A Colour Change With No Edge
Hold a licence at an angle to the light and look across the background artwork. On many designs the colour shifts gradually from one hue to another, with no join, no banding, and no dot pattern where the two meet. That smooth blend is rainbow printing, and it is a security feature hiding in plain sight as decoration.
This guide explains how the split fountain technique produces that blend, why it is difficult to reproduce, and how it works together with the line work it usually sits on. It extends the background artwork discussed in fine line and guilloche patterns on IDs.
How Split Fountain Works
On a press, ink is fed to the plate from a trough called a fountain. Normally one fountain carries one colour. In split fountain printing, the trough is divided and two or more inks are loaded side by side in the same unit. As the press runs, the rollers carry those inks along and they blend gradually where they meet.
The result is a continuous transition laid down in a single pass. That single pass is the important detail: the blend happens mechanically, in the ink film itself, before it ever reaches the card. There is no point where one colour stops and another starts, because at no stage were they separate on the plate.
Why It Resists Reproduction
A desktop printer cannot make this blend, and the reason is structural rather than a question of quality. A desktop printer builds every colour from separate inks laid as dots. To imitate a smooth blend it must vary the dot mix gradually across the area, which produces a halftone pattern rather than a continuous film. Under magnification the difference is stark: the genuine blend is smooth ink, the imitation is a field of dots changing ratio.
Two further problems follow. The dot pattern interacts with a scanner's sampling grid, which is the same effect exploited deliberately in anti-copy features and void pantographs. And any fine line work printed in the blended ink loses its crispness, because the lines are now being reconstructed from dots rather than printed as solid strokes.
Where to Look for It
Rainbow printing is nearly always a background element rather than a foreground one. Typical placements include:
- Across the full width of the card behind the data fields.
- Within a guilloche rosette or a repeating security pattern.
- Along one edge as a band that shifts hue from end to end.
- Behind the ghost portrait, so the blend runs through the secondary image.
Because it is background, people rarely register it consciously, which is precisely why it is effective. An inspector who knows a particular state's card knows what the blend should look like, and a reproduction that gets it slightly wrong reads as off before anyone can articulate why.
How It Differs From Colour Shifting Ink
These get confused, and the difference is worth being clear about. Rainbow printing is a spatial effect: the colour changes as you move across the card, and it looks the same from any viewing angle. Optically variable ink is an angular effect: the colour changes as you tilt the card, and it looks the same at every point.
One is a property of position, the other a property of angle. Cards often carry both, and a checker tilting a document is testing the angular features described in holograms and OVDs on licences rather than the printed blend.
Why the Technique Persists
Split fountain is old technology, used on banknotes long before identity cards existed, and it survives for a practical reason. It costs almost nothing once the press is set up, it needs no special substrate, and it defeats casual reproduction on exactly the equipment most people have. A feature that adds no unit cost and cannot be copied on an office printer earns its place indefinitely.
It also degrades gracefully. Unlike a laminate or a chip, a printed blend cannot stop working, so it remains verifiable for the life of the card. The rest of the printed and physical layers are covered across the ID security features hub.
Frequently Asked Questions
What is rainbow printing on an ID card?
It is a gradual blend from one ink colour to another across the card background, produced in a single press pass with no visible seam between the colours. It usually sits behind the data fields as part of the security artwork.
What does split fountain mean?
The ink trough on a press is divided so two or more inks run side by side in one unit. The rollers blend them as the press turns, so the transition happens in the ink film rather than being assembled from separate colours.
Why can a normal printer not copy it?
A desktop printer builds colour from dots, so it can only imitate a blend by varying the dot ratio across the area. Under magnification that reads as a halftone field rather than continuous ink, and any fine lines printed in it lose their crispness.
Is rainbow printing the same as a hologram?
They are different effects. Rainbow printing changes colour as you move across the card and looks identical from any angle. A hologram or optically variable device changes as you tilt the card, which is an angular effect rather than a spatial one.
Where should I look for it on a licence?
In the background artwork rather than the foreground: behind the data block, inside a guilloche pattern, along an edge band, or running through the ghost portrait area. It is designed to be unremarkable at a glance.
Does the blend fade over time?
Not in the way a laminate wears or a chip fails. It is ink within the card construction, so it stays verifiable for the life of the document, which is one reason issuers keep using such an old technique.