top of San Francisco’s 1,070-foot Salesforce Tower, the new skyscraper dominating the cityscape, the show starts at twilight. The top 130 feet of the tower lights up in yellow, and then dancers move across it. On a clear night, the show is visible for 30 miles. It’s the highest public art installation in the world right now.

This permanent installation, by electrical engineer-turned-artist Jim Campbell, is different from the bands of colored lights atop many skyscrapers. For one, Campbell explains, he’s showing imagery—the ballerina is only the first of what will be a broad palette of images.

For another, the LEDs that create the images shine in toward the building, not out. That might not seem like a big deal, but it’s the difference between images made up of visible dots, like on a Jumbotron, and a wash of color. And it took some serious engineering to make the latter happen.

First, a few numbers. The installation involves 11,136 light fixtures. Each of those fixtures (“hats”) contains two sets of LEDs with a red, a green, a blue, and a white LED in each set.

“I tend to run my LEDs at 10 percent their rated power so they last 10 times as long; putting two sets together lets me do that,” Campbell says.

There’s a reason that most LED installations face the LEDs out—that’s easy, you just stick them on a surface. What Campbell proposed instead was really hard.

“We had designed these rods sticking off the side of the building to hold the LED hats facing in,” Campbell says. And then “the window washers told us they would end up breaking some off every day. It took us six to 12 months to figure out what to do.”

Campbell and his team first considered making the rods too strong to break, but realized that if they did that, a blow to a rod would end up bending the aluminum shell of the building, and that would be even worse.

The group finally settled on a spring that would bend out of the way when bumped by window washers and then bounce back to its original position.

The perforations in the aluminum skin that covers the top six floors of the building caused another issue—some of the light projected by the LEDs would shine through to the other side. They redesigned the LED holders to act as masks to block light coming through the perforations in the building from the LEDs on the opposite side.

Then came the electronics. Campbell explains his design:

“Each set of 12 lights is controlled by a driver chassis. Each driver chassis has an 8-bit PIC microcontroller.

“There are 130 aluminum panels on the building. Each panel has an average of 96 pixels [LED packages], and eight driver chassis—so a total of 1,000 drivers for the 11,136 lights. These are daisy chained together, using a single cable for 300-kilohertz communications and power. The driver chassis and light fixtures were installed onto the panels during their manufacture in Tijuana.

The cables that come out of these panels run to 2-by-3-foot enclosures inside the building; there are 32 of these, each connecting to five or six panels. These have 1,000-watt power supplies, and a data board running a Xilinx FPGA, which converts the data stream from the computer to the protocol of the panel chassis. The data boards are daisy-chained in two groups along catwalks on two levels. Below, on the 62nd floor, a central PC-based computer runs Ubuntu Linux, sending instructions to a communications control system that splits the data and sends it at 11 Mb to the 32 enclosures using a custom communications protocol.”

All of the data and power signals were, of course, carefully calculated, but until the system was turned on for the first time in April, Campbell worried that he had “made too many power compromises and didn’t have enough brightness,” that it wouldn’t be bright enough to be visible. “I lost a lot of sleep over that,” he says, but it runs just fine at his planned 10 percent of the LED’s maximum power threshold, and his system allows him to dial it up to 20 percent if necessary.

So far, the dancer and other imagery are prerecorded. Live imagery is yet to come. Says Campbell:

“We will have six to 12 cameras around the city capturing images of the day—a view of the ocean from Cliff House, a view of the sky, street scenes, plaza scenes, some trees at Golden Gate Park. We will capture images throughout the day, sending them to Amazon’s cloud, and run some algorithms designed to identify visual interesting-ness. For example, at its simplest, when we look at the sky, if it’s all blue, it’s boring; if it’s all white, it’s boring. If it has white and blue it is likely to be interesting. We’ll choose the best half hour of the day at each camera, based on movement and color, to display.”

Eventually, the imagery will also include a clock, with the representations of the hours choreographed by Alonzo King’s Lines Ballet.

And finally, when the main display shuts down late at night, another system designed by Campbell will kick in. In this static display, a set of 36 white LEDs will create a three-dimensional constellation of lights that will look like stars. “It’s quieter, it has a random aspect to it,” he says. “When we turn off the main system late at night, it won’t be a black void up there.”

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