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Proofing and printing documents that include spot inks

In order to test the use of proposed ISO standards to communicate spot colour ink characteristics effectively, an example PDF document (updated 20-02-2018) is available.

The document (see below) includes Silver, Violet and Red inks each with different ink opacity which are used in conjunction with the process inks.

The document's OutputIntent includes CxF data for each of the spot inks and an ICC profile that describes the process inks.

NOTE that many widely used PDF viewers, including Adobe Acrobat, do not support these spot inks and so should not be used as a guide.

Use of CxF/X-4 in PDF documents
Communication of printing characteristics of inks is essential in order to ensure that a printed product has the appearance desired by a print buyer or brand manager. Traditionally inks are thought of as being either process inks or spot inks.

The term 'process inks' is used to describe a set of inks that are frequently used in combination on a printing press (often Cyan, Magenta, Yellow and Black). Process inks are generally characterised in combination by means of characterisation data set usually communicated indirectly in the form of an ICC Profile. For reference printing conditions the ICC profile is included as the Output Intent of a PDF document and in this way the colour aims for the process inks, including the colour of ink combinations, is communicated effectively between the PDF document creator and printer. A PDF/X document created in this way can be proofed and printed reliably without any additional communication between the creator and printer as long as the document includes only process inks. In cases where spot inks are used, PDF/X provides limited direction for these inks and PDF/X documents do not contain sufficient information to allow such documents to be proofed and printed.

ISO 17972-4 defines a subset of CxF (CxF/X-4) that can be used to characterise spot colours and it is hoped that this standard will provide a more reliable means for the communication of spot colour characterisation data. It is usually impractical to print and measure all combinations of spot colour inks and instead each ink is characterised in conjunction with a print substrate by means of its spectral characteristics. Printing and measuring tints of the spot inks provide sufficient data to allow the ink opacity to be determined. The ISO 32000-2 Draft Specification (PDF 2.0) includes an extension that allows spot colour characterisation data to be included in the OutputIntent so that the characteristics of the set of spot inks used in the document can be communicated effectively. In this way the PDF document can be proofed and printed in exactly the way in the creator intends. Note that although these extensions are defined by the PDF 2.0 specification they have been defined in such as way as to allow them to be used in PDF version 1.x.

Spot ink characterisation
Spot inks are characterised for a particular substrate by printing and measuring a chart similar to that shown in the figure below. A number of patches are printed using the ink to be characterised on the print substrate and on a region that has been previously printed with black ink.

The spectral reflectance of each of these patches is measured and these measurement data along with metadata to describe the target and measurement conditions are encoded as CxF/X-4 as described in ISO 17972-4. Most of the resources used in CxF/X-4 files are CxF core resources and are documented as part of XRite's CxF 3.0 documentation.

In addition, a number of required Core Resources and Custom Resources are defined for CxF/X-4 and full details can be found in ISO 17972-4 :2018, Graphic technology -- Colour data exchange format (CxF/X) -- Part 4: Spot colour characterisation data (CxF/X-4).

Extensions to PDF OutputIntent
The PDF 2.0 Specification describes how CxF data can be included in the OutputIntent Dictionary to communicate the characteristics of the spot inks used in the document. When developing the PDF 2.0 specification the ISO committee was careful to ensure that the same method can be used in PDF 1.X documents. The structure is outlined in the figure below.

Use of spot ink characterisation data
Spot ink characterisation data can be used to provide printing aims for each ink by comparing the measurements of a set of patches printed on the press to be used for printing and applying a correction curve when making printing plates to ensure that the printed tints match the characterisation data aims.