Before there was bluescreen or greenscreen, there was yellowscreen, and it was better.
In the 1950s Petro Vlahos created an in-camera, sodium vapor process which filmed actors lit frontally with white light, against a monochrome backdrop, backlit by yellow sodium vapor lamps, using a beam splitting prism that recorded the color image and its monochromatic mask simultaneously on two reels of film. It is basically a dichroic version of Technicolor, invented by Wadsworth E. Pohl, which used prisms to split an image into three color-separated frames.
The sodium vapor process relied on this “magic prism,” which was actually a sandwich of two prisms with spectroscopically calibrated chemical films between them. Walt Disney Studios used one in a single altered Technicolor camera for almost 30 years (from Mary Poppins to Dick Tracy, basically).
In that same era, Vlahos and others kept working on chromakey technology in other chroma, and yellowscreen technology was forgotten, or lost. The process—and the prism—became something of a legend.
Earlier this year the CG researcher Paul Debevec and the VFXheads at Collision Crew dropped the clickbaitiest teaser on YouTube, which made it sound like they’d found Disney’s lost magic prism.
In their video about the sodium vapor process, Collision suggested that part of the reason it didn’t take off was because Vlahos only ever managed to make three prisms, and oh no, it’d cost “tens of thousands of dollars” to make more. Which makes little sense? Youtubers have probably fed more cash than that to a goat to make their videos, and Vlahos and Ub Iwerks won an Oscar for Mary Poppins‘ visual effects. So just money can’t be the reason yellowscreen didn’t take off. Disney kept using a prism regularly, so it’s still a mystery. But anyway, point is, they did not find the prism; and they didn’t figure out how to remake a prism.
What they and Debevec actually managed to do was recreate the process, based on the spectroscopic science, using filters and two cameras. And sure enough, it worked, and is amazing. But I’m still not over the prism.
A few days later, @RustyGellerSteadicam drops this into the video’s comments:
I was one of the last people at Disney to use the Sodium Vapor light system. It was on “Something Wicked This Way Comes” in 1982 or 3. I was an vfx AC at the studio. The prism was held under license from [the J. Arthur] Rank [Organization]. It was a hallowed object. It was kept in a steel box and it was studio policy that 2 AC’s had to be with it at all times when it was removed from the storage locker. We both carried it to the stage, then carefully inserted it into the 2-strip camera. It was never left alone on stage, we took turns leaving for lunch, the john, etc. It hadn’t been used for years but we had a series of tough matte jobs to shoot so they dusted off the old gear. I was aware I was watching a bit of history. The key was the didymium filter in the prism. That thing has to be around somewhere. Technically, Rank would still own it.
Frankly this does not make it sound like there were three. Maybe Disney only had one. But if Rank had them, they’re almost certainly lost. Rank went through three corporate acquisitions, purges, and reorgs, and theoretically exists as a part of ITV. And those suits were not saving magic prisms from the dump.
Could one be made again? Geller mentions a didymium filter, which is now a product category for filtering sodium light wavelengths. But Vlahos’ patent covers the overall process, not the specific fabrication of the prism. It only mentions possibilities–”lower index films may be of such materials as lead fluoride or silicon monoxide, and the higher index films of such materials as zinc sulfide or titanium dioxide”—but no recipes.