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In the context of proofing, color accuracy can simply be defined as the ability to simulate a color on a press sheet or within a CMYK press dataset on a proof print. In the case of an output profile built for a specific printer, ink, paper, resolution and screening, color accuracy is the ability to take a CMYK input value, and, based on the input and output profile print a color-managed version of the patch. It is this patch that is compared to the original. How do you build a system to assess color accuracy?
Color profiles
There are tools that allow for on screen simulations of profile usage and there are techniques for “round tripping” of profiles used for printing but they don’t include the calibration, print to print variation and other potential color errors of an entire system.
Actually knowing the color errors of the entire output system allows one to assess if the color accuracy being achieved meets the target requirements.
Before you begin investigating the color accuracy of a proof print you need to define the parameters to be tested, decide on a the color metric to be used, and finally determine the requirements for internal color acceptance and/or for an industry certifying body like Fogra or IDEAlliance (SWOP, GRACol & G7).
The ISO Standard 12647-7 is the ideal place to identify what parameters are required for a proofing system to meet industry certification. For internal color accuracy testing perhaps a subset of these requirements would be adequate.
Delta-E
The most commonly used color metric to assess color accuracy is Delta E* 76, but others include Delta E*94, Delta E*00, Delta E* and CMC, each giving slightly different results but applicable for different goals.
Selecting Delta E* 76 is a good reference point to start from. If the goal is to obtain proofing certification then one needs to contact the certifying organization for their test forms and requirements.
How to investigate color accuracy
- Step 1 - select the press condition you want to simulate
- Step 2 - make sure your proofing printer has a large enough gamut to handle the simulation you are testing
- Step 3 - use the same dataset that you have collected from your color characterization target
- Step 4 - begin your test!
Harlequin RIP is Fogra certified
Investigating color accuracy
So, how is color accuracy actually investigated? The first step in the investigation is to look at the individual components on the color proofing workflow which includes the input profile, the output profile, the printing device set up including the devices color calibration, and the printer's color gamut on the media being used.
These components need to be understood as they all are additive and therefore influence the final color accuracy.
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Step 1
The first step is to select the press condition one intends to simulate in the hard copy digital proof. This comes in the form a color profile used on the input side of the color transform which can be created using industry datasets found at the ICC website (provided the profile creator is knowledgeable in the procedure), or looking at industry organizations' websites that support different printing types who often provide profiles such as SWOP, SNAP, Fogra, and GRACol to name a few.
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Step 2
The second step is to make sure the proof printer's gamut is large enough to cover the entire press gamut one is attempting to simulate. This is done by first putting the printer in an optimum state (for Harlequin RIPs this means using Global Graphics color tool SetGold Pro) and then printing a color characterization target that has been optimized for the given printing condition. The target is ideal if when viewed in 3-D gamut viewing software the patch distribution is evenly spaced throughout the printer's gamut. This insures predictive value for colors that are in those regions of the color gamut but not the exact patch colors.
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Step 3
The third step is to use the same dataset collected from the color characterization target to build an output profile which represents the best printing for that printer setup.
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Now the testing for color accuracy can begin.
To better understand the printing capability of our system the first test is to simply print two versions of the same color characterization target in succession -with the printer in the optimum state- measure them and compare them. This provides us with the color errors of our system prior to any color management. For a professional inkjet printer measuring 1426 distinct patches the color errors using the Delta E* 76 metric might be for the average 0.26 and for the maximum 2.15.
Comparing datasets
Next it makes sense to understand what errors are the results of the combination of printing and measuring of the target and the output profile .We have the dataset set from measuring the color characterization target from making the output profile and we either collected CIELAB, also known as Lab, or it can be calculated from what data was collected.
This lab data is then used to create a lab color characterization target which in combination with the proper color setup is printed and measured then compared to the original dataset. In essence we have taken patches made up of mixtures of CMYK and then based on the constraints of Black Generation, Black Threshold and Total Ink asked our color management system to find the best color match. The results from this test are usually represented in the following table and graph and the numbers here are representative.
These results show the color accuracy of our printing system including the process errors but now combined with the output profile. This is actually a difficult test because for this target 70+ patches are at the gamut edge and color mapping in this area can be quite challenging to get correct. The critical numbers to look at are the average, the maximum and the TC 95%. The average is important because it is the number that is used to determine overall proofing accuracy.
The maximum is important as it is a good indicator that the process of printing, measuring and making of the output profile did not produce any outliers indicative of perhaps a mistake or damaged print. Finally, the TC 95% is important because it is a good quality metric to determine the color accuracy potential of the output process for 95% of the represented patches. If this number is below 2.0, the generally accepted point at which color differences can be observed, then the color accuracy would be considered very good.
Lab data
| Average ∆E*76 | 0.76 | 960 | >0.5 |
| White Patch | 0.87 | 377 | >1.0 |
| Median | 0.66 | 34 | >2.0 |
| Minimum | 0.07 | 2 | >3.0 |
| Maximum | 3.78 | 0 | >4.0 |
| TC 95% ∆E* | 1.66 | 0 | >5 |
Input profile
The final test of color accuracy is to add the input profile which represents the press data that the proof is trying to simulate and print a target that matches the size of the input data set, today typically the IT8.7/4 of 1617 patches is used, measure it, then compare the measured values of the proof print against the values contained in the press dataset to actually determine the final color accuracy for the newly developed proofing system. The chart and graph above are good summaries for this round of tests as well with the additional emphasis on the white patch value as this value indicates how well the proof is simulating the paper color of the press sheet.
The results will, through the use of metrology, give us qualitative and comparative information about how the proofing system is performing and these values can be compared to proofing certification requirements.
In addition, because we tested and measured several components of our entire color system independently should our color accuracy not meet our final requirements we can investigate and perhaps improve one or more of the components to obtain an overall improvement.
