A similar process is used in making the stencils for silk screen printing.
The process relies on UV-sensitive inks that cure when exposed to light and stick to the plastic sheet, while the ink in the non-exposed areas is chemically removed from the sheet.
Compared to the earlier xerography process used by Disney, the lines can be controlled better and multiple copies made quickly.
The drawings are photographed on high-contrast sheet film, and these negatives are then exposed onto the cels.
This process was used on Disney's animated features, such as The Black Cauldron, The Great Mouse Detective, Oliver & Company, and The Little Mermaid.