Now, the glow extends west to a street-facing installation of vintage neon at the Fabric Workshop and Museum's New Temporary Contemporary annex at 1222 Arch St.
Think of this newest exhibit as a classic winter light display, only made from found objects, each of which represents a fragment of Philadelphia's commercial history.
And rather than hewing to a holiday theme, the signs appear as a series of non sequiturs: A Pontiac Indian leans away from a crowned Pat's King of Steaks sign, as the name "Delilah's" dances enigmatically below. Elsewhere, a neon clock ticks off "Months to Pay," while a ray gun takes aim. An ad for a hair-replacement center depicts a man who is alternately the "before" and the "after," as his neon toupee blinks on and off. And presiding over it all is the Pep Boys trinity - Manny, Moe and Jack.
The signs are culled from Davidson's massive collection, which he calls the Neon Museum of Philadelphia, though no such institution yet exists.
The installation might seem like a strange fit for the Fabric Workshop and Museum (FWM) - but it's not, insists Stephanie Alison Greene, head of exhibitions and publications.
"At the founding of the museum, in 1977, there was a very strong emphasis on hand-screen-printing on fabric. But the museum has become more supportive of hand fabrication in a variety of mediums," she said.
It makes even more sense given that the Fabric Workshop worked with VSBA Architects and Planners to renovate its 118-year-old building - and that VSBA was founded by architects Robert Venturi and Denise Scott Brown. The couple are longtime advocates of what's sometimes called "commercial vernacular" architecture (think: neon signs, diners, and the aesthetics of urban sprawl).
The VSBA team wanted to contrast the museum's classic facade with neon, and called on Davidson to craft an "FWM" sign for the transom windows.
Then they added horizontal neon stripes to visually connect the main museum with the annex.
Finally, in homage to the block's rich neon heritage (including an adult video store that once hummed with lurid signage), Fabric Workshop founder Marion Boulton ("Kippy") Stroud invited Davidson to hang a dozen pieces from his collection for the current installation.
Davidson only wishes he could get more neon connoisseurs to see the light.
He has more than 100 signs in his ever-growing collection of salvaged, purchased, and donated items. He said he had been trying unsuccessfully for 20 years to find them a permanent home. Talks, for example, on installing 50 signs along the Convention Center's 12th Street underpass haven't come to anything as yet.
"A lot of cities struggle to preserve one or two signs," he said. "I've got a hundred, and I'm struggling to find a place to put them up. Each one is a piece of historic Philadelphia. It's folk art, and it's history."
But until he finds an exhibition space, this corner of Center City remains an illuminated beacon.
The neon is certainly appreciated at the Center for Architecture, where an animated faucet drips in the window and a McGillin's Olde Ale House sign warms the meeting space.
Philly architects, following the lead of Venturi and his colleagues, have embraced the art form.
"There's certainly a direct link between architecture - especially architecture in the early to mid-20th century - and the automobile, and neon," said the center's coordinator, David Bender.
He said the lights are pieces of built history, each with its own story. One sign, for Sherwin Williams Paint, shows SWP-label red paint coating a globe, with the words "Cover the Earth." In the '50s, rumors swirled that such signs were Socialist Workers Party propaganda.
The exhibits are also a celebration of Philly's strong tradition of neon, led by such luminary luminaries as Joe Feldman of Ajax signs, the artist behind that quirky hair-replacement ad.
But the trade is fast disappearing, said Davidson, who estimated that at least half the neon shops in the city have closed in the last decade, victims of that scourge of LEDs.
(It's ironic, given that LEDs would be the engine for a very different vision of a Market East sign corridor, outlined in 2011 legislation creating a special electric-billboard district on East Market Street.)
Many of Philly's greatest neon signs have vanished, too - as will the Fabric Workshop exhibition, at the end of March.
Davidson described one storied Feldman masterpiece, for a place called Red Robin Tavern, that supposedly showed an animated bird sipping a martini.
But, he admitted, "I don't really know if there was such a sign. Once they're gone, they become the substance of legend."