Rubylith is a trademark name. It’s a masking film invented by Ulano Corporation, it’s mainly used in the graphic arts industry (silk screen, lithography etc…), where orthochromatic materials are used (sensitive to light apart from the red spectrum side).
a piece of Rubylith masking film in front of a window
In photography we usually don’t use this inactinic masking property in order to create an image. But the Rubylith masking film happens to be a very handy tool to light a darkroom with inactinic natural light.
window blacked out or rather “reded out” by a piece Rubylith film
One can easily create a darkroom out of any room by blacking out all openings apart one that will be “reded out” with such a translucent masking film, in order to obtain an inactinic natural light source for the darkroom.
It’s also used in the making of “darkboxes”, which are nothing more than small foldable and portable darkrooms. These are mainly used in old/historical photographic processes at least those requiring immediate development procedures needed to be carried out on location.
Needless to mention what’s keeping me busy these days…