By Robert Strauss

"Come join the Revolution!" bellow three guides in Colonial garb just steps from Independence Hall. Their stentorian shouts could refer not only to the theme of the multimedia show about to begin, but to its underlying technology as well.

Five nights a week through Labor day, the production, the Lights of Liberty Show, will blend modern technology with 200-year-old history to take visitors on a six-block walking tour of Independence Historical National Park.

Lights of Liberty, which opened in 1999, calls itself the world’s first ambulatory sound-and-light show. For an hour visitors walk through and near historic structures like Ben Franklin’s House, Carpenter’s Hall, the Second Bank of the United States and Independence Hall itself, viewing Revolutionary-era scenes projected on the buildings and hearing multichannel sound through headsets.
The technological innovations were driven largely by the limitations imposed by the park site: any such show would have to avoid physical damage or modification to the 18th-century structures and the grassy areas surrounding them.

"We had to figure out how to work within the National Park Service restrictions and still give a state-of-the-art show," said Ann Meredith, the president and executive producer of Lights of Liberty, the nonprofit groups that organizes the event. "So we went into heavy research and development, and that can be dangerously uncharted waters."

Ms. Meredith and her planners first tried to follow the IMAX theater’s example and use infrared technology. After a year of trials, that approach was judged unsatisfactory because it required obtrusive infrared panels to be built all around the park. Radio-frequency technology was also considered but that, too, required digging in Independence Square and putting antennas on buildings surrounding the park. The radio receivers they experimented with also made the headsets too heavy, "creating a medieval torture device," Ms Meredith said.

In the search for "something creative that would be miniaturized, lightweight, and wireless," she said, she turned to the National Aeronautics and Space Administration, which directed her to Aeptec Microsystems and the California Institute of Technology, which came up with a digital solution.

The headsets designed by Aeptec for the Lights of Liberty Show are the equivalent of microprocessors that work with modified MP3-type audio technology. Each headset is individually programmed by a staff member for the user’s language and show time and it is checked out at a building near the park.

On a walking tour enhanced by high-tech sound and light, the nation is reborn in Philadelphia.
There is an English version of the tour for adults (with Ossie Davis as the lead character, a free black man of the era named James Forten), an interpretation for children narrated by Whoopi Goldberg and translations into Spanish, German, Japanese an Italian.

As visitors are led to the front of Franklin’s home, each headset is started by an infrared signal from the master computer at the building where the headsets were checked out. Staff members keep the visitors moving at a leisurely pace in sync with the recorded tour.

For the visual part of the production, 56 computerized projectors splash historical scenes on the facades of the buildings where the stories being told took place. The scenes were inspired by vintage paintings or drawings or derived from written sources.

"The alternative we had to do was video, but in effect then we would be creating a film, and you would feel you were in a drive-in movie of a sort," Ms. Meredith said.
Because the Park Service would not grant Lights of Liberty permission for a permanent installation, getting the show in place is an enormous nightly undertaking, Each evening it takes 50 stagehands nearly two hours to cart the projectors, their computer controls, 275 light fixtures of various sorts and more than 7,000 feet of cable into Independence Park from a garage a few blocks away.

The Park Service allowed Lights of Liberty to install a few power boxes in unobtrusive areas, but that was it.

At the end of the tour, "God Bless America" is played and a projection of part of the Declaration of Independence with John Hancock’s signature covers the second-floor windows of Independence Hall. Nearly everyone on the tour sings along, Ms. Meredith said-disconcerting, perhaps, for those walking through Independence Square without headphones, although sometimes they, too, join in.

"The technology is great," Ms. Meredith said, "and it helps people get into the spirit which is what we wanted to create here."

Ms. Meredith hopes to convince the Park Service that the projection equipment should be put in the park permanently. Her proposal is to place it on hydraulic lifts that go underground when the gear is not in use. It would probably cut the set up crew from 50 people to three or four, she said, and lower the price of the tour.

Still, "what is important is that the technology allows us, even now, to provide a three-dimensional lifelike experience that was never available before," Ms. Meredith said. "It is history alive, which is the best way to view it."