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New for February 2021: Rubylithography was and still is a means of creating a photo mask. Back in the 1960s and 1970s, it was the state-of-the-art for etching stripline and microstrip circuits. "rubylith" is a registered trademark of the Ulano Corporation, first registered in 1961. The magic of rubylithography is that a film of red tape is bonded to a clear sheet of polyester film. The red color was chosen to block blue or green light in a darkroom. By cutting and peeling away areas of the red layer, a photomask was formed that could serve to expose photoresists in contact lithography, or photo-emulsions when reducing the size of a mask for improved resolution.
In the early days, a technician would have to cut the film by hand with an X-Acto knife. Later, machines were developed that could do the cutting from computer commands, and the technician would only have to peel off the ruby film, thus rubylith entered the realm of computer-aided design.
Here you can see the tedious process of cutting a ruby, by hand:
By 1982, software was developed to drive a Gerber photo-plotter, spelling the end of the more labor-intensive rubylith technique. However, rubylith lives on in the world of graphics design, you can still buy a sheet made by the original manufacturer, Ulano.
Learn a little more about rubylith on Wikipedia.
In the 1980s. rubylith fell out of favor, with the advent of Gerber photoplotters and software to drive them. Learn about one such design tool, "BAA", here.