Knowledge base

ICC profiles for industrial inkjet: colour that stays the same from file to product

By Piotr Jędrzejczyk, AssembleX Industrial Services

An ICC profile describes how one specific combination of machine, ink and substrate turns digital values into colour. Without it, the same file looks different on every line.

What an ICC profile is

An ICC profile is a small file that describes the colour behaviour of a device or process. It links the device's own values (for a printer: the amounts of each ink) to a device-independent colour space, usually CIE Lab, called the profile connection space.

To print a design, the colour management module converts the colours of the file through two profiles: the profile of the source (for example sRGB) into Lab, and then from Lab through the profile of the printer into ink values (Figure 1). A colour in the file is thus reproduced as closely as the printer can physically get.

One profile describes one combination

In industrial inkjet the colour does not come from the ink alone. It results from the whole stack, and a profile is valid only for the combination it was measured on. Change one element and the profile no longer describes reality.

• The substrate and its surface: tile body, coated steel coil, glass, PVC.
• The glaze, engobe or primer under the print.
• The ink set and the ink limits.
• The printhead type, resolution and drop sizes.
• The firing or curing conditions.

Printable colours: the gamut

No printer can reproduce every colour. The range it can reproduce is its gamut. Colours of the design that lie outside the printable gamut are mapped to the nearest reproducible colour, according to the rendering intent chosen for the conversion (Figure 2). Perceptual compresses the whole range; relative colorimetric keeps the colours inside the gamut exact and clips the rest.

How a profile is built

1. Stabilise the machine. Mechanical setup, jetting and linearisation come first. A profile built on an unstable base records the faults.
2. Print a profiling chart. A target with hundreds of colour patches covering the gamut, printed with the production settings and ink limits.
3. Process it as in production. Fire or cure the samples. The profile has to describe the final colour.
4. Measure all patches with a spectrophotometer..
5. Generate the profile in profiling software, and set the ink limits and total ink so that the print stays stable and does not oversaturate..
6. Validate. Print an independent chart or real images, measure them and calculate the colour difference ΔE against the reference (Figure 3).
7. Apply it in the RIP and document it, together with the exact conditions it is valid for..

Keeping colour stable

A profile is a snapshot. Inks, heads and kilns drift, so colour has to be checked on a schedule against a reference tile or sample. If ΔE rises above the agreed tolerance, first check the machine (jetting, linearisation, ink condition) and only then rebuild the profile. A profile is not a way to hide machine faults.

Common mistakes

• Profiling before the machine is linearised and stable.
• Measuring unfired or uncured samples.
• Copying a profile from another line or another glaze and expecting the same result.
• Ink limits set too high: the print oversaturates, colours merge and the process becomes unstable.
• No reference and no re-check schedule.

A profile builds on a linearised machine. If the colour on your line drifts or differs between heads, start with linearisation.

• Linearisation in industrial inkjet: why every head needs its own curve →
• Stitching between printheads: how to make many heads print as one →
• Reading print defects: a field guide from file to finished product →
• Drop size and grey levels: how an inkjet head makes tones →
• RIP and print strategy: preparing a file so the machine can print it well →