Rainbow Printing
Rainbow printing, also called iris printing or split-fountain printing, places two or more ink colors in the same inking unit of a press so they blend gradually across the sheet. The result is a continuous color transition laid down in a single pass, with no screening, no dots, and no visible boundary between colors.
| Also called | Iris printing, split-fountain printing |
| Technique | Two or more inks in one fountain, blending across the roller |
| Result | A seamless gradient with no screening |
| Requires | A specialized offset press with a divided ink fountain |
| Usually applied to | Guilloche patterns and background artwork |
| Defeats | Color copying and desktop reproduction |
| Found on | Banknotes, passports, visas, certificates |
How the gradient is made
A conventional press runs one ink per unit. Rainbow printing divides the ink fountain into sections, loads different colors into each, and lets the oscillating rollers blend them where they meet.
The blend is physical. Ink of one color mixes with ink of the next in the roller train, so the transition is genuinely continuous — a gradient of actual pigment rather than an illusion built from dots.
That distinction is the whole security value. A color copier or desktop printer reproduces every color it sees, but it does so by screening: laying down tiny dots of cyan, magenta, yellow and black in varying densities. Magnify a copied gradient and you see a dot pattern. Magnify a genuine rainbow print and you see continuous ink with no screening structure at all.
The colors match. The mechanism does not, and the mechanism is visible under magnification.
Why rainbow printing matters for identity verification
Rainbow printing is usually applied to the guilloche background rather than existing as a separate element, which means the two features are checked together and reinforce each other.
A reproduction has to defeat both simultaneously: continuous fine lines and a screening-free color transition. Copying equipment fails at both for the same underlying reason — it works by breaking a continuous image into discrete samples, and both features are defined by continuity.
For a identity document verification system working from a phone photograph, the gradient is assessable where texture is not. Smoothness of transition, absence of banding, and absence of a dot structure are all measurable from an image, provided the capture has enough resolution to see them.
Genuine rainbow print vs a color copy
| Genuine | Color copy | |
|---|---|---|
| Color transition | Continuous pigment blend | Simulated with screened dots |
| Under magnification | Solid ink, no dot structure | Visible CMYK rosette or dot pattern |
| Banding | None | Frequently visible steps |
| Fine lines within the gradient | Stay separated | Merge or break up |
| Equipment required | Divided-fountain security press | Any color copier |
What rainbow printing can’t do
It needs magnification or a good capture. At arm’s length a copied gradient looks convincing. The distinction lives in the dot structure, which requires either a loupe or a high-resolution image.
It is not tactile. Unlike intaglio, there is nothing to feel, so it contributes nothing to a quick physical check.
It says nothing about the data. A genuine gradient sits happily behind altered printed values. It attests to how the document was produced, not to what it says.
Compression destroys it. Image compression in a mobile capture pipeline smooths exactly the fine structure that distinguishes real from copied. Capture quality is the limiting factor, not the feature.
Frequently asked questions
What is split-fountain printing?
The same thing as rainbow printing. The ink fountain on the press is divided into sections holding different colors, and the rollers blend them where they meet, producing a continuous gradient in a single pass.
Why can’t a copier reproduce rainbow printing?
A copier can match the colors but not the mechanism. It builds color from screened dots, so a copied gradient shows a dot structure and often visible banding under magnification, where a genuine one shows continuous ink.
Is rainbow printing the same as optically variable ink?
No. Rainbow printing produces a fixed gradient across the surface that looks the same from every angle. Optically variable ink changes color depending on viewing angle. They are sometimes used on the same document and are often confused.
Where does rainbow printing appear on a passport?
Most often in the background artwork of the data page, applied to the guilloche pattern. It is also used on visa pages and around portraits.
