Masking sheets are used to prevent light from exposing the non-image areas of a printing plate or film. This ensures that the non-image area remains ink free. Masking sheets are flat sheets of yellow or orange opaque masking material. They are also called knockout film. Masking sheets are printing-plate-sized, goldenrod-yellow paper with a printed grid and are used to assemble and position negatives for exposing plates.
Masking sheets prevent actinic light from reaching the plate: Actinic light is the chemically active light that exposes an emulsion. They prevent light from reaching selected areas of film or a printing plate and ensure that the non-image area remains ink free.