Hey, here we go... I've serendipitously found a reference stating that the sodium-matte process
was used in
The Black Hole after all. It also confirms what I recall reading about the uniqueness of the equipment that was used for it. It's a page listing the "Top 50 movie special effects shots," and the entry is way down at #6,
The Birds:
http://www.denofgeek.com/movies/177951/top_50_movie_special_effects_shots.html
An extraordinarily complex piece of compositing (shown in the clip with inserts removed) which demonstrates Hitchcock's continuing urge to push the lackadaisical state of the art. The flapping of the birds' wings caused too much fringing for conventional blue-screen work to be utilised, and Hitchcock was forced to turn to the 'yellow screen' or 'sodium vapour process'. Only Walt Disney studios have ever been equipped for this process, and indeed only one camera has ever been rigged for it. SVP involves filming the subject against a screen lit with powerful sodium vapour lights utilising a very narrow spectrum of light. Unlike most compositing processes, SVP actually shoots two separate elements of the footage simultaneously using a beam-splitter; one reel exposed is regular photographic stock and the other an emulsion sensitive only to the sodium vapour wavelength. Very precise mattes are obtained from the latter, allowing the subject to be pulled out of the background and combined with any other in a later run through an optical printer. The fringing or 'matte line' effects are negligible compared to blue-screen work, but the very precise conditions under which the footage must be shot mitigated against its wide usage. Disney, to whom many shots in The Birds was farmed out, used the process in numerous films including Mary Poppins (1964), Freaky Friday (1976)and The Black Hole (1979).
Offhand, I'd say that the "window" shots I was so impressed by, with the ship, black hole, and starscape visible through the windows with the actors moving in front of them, were most likely done with sodium mattes, explaining the lack of matte lines. Come to think of it, I suppose the reason the Art Cruikshank interview only mentions bluescreens is because he was discussing the miniature effects rather than on-set composites such as the window shots. Given what I've seen and read about the sodium vapor process, it stands to reason that it would've been used for those kinds of shots, with actors moving in front of the "yellow screen," as in
Mary Poppins.
So that's reassuring. I was sure when I saw those shots that they couldn't be bluescreen, because they were just too free of matte lines. I could've sworn I'd never seen bluescreen work that looked quite like that. I was afraid I was losing my eye for detail.
It's a shame it wasn't feasible to use sodium mattes more often. What I recall is that the beam-splitter prism just sort of happened by accident to be able to split off that particular wavelength, and nobody ever figured out how to make another one that could do the same. That's both amazingly cool and rather sad.
And my discovering this is more of that weird synchronicity. For years, I never saw or read anything about the sodium vapor process and wasn't even sure I remembered its existence. Then, a week after I started this thread wondering about it, I happened across a TV documentary that talked about it, and now a week after that, I happened across a website that talked about it. It's weird the way the universe seems to work sometimes.