Contrary to the opinion of a few million formerly stoned yuppies, Pink Floyd was long past its prime by the time it got around to recording Dark Side of the Moon. In fact, the band pretty much peaked at its debut, The Piper at the Gates of Dawn, an album that managed to encapsulate all that was mystical, freaky and vaguely terrifying about psychedelic rock before the group grew to represent everything contrived, pretentious and boring about it. It was basically downhill from there, for one specific reason: Right after Gates of Dawn, the three other wankers severed ties with the one guy tethering them to the acid-soaked netherworld, the crazy diamond himself, singer-guitarist Syd Barrett. Since Barrett subsequently walked through the doors of perception and slammed them shut behind him — and because the still-sane members went on to release two sales chart evergreens — the early era of the Floyd is remembered about as well by the general public as the Beatles’ skiffle period.

For that reason, the stuff contained on London 1966/1967 lives up to its “rare” billing. But here’s a truth-in-advertising warning: While the DVD does feature “full length performances from the classic late ’60s Pink Floyd lineup,” those performances — two epic-length instrumentals — are mostly audio. Yes, there are clips of Barrett, Nick Mason, Roger Waters and Rick Wright playing together, but the majority of the footage on the main program is of Swinging Londoners shimmying their brains out at the height of the British youth movement. Director Peter Whitehead’s expressed goal at the time was to capture the counter-cultural revolution as it happened, not document Pink Floyd — who hadn’t even put a record out yet — in their larval stage. As a snapshot of a city bristling with frenetic energy, Tonight Let’s All Make Love In London — the title of the original documentary from which the disc’s 31 minutes are culled — is a worthy addition to an admittedly bloated canon. Really, the band is incidental to the actual film, although the nightmarish, space-tripping soundscapes it conjures up do provide an interesting juxtaposition to the images of blissful flower children enjoying their brief moment of empowerment.

But in using Pink Floyd as the sonic representatives of the so-called “psychedelic summer,” Whitehead inadvertently found the perfect metaphor for the fallout of the ‘60s. Take Barrett, seen here mainly with his head down, exploring his already fried subconscious with his guitar and an effects pedal: After leaving the group he recorded a pair of great, ragged psych-folk albums — both informed by his genuine madness — before disappearing into seclusion, where he’s remained for the past three decades. Meanwhile, his ex-bandmates took his visions, filed down the edges and got rich. That, in microcosm, is how the baby boomers are split today: Those who were serious about “mind expansion” expanded theirs right out of this plane of existence; everybody else is going to business meetings, secretly smoking joints on the weekend, and listening to The Wall. It’s debatable which fate is worse.