Guhan S K - ISO Quality

Maths and me @ ISO Quality

Project Quality check - testing tool
Aim : Testing tool for checking ISO standard with help of Excel worksheet to get instant document report.
Time line : 2014-2019

Quality on ISO standard 12647-covers the basic points to be consider in each category and here I explained only the project details on worksheet that help to test printed elements with ISO standard. Collection of formulas put to gather compiled and designed in Excel worksheet to get easy documentation.

Important standards for print production are ;

ISO 12647-1 - Parameters & measurement methods
ISO 12647-2 - Offset Lithographic process
ISO 12647-3 - Coldset Offset Lithography on NP (Newsprint)
ISO 12647-4 - Publishing Gravure
ISO 12647-5 - Screen Printing
ISO 12647 - 6 Flexo Printing
ISO 12647 - 7 Proofing process from digital data

ISO 2846 -1 - Ink color and transparency
ISO 3664 - Lighting condition for viewing color copies
ISO 15930 - X PDF/X data exchange in print production ( like PDF/ X-4)
ISO 1507G - ICC color profile format
ISO 12640 - data format from which ICC profiles are calculated

What is Specifications ?

A detailed description of the criteria for a piece of work. Specifications for printing can include characterization-data, ICC-profiles, PDF preflight criteria, calibration targets for the print process, etc.
Example ;

SWOP – Specifications for Web Offset Publications
SNAP – Specifications for Newsprint Advertising Production
GRACoL – General Requirements for Applications in Commercial Offset Lithography
First – Flexographic Image Reproduction Specifications & Tolerances

There are number of testing methodology and specifications to analysis standard quality in printing industry. For example methodology - ICQC's used with IFRA specification for Web-offset printing and for Sheet-fed offset G7 methodology combined with GRACoL / SWOP specifications is used by this work group. PSO Process standard offset printing by FORGRA work groups, System Brunner, packaging GMI are some the work group were involved in printing quality to support archive ISO standards.

Comparison between GRACoL-G7 & IFRA-ICQC;

Here G7 mainly used in western countries for Sheet-fed offset printing and IFRA ICQC for Euro, Asian countries for Web-offset printing. [ Note : GRACoL, IFRA are not standards, they are specifications]

Idealliance Expanded Color Gamut (ECG) Target G7

Idealliance ECG project goal is to make 7 color process printing as predictable and normal as current CMYK printing specifications such as GRACoL and SWOP. The objective is to create a standard characterization data set for use with a set white point using CMYK+OGV printing inks.

Target white point: L* = 95, a* = 0, b* = 1

Tolerance: < 2.0 dE00

Printers may user either AM or FM screening 175 lpi

Substrate ; Coated paper art.
TAC = 340

Recommended screening options:

• FM/CMYKOGV

• AM/CMYKOG + FM/V

• AM/CMYK + FM/OGV

Ink Color Recommended Values

Colors L* a* b*
Cyan 49.0 -31.0 -59.0
Magenta 45.0 79.0 9.0
Yellow 90.0 -5.0 103.0
Black 10.0 0.0 0.0
Red 45.0 74.0 55.0
Green 42.0 -73.0 24.0
Blue 16.0 21.0 -45.0
Orange 70.0 55.0 82.0
Green7 66.0 -73.0 -1.0
Violet 24.0 46.0 -57.0

Ink Color Status T- Density Solid 100%

(Approximate)

Cyan 1.85
Magenta 1.85
Yellow 1.20
Black 2.00

WAN-IFRA ICQC Target

The International Color Quality Club (ICQC) of WAN-IFRA is conducting this test method for ISO standard.

Target white point: L* = 82, a* = 0, b* = 3

Tolerance: < 2.0 dE00

Newsprint or paper shade (or color of the paper) is measured in accordance with light source D50, measuring geometry 45°/0° or 0°/45° and black backing.

Printers use AM screening with 120 lpi

Substrate ; Newsprint (NP)
TAC = 220

Color specifications;

Colors L* a* b*
Cyan 57 -23 -27
Magenta 54 44 -1
Yellow 78 -3 58
Black (K) 36 1 4
Green, Y + C 53 -34 17
Blue, C + M 41 7 -22
Red, M + Y 52 41 25
4c-Black, 34 1 2
White shade 82 0 3

Ink Color Status E- Density Solid 100%

(Approximate)

Cyan 0.83
Magenta 0.85
Yellow 0.90
Black 1.05

Excel worksheet for Colour quality report generation:

In color management, profiling is an analysis characterization of colour reproduction, we measure and record our test specimens color in form of data as density and tristimulus values like CIELAB. Excel application helps in manipulating this data and visualizing them in form of charts and graphs.

In 2015 ICQC competition for category 1 we uploaded data for pre-check and received pdf test results after a few minutes. We decided to replicate one that give instant report for consecutive test, thus prepared Excel worksheet which satisfied all testing conditions.

Formulas and documentation work uplift our project to the next level, in 2018 for test condition category 4 Non standard method, it is a challenging aspect for us to modify the worksheet. When there is no pre-check facility provided yet by others - we tried our worksheet to handle such complex calculations with 6 sets of test data at a time.

From our Kochi branch we shared the file for testing to our participants.

Advantages of our in-house colour profiling worksheet:

This worksheet is compatible for working with all categories that specified by ICQC
By simple modifications we can able to adapt any TVI settings
It works for both heat-set and cold-set inks
We can make many number of tests with multiple files, no restriction for pre-check.

This worksheet is for study purpose only - No download access to this sheet.

We have prepared worksheet to test run for quality checking and attached documentation pdf file for sample.

This project worksheet can be used by modifying profile settings for any category as well as for cold set or heat set inks ISO 12647-3 standard.

Raw material - Paper:

We can check - delta ΔE, whiteness-index of Hunter, measure energy in kelvin, find position in chromaticity diagram - of our newsprint used. WI value below 100 has < 5000K energy are called warm white (yellowish), and above 100 has > 5000K energy called cool white (bluish). Reason for selecting bright colour paper is to increase our gamut volume in the printed copy, substrate that we use is also important to get more colours in printing.

Raw materials used should be in ISO standard News paper with 80% utilization of newsprint, news-shade has to match with standard. Comparing La*b* values we use ΔE (deltaE) = √(L1-L2)^2+(a1-a2)^2+(b1-b2)^2

Mid-tone spread deviation between maximum gain and minimum gain here 20.81 is max and 17.64 is min in 40% mid-tone area so we get 3.17 as result compared with standard.

Dot gain in Mid-tone and highlight (usually 40% & 70%) of all process color is measured. We already crossed dot gain formula in quality production, controlling gain in tint area is important.

Tonal value increase:

Plotting this type of curve is common and we all know the details about this TVI graph. But below I represent TVI curve in different mode, TVI based on CIELAB value with hue angle and its lightness.

When we are familiar with hue angles of NCS (natural colour system) then - CIELAB hab,e = {25,92,162,220,271,337} {R,Y,G,C,B,M} from this data - Yellow 92° is already in the graph but found more compression in saturation , Cyan 220° shifted more in highlight area, Magenta 337° shifting like cyan in highlight area. Above graph is the example reading from TVI ugra test strip. This shows our raw materials performance as well as our machine.

Here is another example from dot gain formula - three lines gray, red and dot-red, CMYK colors in

40% mid-tone density is projected. Red lines indicate actual reading measured and dot-red is based on actual SID by formula and gray for reference gain value for density.

Excel worksheet visualize our results in unpredictable manner and also provide solution for errors.

Gray balance:

Analyzing gray balance is excitement for us when we got the result as same like ICQC. Calculation for finding target based on measured data is different from other module.

Gray balance the quality of we produce gray level in light, medium and dark area are measured and compared with standard.

Colour Space:

When two images look alike, purpose is the same, but method of plotting is different. The second image gamut is actual position of our colour stimuli in chromaticity diagram 2D chart, it gives a better understanding than the previous one. Excel also has some limitations that are more advanced like calculating gamut volume, this is possible now with high level programs.

We can calculate gamut area and match with reference gamut using excel worksheet.

Color space and conformity - Our measured samples with color element is checked against ISO standard.

Registration mark:

Finally we added registration part in this documentation, using coordinate geometry it is also possible to measure the distance between points in the plane.

Color Registration is also in standard max of 200 ʮm tolerance is accepted between colors.

Ink trapping can be plot using CIELAB values.

Data collection:

Table content are data set (Characterization data set) from test elements provided here for sample.

This work sheet is a project work mainly to understand the concept in ISO standard quality printing - This designed Excel worksheet used for both test targets FOGRA color control strip and IFRA cuboid element.

One of the best ways to achieve consistent and predictable color quality on a printer is to calibrate it and create a custom ICC profile for it. But the work doesn’t stop there. To maintain consistent and predictable color quality, you need a verifiable color management strategy that centers around process control.

When you choose to implement process control into your color management strategy, you will ultimately save yourself money in ink and media as well as save time by not having to repeat jobs.

Process Control Is Cyclical

Printer profiles must be maintained; in much the same way that a car does. When you buy a vehicle, at some point you need to take it in for maintenance which often includes an oil change. Once the oil has been changed, the maintenance provider will often place sticker on your windshield as a reminder of when the next service is due. That’s because the maintenance needs to be ongoing. If the car only gets one oil change and it’s never checked again, its engine will eventually seize up and fail. Through everyday wear and tear, the amount of oil will decrease and parts will begin to deteriorate.

The same is true for a printer profile. Like a car, a printer is a machine that is in constant operation and will experience wear and tear over time. That can affect a profile’s performance; which is why the printer must be maintained and profile verified.

The Color Management Pyramid’s Role In Process Control

PRINTING United Alliance’s and Idealliance’s iLearning+ color management program, suggest often refer to the Color Management Pyramid as a guide on how to build a custom ICC profile. The pyramid is based on agreed upon industry best practices for color management.

To use it, you start at the bottom of the pyramid and work your way up. The first three levels of the Color Management Pyramid are important elements of process control. We refer to it as stabilizing the system.

The Environment: If you refer to the pyramid image, the first consideration is the environmental condition in the printing space. You want to make sure your printer has clean power and is absent of dust and dirt. Temperature and humidity should also be monitored carefully. The optimum temperature in your printing environment should be around 70 degrees Fahrenheit and the relative humidity should be non-condensing and around 50%.

The Printer and Media Settings: You’ll want to ensure that the settings are correct for the print head height and alignment as well as your media feed and UV heat lamp settings if applicable. Don’t forget to perform a nozzle check daily too. It’s one of the easiest and most important checks you can make on your printer.

The RIP and Print Settings: These settings refer to the preferred options in your RIP software and on your printer. They include resolution, pass count, variable dot settings, etc. Understanding and applying the optimal settings for your RIPs and printers will help you set up your system for success.

Once your system is stabilized, you can follow the steps in the rest of the Color Management Pyramid to create a custom ICC profile (which will be explained in a future article). A custom ICC profile serves as a baseline that describes a printer’s known good print condition at a specific point in time. But, as explained before, it must be verified frequently because it can be affected by changes in variables in the color management process.

Verification for Process Control

There are many variables that can affect a profile of a printer. They include replacement of printer parts, changes to inks or substrates and the environmental variances (temperature or humidity) as a whole. Therefore, it’s important to frequently verify your custom ICC profile to make sure it’s still performing at your preferred baseline.

To verify a profile, you need a spectrophotometer that is supported by a piece of software that can collect and analyze spectral data. You also need to choose a control target to measure against. This target should be a specific guideline such as GRACoL®. You can find free downloads of control wedges to download and measure on the Idealliance website.

Once you choose a target, you can print it out with the ICC profile applied. Measure the target into the software program and choose the guideline to measure against. In most programs, you can view the results numerically and pictorially.

The software will show you the Delta E number for each swatch of color on the control strip compared to the guideline so that you can see the mathematical difference between the swatches. These numbers and visual charts will help you determine whether your custom ICC profile is still performing as expected or if it’s time to re-calibrate your printer.

Keep The Cycle Going

Whether you have to re-calibrate or not, you need to frequently check up on your printer and profile. Ongoing maintenance of process control is necessary to maintain consistent and predictable color quality.

It’s a good idea to make one person responsible for verifying a printer’s performance over time. Having a strict routine where someone is logging results with time and date stamps will ensure that the necessary steps are taken to keep your printer performing at its best.

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