SF-03
Split-duct (rainbow) printing
Two or more inks laid down in one pass, blending into each other across the sheet without a join.
Split-duct (rainbow) printing ×4
Specification
- House code
- SF-03
- Method
- Split ink duct, single pass
- Apparent at
- ×4
- Deters
- Reproduction by a process that separates the image into fixed inks.
- The recipient’s check
- Follow a single continuous line across the sheet. Its colour changes gradually along its length, with no seam and no join anywhere.
The ink duct on a press is a long trough running the width of the sheet. Divide it with thin blades, put a different colour in each compartment, and the rollers will carry all of them at once — mixing only where the compartments meet. A line crossing the sheet therefore changes colour continuously along its length, and it does so in a single impression.
The effect is called a rainbow or iris, and it is deceptively hard to imitate. A conventional four-colour reproduction reconstructs colour from separated inks laid in dots; the gradient it produces is built rather than blended, and under any magnification the difference is plain.
In practice
Split-duct work is usually combined with a guilloche, so the colour transition happens along curves rather than across a flat panel. It is one of the few security features that is also unambiguously decorative, which makes it popular on documents that people are meant to want to keep.