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 be cut and peeled away from the bottom layer. The top layer's colour is light-safe for orthochromatic films (which are sensitive to blue and green 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 produced a yellow-(amber-)coloured masking film called Amberlith that was light-safe only for blue-sensitive emulsions. They discontinued production in late 2007.[1]