Wikipedia
right|thumb|200px|Rolls of lithographer's tape (these rolls are made by 3M). The roll in the back is 1/4 inch, the one in the front, 1/2 inch width. These products are often called "Rubylith" tape because Rubylith has become a generic term for coloured masking films. Rubylith is a brand of masking film, invented and trademarked by the Ulano Corporation. Today the brand has become genericized to the point that it has become synonymous with all coloured masking films.
Rubylith consists of two films sandwiched together. The bottom layer is a clear polyester backing sheet; the top layer is a translucent, red-(ruby-)coloured, sheet. The top layer can cut with a knife and peeled away from the bottom layer. The top layer's colour is light-safe for orthochromatic films (which are sensitive to blue light but insensitive to red light).
Rubylith is used in many areas of graphic design, typically to produce masks for various printing techniques. For example it is often used to mask off areas of a design when using a photoresist to produce printing plates for offset lithography or gravure. It is also frequently used during screen-printing.
Ulano also produces a yellow coloured, masking film called Amberlith, that is not light safe but easier to use for masking when not employing a photoresist.
The physical layouts of the first generations of Intel microprocessors (at least the Intel 4004, 4040, 8008, 8080, 8085, and 8086) were designed by physically cutting sheets of Rubylith to create the different required artwork for production of the integrated circuits. The finished Rubylith artwork was photo reduced up to 100 times and then step and repeated on to glass plates for production use. Today the process is done at a 1:1 scale using the output of computer-aided design systems.
Certain digital image editing programs that have masking features may use a red overlay to designate masked areas, mimicking the use of actual Rubylith film.