Archive for the ‘Color Management’ Category

The popular “Heidelberg Learning tips” inputs on Dampening solutions in offset printing is rendered below for the benefit of readers.  This is second part of the series.

3. Water Hardness

The proportion of lime in the water can cause problems during printing, for example:

  • The inking rollers run blank (calcification)
  • Deposits on the rubber blanket
  • Impact on the pH-Balance
  • Fluctuation in the pH-Balance

The proportion of chloride, sulphate, or nitrate is too high, which in addition leads to corrosion. The overall hardness of the water may be measured simply by using test strips. Dip the hardness-strip briefly (1 second) into the water, then read the results after two minutes.

In order to ensure that the dampening solution preparation possesses the ideal degree of hardness, the principle of reverse osmosis for water desalinisation is used. In the process, the water is pressed against a membrane.

Water treated like this, emerges with a very low residual salt content. Subsequently, this osmosis water is reconditioned with salts, until it reaches a degree of hardness ranging from 8° dH to 12° dH.

4. pH-Balance

“pH” is derived from the Latin (Potentia Hydrogenii) and represents a logarithmic description of the concentrations of hydrogen ions. In other words, the pH-Balance is a measure used to determine the acid or alkaline content of aqueous solutions.

What type of acid or base is involved cannot be determined. A liquid with a pH-Balance of 5 has 10 times more acid than a liquid with a pHBalance of 6. As a general rule, dampening solution additives are buffered, in order  for the most part  to neutralize external influences.

A pH-Balance measure does not tell us very much about the quality of the dampening solution. The measure only shows, whether an additive is present or not. Naturally, in order to determine the quality of the dampening solution, its conductivity should also be determined.

you can buy the pH Meter from Amazon, by clicking the above image.

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Just when you’ve all cozied down with PDF 1.7 what happens?  Yes, that’s right.  A new standard rears its head.

Around the middle of 2017 the ISO committee will publish PDF 2.0 (ISO 32000-2). So by the end of 2017 you’ll probably need to be considering how to ensure that your workflow can handle PDF 2.0 files correctly.

As the primary UK expert to this committee I thought I’d give you a heads up now on what to expect.  And over the coming months via this blog and our newsletter I’ll endeavor to keep you posted on what to look out for as far as print is concerned.  Because, of course, there are many aspects to the standard that do not concern print at all.  For instance there are lots of changes in areas such as structure tagging for accessibility and digital signatures that might be important for business and consumer applications.

As you probably already know, in 2008 Adobe handed over ownership and development of the PDF standard to the International Standards Organization.  Since that time I’ve been working alongside other experts to ensure that standards have real-world applicability.

And here’s one example relating to color.

The printing condition for which a job was created can be encapsulated in professional print production jobs by specifying an “output intent” in the PDF file. The output intent structure was invented for the PDF/X standards, at first in support of pre-flight, and later to enable color management at the print site to match that used in proofing at the design stage.

But the PDF/X standards only allow a single output intent to be specified for all pages in a job.

PDF 2.0 allows separate output intents to be included for every page individually. The goal is to support jobs where different media are used for various pages, e.g. for the first sheet for each recipient of a transactional print job, or for the cover of a saddle-stitched book. The output intents in PDF 2.0 are an extension of those described in PDF/X, and the support for multiple output intents will probably be adopted back into PDF/X-6 and into the next PDF/VT standard.

But of course, like many improvements, this one does demand a little bit of care. A PDF 1.7 or existing PDF/X reader will ignore the new page level output intents and could therefore produce the wrong colors for a job that contains them.
In my next post I’ll be covering changes around live transparency in PDF 2.0.  Bet you can’t wait!
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The background
The last few years have been pretty stable for PDF; PDF 1.7 was published in 2006, and the first ISO PDF standard (ISO 32000-1), published in 2010, was very similar to PDF 1.7. In the same way, PDF/X 4 and PDF/X 5, the most recent PDF/X standards, were both published in 2010, six years ago.

In the middle of 2017 ISO 32000-2 will be published, defining PDF 2.0. Much of the new work in this version is related to tagging for content re-use and accessibility, but there are also several areas that affect print production. Among them are some changes to the rendering of PDF transparency, ways to include additional data about spot colors and about how color management should be applied.

*source: http://blog.globalgraphics.com/getting-to-know-pdf-2-0/

In the light of the established collaboration between the European Color Initiative (ECI), Fogra and the German Printing and Media Industries Federation (bvdm), the Fogra characterization database has been extended to three newly developed sets of characterization data.

While FOGRA48 addresses heat set printing on improved newsprint stock FOGRA49 and FOGRA50 cover standardized offset printing based on FOGRA39 with additional surface finishing namely matt and glossy laminated sheet-fed offset prints using OPP films.

The new printing condition FOGRA48 and the associated ICC profile “PSO INP Paper (ECI)” for heatset web offset printing on improved newsprint are based on a test print series conducted by the ECI web offset working group (WOWG). Several European web offset printers contributed print samples on typical improved newsprint papers. Upfront the working group had agreed upon aim values for the solid coloration and the tone value increase  (TVI) of the printing inks cyan, magenta, yellow and black. As for all other offset profiles provided by the ECI, the aim values for the TVI are taken from the international standard ISO 12647-2:2004. For the dot gain of cyan, magenta and yellow curve C (19%) applies, and curve D (22%) for black, measured in a 40% patch of a control strip. With the new profile the ECI completes the range of standard profiles for web offset papers.

The characterization data for lamination are based on test prints of the ECI coating working group, which included several types of coated paper and cardboard, as well as different lamination films and machines. Based on measurements before and after lamination, typical colour changes have been determined and applied to the FOGRA39 data set. This ensures an optimal correspondence between unlaminated and matte (FOGRA49) or glossy (FOGRA50) laminated data sets. In addition, the Fogra research project (No. 32.152) examined the effect of varnishing, which leads to similar, but smaller colour shifts than film lamination. The resulting varnished gloss depends strongly on paper and ink gloss and affects saturation and contrast. Since there are considerable differences in these material properties, no standard data sets for varnishing are made available as “FOGRAxx data”. However for individual high-quality production, the varnish data sets are available on the Fogra project web page. This applies also to UV varnish, which has about half the effect of film lamination. Here, FOGRA49 and FOGRA50 might serve as a rough estimate of what to expect. Again, for high-quality varnished jobs it is preferable to establish individual characterization data. This could be done by appropriate data adjustments based on the newly provided data sets.

Source: ECI.org

Universal SmoothnessTest.tif (2,3 MB)


This test image contains 16,7 million colors of a
24-bit RGB file. Convert this test image with the designated ICC profile. So you can evaluate very quick and easily the color reproduction and smoothness quality of the output profile.

CIE Lab & LCH

This is a very basic introduction to two related colour models which are becoming increasingly important in the world of colour reproduction. A colour model is merely a way of describing colour. These are among the tristimulus (three-dimensional) colour models (‘spaces’) developed by the C.I.E.

What is the CIE? C.I.E. is short for ‘Commission Internationale de l’Eclairage’, which in English is the ‘International Commission on Illumination’.  A professional scientific organisation founded over 90 years ago to exchange information on ‘all matters relating to the science and art of lighting’. The standards for colour spaces representing the visible spectrum were established in 1931, but have been revised more recently.
For those of us involved in creating colour which will be reproduced on a printed page, it is easy to forget that there are other industries which need to accurately describe colour! RGB or CMYK descriptions won’t be of any use to paint or textile manufacturers! Terms such as ‘maroon’ or ‘navy blue’ won’t be precise enough.
There are many CIE colour spaces, more correctly known as models, which serve different purposes. They are all device independent, unlike RGB or CMYK colour spaces which are related to a specific device (camera, scanner, or press, etc.) and/or material type (paper, ink set, film emulsion or lighting, etc.). These RGB and CMYK spaces usually do not cover the entire visible colour spectrum or gamut. The CIE also specify lighting conditions.

The CIE LCH Colour Space or Colour Model.

This is possibly a little easier to comprehend than the Lab colour space, with which it shares several features. It is more correctly known as  L*C*H*.  Essentially it is in the form of a sphere. There are three axes; L* , C* and H°.

The L* axis represents Lightness. This is vertical; from 0, which has no lightness (i.e. absolute black), at the bottom; through 50 in the middle, to 100 which is maximum lightness (i.e. absolute white) at the top.

The C* axis represents Chroma or ‘saturation’. This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 or more at the edge of the circle for very high Chroma (saturation) or ‘colour purity’.

If we take a horizontal slice through the centre, we see a coloured circle. Around the edge of the circle we see every possible saturated colour, or Hue. This circular axis is known as H° for Hue. The units are in the form of degrees° (or angles), ranging from 0° (red) through 90° (yellow), 180° (green), 270° (blue) and back to  0°.

The LCH colour model is very useful for retouching images in a colour managed workflow, using high-end editing applications. LCH is device-independent. A similar colour model is HSB or HSL, for Hue, Saturation and Brightness(Lightness), which can be used in Adobe Photoshop and other applications. Technically this is ‘device-dependent’, however it is particularly useful for editing RGB images. For example to edit a green:  Adjust the Hue angle by increasing it to make it ‘bluish’ or by reducing it to make it ‘yellowish’; Increase the Saturation (Chroma) to make it ‘cleaner’;  increase the Brightness or Lightness to make it lighter. Go on give it a try!

The CIE Lab Colour Space or Colour Model

CIE Lab colour space used in ICC Colour ManagementThis is more correctly known as  L*a*b*.
Just as in LCH, the vertical L* axis represents Lightness, ranging from 0-100.  The other (horizontal) axes are now represented by a* and b*.  These are at right angles to each other and cross each other in the centre, which is neutral (grey, black or white). They are based on the principal that a colour cannot be both red and green, or blue and yellow.
The a* axis is green at one extremity (represented by -a), and red at the other (+a).
The b* axis has blue at one end (-b), and yellow (+b) at the other.
The centre of each axis is 0. A value of 0 or very low numbers of both a* and b* will describe a neutral or near neutral. In theory there are no maximum values of a* and b*, but in practice they are usually numbered from -128 to +127 (256 levels).

CIE Lab is extensively used in many industries apart from printing and photography. It’s uses include providing exact colour specifications for paint (including automotive, household, etc.), dyes (including textiles, plastics, etc.), printing ink and paper. Nowadays it is becoming of increasing importance in specifying printing standards such as in ISO-12647, where it is usually used instead of densitometry.
For example Paper Type 1 (115gsm gloss coated white, wood-free) has ‘Paper Shade’ described as ‘L* 95, a* 0, b* -2’. So the L*95 is very light, the a*0 neutral, and the b*-2 very slightly ‘blueish’.
Paper Type 5 (115gsm uncoated yellowish offset) is described as ‘L* 90, a* 0, b* 9’. So it is a darker, more ‘yellow’ paper. If you compare the different Lab values for Type 1 & 5 you will understand the descriptions.

Lab measurements can be used to control printing, typically by monitoring a 3-colour neutral grey mid-tone patch. It is also very useful for specifying a spot colour, perhaps an important “house” or “corporate” colour such as “Coca-Cola Red”. The same colour definition could be used for printed matter, vehicles, clothing, buildings, and of course tin cans.

To obtain CIE Lab measurements from an RGB image in Photoshop etc., you will need to have assigned the correct ICC profile to that image.

In ICC Colour Management  CIE Lab is often used as the Profile Connection Space (PCS) where it provides a link between two colour profiles, such as Input RGB (scanner or camera) and Output (CMYK or RGB press or inkjet). All ICC profiles contain a PCS. In an input  profile the tables will convert the scanner’s or camera’s RGB space to the PCS (Lab). An output profile will convert the  PCS (Lab) to the digital printer or printing press colour space (CMYK). The other PCS colour space is CIE XYZ, which is often also used by spectrophotometers to report colour, see the nextarticle.

Delta E Differences and Tolerances.

The difference between two colour samples is often expressed as Delta E, also called  DE, or ΔE. ‘Δ’ is the Greek letter for ‘D’. This can be used in quality control to show whether a printed sample, such as a colour swatch or proof, is in tolerance with a reference sample or industry standard. The difference between the L*, a* and b* values of the reference and sample will be shown as Delta E (ΔE). The resulting Delta E number will show how far apart visually the two samples are in the colour ‘sphere’.

Customers may specify that their contract proofs must have tolerances within ΔE 2.0 for example. Different tolerances may be specified for greys and primary colours. A value of less than 2 is common for greys, and less than 5 for primary CMYK and overprints. This is somewhat contentious however. Proofing RIPs sometimes have verification software to check a proof against  a standard scale, such as a Ugra/Fogra Media Wedge, using a spectrophotometer. Various software applications are available to check colour swatches and spot colours, proofs, and printed sheets.

Delta E displays the difference as a single value for colour and lightness. ΔE values of  4  and over will normally be visible to the average person, while those of 2 and over may be visible to an experienced observer. Values for neutrals are lower. Note that there are several subtly different variations of Delta E: CIE 1976, 1994, 2000, cmc delta e. When no particular ‘flavour’ of DE is mentioned, it is usually DE76 (as in 1976).  The later DE2000 is more perceptually uniform, and becoming more common.

Information source:

http://www.colourphil.co.uk/lab_lch_colour_space.html

Converting raster images from an RGB colorspace into a print CMYK colorspace has two significant impacts:

1) Typically a compression and alteration of colors as the image is transformed from the original RGB gamut to the different gamut used for CMYK presswork.

2) The on-press printability of the imagery in terms of color stability, press performance/runnability, and ink usage (i.e. cost).

Converting images from one CMYK separation condition into a different CMYK separation condition by reseparating files is primarily intended to enhance the printability of the imagery while maintaining the appearance of the original CMYK
imagery. Put another way, reseparating CMYK files is effectively a way to optimize press forms.

Under Color Removal & Grey component Replacement (UCR & GCR)

The principle of RGB to CMYK separation:
In order to go to press, RGB color images must be converted to their process color counterparts; cyan, magenta, and yellow. An achromatic black channel is added because if the color black in presswork is just made from CMY it can often appear “muddy” or “patchy.” Also, making dark colors from the three chromatic process colors can lead to a higher than desirable volume of ink on the press sheet.

Neutral colors made up of three process colors are also more difficult to maintain consistent on press as solid ink densities normally vary through the run compared with a neutral made primarily of a single black ink. The net effect of introducing black ink in process printing is a reduction of ink usage/costs, stabilization of color (especially gray
tones), and and better printability.
The conversion process is done by taking the 3 channel RGB image, passing it through a 3 channel device independent CIEL*a*b* profile connection color space where the RGB is converted to CMY and the black channel added, and finally
outputting the result as a 4 channel CMYK image.