Sodium vapor process

The sodium vapor process (occasionally referred to as yellowscreen) is a photochemical film technique for combining actors and background footage.

It originated in the British film industry in the late 1950s and was used extensively by Walt Disney Productions in the 1960s and 1970s as an alternative to the more common bluescreen process.

This increased accuracy ultimately renders the matte "lines" almost invisible, though as with bluescreen, its use may be signaled by hard separation or mismatched coloration and contrast between elements, or in this case, a telltale white/yellow fringe.

At the time of its use, the sodium process yielded cleaner results than did bluescreen, which was subject to noticeable color spill (a blue tint around the edges of the matte).

The process was used in the 1970s for scenes in Island at the Top of the World, Gus, The Apple Dumpling Gang, Bedknobs and Broomsticks, Freaky Friday, Escape to Witch Mountain, Pete's Dragon, and The Black Hole.

Geller, who was part of the camera and electrical team, also states that the key to the process was a didymium filter inside the prism and believes that Rank should still have it.

On April 7, 2024, production studio Corridor Digital published a video showcasing a modern implementation of the sodium vapor process using a custom filter setup developed by Paul Debevec.