John McDaid

Applications Technical Lead,

Sales and Technical Services.

Graphic Arts & Print Markets

KAYELL AUSTRALIA

PROFILES 101

INTRODUCTION

Ten million. This is the estimated number of colours human photopic vision can differentiate. (1)

Therefor unsurprisingly accurate and consistent colour reproduction is often considered difficult, time-consuming and frustrating. And while the introduction of colour management systems (CMS) have improved overall process control often good colour management practices are still misunderstood and ignored.

The ICC (International Colour Consortium) glossary defines colour management (digital imaging) as; (2)

communication of the associated data required for unambiguous interpretation of colour content data, and application of colour data conversions, as required, to produce the intended reproductions. [ICC.1]

NOTE 1 Colour content may consist of text, line art, graphics, and pictorial images, in raster or vector form, all of which may be colour managed.

NOTE 2 Colour management considers the characteristics of input and output devices in determining colour data conversions for these devices.

Simply, the core function of a CMS is to maintain a colour appearance when reproduced under a different condition. The four basic components of an ICC based CMS are; profile connection spaces (PCS), colour management modules (CMM), rendering intents and profiles. (3) As an end user interaction with these components is typically necessary only in selection within the CMS, however often the generation of custom profiles are required. Profiles serve purpose in both the communication of data but also the definition of its intended preproduction. Without profiles a CMS knows nothing about the different devices they drive, it is the profile that describes the device behaviour. The analysis of a device's characteristics is known as profiling.

Accurate profiling for proofing & separation profiles often required time-consuming ‘finger-printing’ of a press, commonly with large-scale test charts. The process is time-consuming and often cumbersome as a constant state of the press or device is necessary to ensure accuracy in the profile generation. Also, with a change to the substrate, inks or process re-generation for each profile is required.

Above: A ECI2002 Characterisation Chart

PROFILES

Conceptually profiles as part of a CMS are quite simple; it is their anatomy that can be complex. Profiles can describe a single device, a class of devices or a abstract colour space. Regardless of what it describes a profile essentially is a lookup table, with one set of entries that contains device control signal values (RGB or CMYK) and another set that contains the actual colour expressed in the PCS that those control signals produce. (4)

As RGB and CMYK values are ambiguous to each different device, a profile provides meaning, without changing those values. The profile won’t change the behaviour of the device, instead merely describe that behaviour.

Profiles can be broken into a number of classes. Seven in total as defined by the ICC. (5) One important factor of profiles is whether they allow one way or two-way conversion. (Eg: device space to PCS and from PCS to device space.) Below are the seven profile variations and their conversion status.

•    Input Profiles: (EG: RGB -> Lab)

•    Display Profiles: (EG: RGB <-> Lab)

•    Output Profiles: (EG: Lab <-> CMYK)

•    Device Link profiles (EG: CMYK -> CMYK)

•    Abstract profiles (EG: Lab -> Lab)

•    Space profiles (EG: Lab -> LCH)

•    Named Colour Profiles (EG: Pantone 256 = Lab(x,x,x))

HISTORY

In the late 1980’s and early 1990’s colour management systems were proprietary ‘closed loop’ systems. They used direct transformations from device to device, often with look-up tables (LUT). The development of these systems was enormously laborious, and the subsequently generated profiles were not cross-compatible between other systems. In 1993 Apple Computers introduced their colour management utility ColourSync into their Macintosh operating system in the hope of addressing this incompatibility. Building upon this eight industry-leading vendors (Apple Computers being one) formed the International Colour Consortium, or ICC ‘for the purpose of creating, promoting and encouraging the standardization and evolution of an open, vendor-neutral, cross-platform colour management architecture and components.’ (6)

Rather than the specified direct device to device transformation their proposed solution was that of a two-step conversion. First the forward transformation of the source input device space to a profile connection space (PCS) then the transformation from the profile connection space to the destination device space. 

ANATOMY OF A ICC PROFILE

With each new release of the ICC profile format specification (currently version 4.3.3.0:2010 with the preliminary specification of iccMAX:2018 released at the time of writing), the International Colour Consortium continually update and improve specifications. However, many architected elements remain constant.

The ICC Profile format is a binary format with an extension of ‘.icc’ or ‘.icm’. All valid profiles require a 128-byte header, a tag table identifying the tags present in the profile, a description string, the numerical values of the media white and the colour processing elements and other tags as defined for the profile class by the specification. Profiles may optionally include other valid tags, together with private tags not defined in the specification but registered in the ICC Signature registry. (7)

Example of tags contained within a.icc profiler header.

As ICC profiles are compiled in binary and displayed using hexadecimal notation an application is required to decompile and phase the data into a human-readable format, though not all data is typically available as profile inspector applications usually restrict access to the internal profile structure.

OUTPUT PROFILES

In the print industry output profiles, typically printer profiles are the most common profile type for operators to interact with. Output profiles are device dependent, meaning their colour space is dependent upon the output device and subject to other variables such as media, inks and RIP parameters. There are generic profiles, such as SWOP (Specification for Web Offset Publications) that determine how a press should behave, or for common mainstream proofing systems, however, these generic profiles are useless for unstable systems. (8)

For the most accurate results determining the output devices colour space (its ‘gamut’) by the technique of profiling is best practice.

OUTPUT PROFILE GENERATION

Each profile generation application has their own methodology for output profile generation. However, most follow a few important steps.

1.    Optimisation (Calibrating for the media type, known as Linearization)

2.    Characterization (Printing of known RGB or CMYK valued patches via the output device)

3.    Measurement (Printed patches are measured with a Spectrophotometer device then compared to     

       the target data).

LIMITATIONS

Colour management systems incorporating ICC based profiles face some limitations.

First and foremost, the most significant shortcoming is the reliance on the constant state of the device. The profile is specific to one specific device, one substrate, one ink-set and the parameters as set in the RIP. Once any of these change the profile is no longer accurate, and the process of regenerating the profile is required.

Secondly, profile generation software can not cover the entire set of possible device signals. Limitations on profile size and the time/effort required would be unrealistic; therefore, interpolation is unavoidable.

Also, an ICC based CMS employs a two-step conversion process data is lost in during the conversion of spectral data to the profile connection space, XYZ and to LAB

CONCLUSION:

ICC based profiles in colour management systems have come a long way since their first implantation in the early 1990’s, introducing more flexibility than that of its predecessors. And while standards continue to develop, new developments by leading software vendors are continually producing innovations in which future colour management systems will base their fundamentals.

REFERENCES

1. Judd; Wyszecki (1975) Colour in Business

2. International Colour Consortium (2004) White Paper #5

3. Fraser; Murphy; Bunting: (2005) Real Word Colour Management

4. Fraser; Murphy; Bunting: (2005) Real Word Colour Management

5. CHROMiX (2005) Colour News Issue #22

6. Stokes: (1997) Morovic: (2008) Colour Gamut Mapping

7. Green: (2010) Colour Management, Understanding and using ICC Profiles

8. Fraser; Murphy; Bunting: (2005) Real Word Colour Management