For most of movie history, set backdrops had been opaque (non-transparent) sheets of fabric stitched together to form larger, set-sized sheets.
In 1950, photographer M.B. Paul was profiled creating transparent set backdrops from actual photographs. Because they were transparent, they could be lit from the back as well as the front.
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The backdrop below may be from a movie called “Beloved Over All,” later retitled, “Our Very Own.”
Source: Popular Mechanics January 1950