PDF/A and Colors

This article deals with color spaces, ICC profiles and output intents.

The aim of this article is to provide a compact overview on the subject of “PDF/A and colors”. More detailed information about this topic is available on the PDF/A Competence Center’s website. Especially recommended is the Technical Note “TN0002 – Color in PDF/A-1”.

The article provides several user examples based on Adobe Acrobat 8 Professional. There are also numerous other PDF/A tools which are not mentioned here.

Goal of PDF/A

What makes PDF/A unique and special? The following items are important for long-term archiving:

Retaining the visual appearance

The visual appearance should be guaranteed over a long period of time. In addition, it must be ensured that the PDF file can be (correctly) visually reproduced on any (including future!) output device.

Measures

In order to achieve this, all of the necessary resources must be embedded in the PDF file, for example text, vector graphics, images and fonts.

Device independent color definitions

The colors must also be exactly defined. This is achieved by means of ICC-based colors and device colors with output intents.

Colored objects in PDF/A

There are numerous object types in PDF files that can be colored.

Page objects

The category “page objects” includes images, vector graphics, text, Type3 fonts and patterns.

Annotations

Comments / Annotations also include colored objects that one may not expect at first, for example form fields, digital signatures, links (visible borders) and of course the annotations themselves.

Please note: Annotations are always defined in RGB!

Color models in PDF

Additive Colors (RGB)         Subtractive Colors (CMYK)

Primary colors:                                     Primary colors:
Red, Green, Blue                                   Cyan, Magenta, Yellow, BlacK (Key)
Uses: Monitors, projectors              Uses:Printing

Device colors are not explicitly defined

Device colors only describe a portion of an (undefined) color (for example 80% red, 80% green, 0% blue). Device colors result in varying colors when reproduced on different output devices. Even the color of the paper can have an influence.

Due to this, PDF files can look different according to the output device:

Laser color printer

Newspaper print

Different color ranges (Gamut)

The divers color ranges of the individual color spaces are responsible for these color variations. This can be demonstrated by using 3D-models:

AdobeRGB vs. sRGB

IsoCoated vs. IsoNewspaper

PDF color spaces

PDF works with the following color spaces:

Device colors (device dependent)

Device dependent color spaces include RGB, CMYK and Grey.

CIE-based colors (media-independent)

Media-independent colors are CIELAb, the calibrated colors CalRGB and CalGray, as well as ICC-based colors (RGB, CMYK and Grey)

Special colors

In addition, special colors can be found in PDF files, including spot colors, (for example pantone, HKS), DeviceN, separation, indexed colors as well as patterns.

ICC Profiles

This standard format is used to characterize the color properties from input devices (cameras, scanners), viewing devices (monitors) and finally output devices (color printers, print processes).

ICC profiles are defined by the International Color Consortium. There is also an ISO standard (ISO 15076). ICC profiles are used in PDF for defining ICC-based colors and as output intents (OutputIntent).

More information on this topic can be found on the International Color Consortium website.

Solution 1: ICC-based colors

Here, a device independent color definition is used for every object with help of an ICC profile. One has the possibility of embedding the definition when constructing the object (like in Photoshop) or allocating the color spaces in Distiller.

Note: not all applications support an integrated color management.

Solution 2: Output intent (OutputIntent)

The characterization of all device colors is implemented using a single ICC profile (valid for the entire document). With PDF/A, the ICC profile of the output intent must always be embedded (a simple reference is not permitted).

All types of ICC profiles are possible in PDF/A:

Input profile

Monitor profile

Output profile

Allocating the OutputIntent

With Acrobat 8.0 Professional one can select the desired OutputIntent during the conversion process, when PDF/A is created by means of Preflight.

In Acrobat Distiller 8.0 Professional, the OutputIntent can be identified when setting up the job options.

Distiller: OutputIntent

Note: Distiller 7.0 Professional creates non-conforming PDF/A files, because it uses provisional parameters only.

Standard profiles

There are common standard profiles depending on the program or area of application.

Office documents:

Office files use sRGB (defined by Microsoft and HP).

Images from digital cameras

Low-end cameras frequently work with sRGB, whereas high-end cameras often employ AdobeRGB.

Graphics

As a general rule, graphics use the monitor’s profile or AdobeRGB.

Sheet-fed offset

When printing with the sheet-fed offset method, ISO coated V2 (ECI) and ISO Uncoated (ECI) are common profiles.

Web-fed offset

ISO WebCoated (ECI) or SWOP (in the USA) are usually applied with web-fed offset printing.

Newspaper print

ISO Newspaper (IFRA) is a special profile for printing newspapers.

Permitted color spaces

Output inten­t     Device Gray     Device RGB     Device CMYK   Calibrated ICC-based
Grayscale                      ✓                                    –                              –                                          ✓
ICC Profile
RGB                                    ✓                                    ✓                             –                                         ✓
ICC profile
CMYK                                ✓                                   –                             ✓                                         ✓
ICC profile
No output intent        –                                   –                              –                                          ✓

Links

International Color Consortium

sRGB
http://www.color.org/srgbprofiles.html
http://www.srgb.com/srgb.html

AdobeRGB

http://www.adobe.com/digitalimag/adobergb.html

http://www.adobe.com/support/downloads/iccprofiles/iccprofiles_win.html

http://www.adobe.com/support/downloads/iccprofiles/iccprofiles_mac.html

Offset and gravure profiles
http://www.eci.org/eci/en/060_downloads.php

By Stephan Jaeggi, PrePress-Consulting

Translated from the German original by Reeves & Partner GmbH

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