Curious. I rent a darkroom space that has been in use since the 80's. I was cleaning off a shelf and found an envelope marked Ruby Lith Filters. They are round and are about 3 inches in diameter, was wondering what is their purpose. I have guesses but figured someone could fill me in.
they block light, used in alternative processes, and years ago many enlargers had a red filter that would swing out away from the light source and back when one wanted to block the light but see the image.
Frankly that use was always a mystery to m e, as it is very hard to see.