Posts Tagged ‘Density’

ISO 12647-2 Presswork Procedure

Quite often we get the question of how to achieve ISO-12647-2 standards in press (not for certification purpose ofcourse).  So decided to write down the steps need to be followed.

1. We have to make a test form with linear plates (test form high-res file is with Mr.Saravanan of Heidelberg). A linear plate means, input and output value has to be same. If we give a value of 50%, the plate should have 50% only (+ or – 1% tolerance)

2. Print this linear plate in the press with the pre-requisites below with varied densities across the sheet

3. After the print is allowed to dry 3 hours, measure the sheet and find the optimum density for the press using Delta E and Contrast method.

4. Then print another test chart with the calculated optimal density (linear plate) and calculate the dot gain deviation.

5. The dot gain deviation has to be calculated and compensated in the RIP as per ISO 12647-2 standards

6. The same test chart to be printed with revised dot gain curve

7. The iteration should continue until the best results are achieved as per ISO 12647-2 standards

8. The Pre-press need to be set as per the “ISOcoated V2.icc” and proof need to be FOGRA 39 standard in V3 media wedge.
Pre-Requisites:

But to do the above exercise, we need to have the below

• Should have a densitometer and also a plate (Calibrated one. Prefered instruments are Ihara R730, Techkon Spectrodens for press and Ihara Accudot or Techkon spectroplate for plate dot measurement)

• you need to print minimum of 3000 sheets of your full paper size of 115/135 gsm art glossy paper with Fresh CMYK ink in the inking unit. When the target is the ISO 12647-2 standard, the used ink series should be ISO conform. Values given below for reference. We can check Ink manufacturer for the confirmation.

• The printing press has to be in a well maintained condition

• Blankets need to be in a clean and proper condition

• New blankets should be washed with water and cleaning solvent

• Verify the blanket tension with a torque key (for tension values check the machine manual)

• The Packing has to be done as per the standards

• The Pressure between blanket/plate have to be in a range of “kiss print”

• The Pressure between blanket/substrate have to be in a range of “kiss print”

• The Ink deck have to be washed clean

• All rollers have to be well adjusted

• Ink- /dampener rolls have to be well adjusted especially to the plate

• The ink distributor roller have to be checked

• Verify the machine manual for adjusting the rollers according to the machine manufacturer

• The pH-value of the fountain solution should be at a level between 4.7 – 5.3

• The percentage of alcohol in the dampening solution should be at a level between 5 – 10%

If you have any doubts are clarifications, please feel free to come back to us colorsherlock@live.com

 

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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.

Do you know the Difference between GRACoL, GRACoL 7 and G7?

There has been some confusion about the terms “GRACoL” “GRACoL 7” and “G7”, which many tend to use interchangeably even though they have very different meanings!

GRACoL®, in the generic sense, refers to the IDEAlliance Committee that began in 1996 as a graphics arts task force that was formed to develop a document containing general guidelines and recommendations that could be used as a reference source across the industry for quality color printing. The mission of GRACoL is to improve communications and education in the graphic arts by developing best practices that reflect the influence and impact of new technologies in the workflow of commercial offset lithography. GRACoL is a registered trademark of IDEAlliance.

GRACoL 7 is the current version of the GRACoL Publication. The GRACoL 6 Guidelines was published in 2002. Since that time, the IDEAlliance GRACoL Committee conducted a series of research press runs and developed a new set of process controls based on principles of digital imaging, spectrophotometry, and computer-to-plate (CtP) technologies in ways that have not been done previously in the US. GRACoL 7 will explain the rationale behind the new visual-appearance-based GRACoL press and proofing system, gray balance characterization and calibration methods. In addition, GRACoL 7 will specify a definition for gray balance and recommend characterization data for commercial offset printing on a #1 coated sheet. These recommendations are intended to stabilize visual output but not to eliminate process control best practices based on SIDs or TVI/Dot Gain as defined in the ISO 12647-2 printing standard.

G7™, the IDEAlliance proof-to-print process, is based on principles of digital imaging, spectrophotometry, and computer-to-plate (CtP) technologies. G7 is currently being applied to many types of printing including commercial and publication printing, newsprint and even flexo. This new methodology utilizes the existing ISO 12647 Standards as the basis for good printing. G7 requires printing with inks defined by ISO 2846-1 so that the dry solids measure as close as possible to the ISO CIELab values for seven colors ─the four primary colors and three 2-color overprints specified in ISO 12647. Because our goal is to specify a simple calibration process that will help the printers reliably achieve a close “visual match” from proof to press, G7breaks from tradition by focusing on colorimetric data for gray balance in the mid-tones rather than on densitometric aims, i.e. dot gain, for each color. G7 is named for its gray scale calibration technique and the 7 ISO ink colors it requires. G7 is a trademark of IDEAlliance. Although G7 was developed by the efforts of the GRACoL Committee, it should not be confused with GRACoL or with GRACoL 7.

sRGB v4 Preference:

The sRGB v4 ICC preference profile is a v4 replacement for commonly used v2 sRGB profiles. It gives better results in workflows that implement the ICC v4 specification. It is intended to be used in combination with other ICC v4 profiles.

The advantages of the new profile are:

  1. More pleasing results for most images when combined with any correctly-constructed v4 output profile using the perceptual rendering intent.
  2. More consistently correct results among different CMMs using the ICC-absolute colorimetric rendering intent.
  3. Higher color accuracy using the media-relative colorimetric intent.

A typical use case would be to print sRGB images captured with a digital still camera. In this case a user could open the image in Adobe® Photoshop®, assign the sRGB v4 profile as shown in Figure 1. Adobe® Photoshop® is either a registered trademark or trademark of Adobe Systems Incorporated in the United States and/or other countries

Assigning the sRGB v4 ICC preference profile.

The user would then convert the data to printer specific values with the sRGB v4 profile as source, the v4 printer profile as destination, and selecting the perceptual rendering intent, as shown below

Conversion from sRGB v4 ICC preference profile to v4 printer profile in Photoshop using the perceptual rendering intent.

The three available rendering intents of the sRGB v4 profile should normally be used as follows:

  • The ICC-absolute colorimetric rendering intent should be used when the goal is to maintain the colors of the original on the reproduction,
  • The media-relative colorimetric intent should be used when the goal is to map the source medium white to the destination medium,
  • The perceptual intent should be used when the goal is to re-optimize the source colors for the reproduction medium while maintaining the “look” of the source image

The re-optimization performed by the perceptual rendering intent of the profile is a result of using the Perceptual Medium Reference Gamut defined in the ICC v4 specification as an intermediate rendering target. No perceptual reference medium is defined for v2 profiles.

Advice for caution

Ideally the ICC v4 profile should not be combined with ICC v2 profiles. If that is unavoidable, see the intermediate-level ICC White Paper 26 ‘Using the sRGB_v4_ICC_preference.icc profile’ for additional information and recommendations.

The sRGB v4 profile will in some cases not produce the same results as would be obtained using an sRGB v2 profile. Differences will depend on the particular sRGB v2 profile and rendering intent used. See the intermediate level white paper for additional information and recommendations.

If sheets are pulled freshly printed from the delivery and measured, the ink is still wet and has a shiny surface. While drying, the ink penetrates the paper (absorption) and loses its gloss. This not only changes the color tone, but also the density. It is only possible to a limited extent for the press operator to use densitometry to compare wet sheets with the reference values, which also refer to dry ink.

Why Polarizing filters?

To get round this problem, two linear polarizing filters at right angles to one another are placed in the path of the densitometer. Polarizing filters only permit light waves oscillating in a certain direction to pass. Part of the resultant aligned beam of light is reflected by the surface of the ink, but its direction of oscillation remains unchanged. The second polarizing filter is rotated 90° in relation to the first, which means that these reflected light waves are blocked.

Fig1. Polarizing filter

However, if the light is only reflected after it penetrates the film of ink, either by the ink or the paper, it loses its uniform direction of oscillation (polarization). Consequently, part of it passes through the second polarizing filter and can be measured.

Filtering out the light reflected by the glossy surface of the wet ink thus has the effect of making the densitometric measurement values for wet and dry ink roughly equivalent.

Brilliant colors true to the original on a premium surface – that ̓s what distinguishes a high-quality print product. Many elements in prepress and printing impact on a product ̓s color fidelity. The various input and output units in a print shop ̓s production process, consumables and other factors can cause deviations in color. A consistently employed color management system eliminates this problem.

In this series of blog tips we ̓ll show you the most common sources of error and give you tips on color management. We ̓re pleased to have caught your interest.

What is Color Management?

The color management process enables an identical color impression to be produced error-free on various output units, such as monitors, proofers and printing presses.

a. World of Color Management

With a consistently employed color management system, a template made on any input unit can be reproduced virtually identically at any output unit. Color management systems can also harmonize a range of devices, such as scanners, digital cameras, monitors, printers, filmsetters and platesetters. The color is then shown according to the print conditions, for example.

The print result is at the heart of the color management process, because the possibility of making corrections directly at the printing press is limited.