Rubylith

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).