Visitors to Philadelphia's historic district should expect to encounter the Liberty Bell unchanged by time. But it's just smart business to renew other tourist attractions around Independence Mall from time to time.

So it's good to see an ambitious update in the works for one of the signature offerings in and around Independence National Historical Park - the Lights of Liberty sound-and-light show depicting the nation's founding.

When the $10 million project is completed next spring, visitors will be treated to a new digital, 3-D version of the roving outdoor performance, with images projected on historic buildings and a soundtrack over headsets.

In addition, Lights of Liberty, run by the Historic Philadelphia Inc. nonprofit, this summer will add a year-round indoor movie on the struggle for independence shown in 3-D and 360-degree panorama. The Liberty 360 attraction debuts over the July Fourth weekend at the Public Ledger Building, Sixth and Chestnut Streets.

Announcing a $5 million grant toward the cost of the project on Monday, Gov. Rendell continued in his role as founding patron of Lights of Liberty, which launched in 1999, while he was mayor.

Two other bricks-and-mortar attractions are slated for the historic district: the planned American Revolution Center, and the President's House exhibit now being built at Sixth and Market Streets.

But it was the sound-and-light show - originally championed by the late city planning giant Edmund Bacon - that offered the first tangible sign of rebirth around Independence Mall in the late '90s. The makeover of the mall itself followed, giving the city's growing tourism industry yet another boost.

Just as Historic Philadelphia in recent years transformed Franklin Square into a playdate destination with its playground, carousel, mini-golf, and snack bar, its updating of Lights of Liberty should give first-time and seasoned visitors another good reason to tarry a while longer in the city's historic district.