How Many Patches
We frequently deal with color management topics in a large-scale overview kind of way; This time we are narrowing in on a specific topic that most of you will run into from time to time.
How many color patches should I use in my profiling target, control strip or heat map? Like most topics in color management, the answer is "it depends". This article explores the different factors that can influence how many patches or colors you would choose to use when designing a target.
What kind of printer?
A modern inkjet printer is very stable and on most media has a predictably smooth response to color to different levels of ink throughout the color range. If you are printing through a print driver, the ink linearization has already been set down by the manufacturer. This kind of printer needs only a relatively small number of patches to adequately capture the behavior of the printer, say 1000 patches or so.
A printing press is on the other end of the spectrum, with color varying throughout the press run and across the sheet. There you would want a large number of patches and to take several samples of the press sheet throughout the run, averaging the measurements to ensure you have a good representation of the entire run.
Somewhere in the middle will be your digital presses, silver halide processes and the like. Know the shortcomings of your particular printing process and determine your patch count accordingly. 2000 patches is the approximate number to shoot for in this category.
Looking for a specific patch set?
Your profiling software will provide you with built-in or preferred targets appropriate for that software. In addition, in the printing industry there are certain collections of patches that are already well known and used. For CMYK profiling, the IT8/7-4 target has 1617 patches and is widely used as a profiling target for presses and proofers. Ironically, this popular patch set has only one neutral patch. The G7 calibration method requires a "P2P" target of 300 patches which greatly samples the neutral and near-neutral areas.
At CHROMiX we have developed several targets that combine these two into a single target called a "OneRun" target. These were originally intended for use with our Curve software, but can also be used in a variety of situations. Duplicate patches are eliminated so the patch count is minimal, and the target samples important areas of the gamut nicely.
Smoothness vs. Accuracy
As we have noted in earlier newsletters, more is not necessarily better.
It's always best to get data points around any portions of the color gamut that your printer has trouble with, or where inks are transitioning from one to another, and reacting in a sudden, different way. More patches are needed to capture this "non-linear" behavior. But having a huge number of patches (4000+) can make your transitions less smooth as the profile has to be true to every data point that was measured.
A smaller set of patches means that the profiling software will mathematically estimate the "in-between" areas and provide a smoother profile. Smoothness ensures that saturated gradients from one color to another are natural and even - not pixelated or showing obvious blocks of color.
A balance between smoothness and accuracy is needed and the number of patches used is a large contributor to the smoothness of a profile.
How important are neutrals?
Profiling software is so advanced these days that you can get excellent results using the default settings in many cases. However, if accurate neutrals are particularly vital, you can choose a patch count that intentionally over-samples the neutrals. X-Rite's i1Profiler software uses near neutral patches to "pad" the extra patches from one layout of patches to the next largest set. If you are shooting for a patch count around 1200 patches for an RGB target, for example, and you want the most neutrals, then choose 1215 to get 3 columns of near neutrals. A patch count of 1216 will give you none.
CMYK target generation is similar. Check the patch preview illustration while increasing the patch count one at a time. You can see the new patches added to the list, and you can watch when the preview "jumps" to the next largest set.
Note that you can also choose to over-sample your favorite corporate colors, or flesh tones, or other key colors, and add them to your patches. ColorThink Pro allows you to sample sections of an image at will and save them as a color list. The color list can be added to the reference colors for the patch list.
i1Profiler has a similar feature for adding image colors when making a monitor profile.
If the target is to be used for profiling, you will want to lean toward a large number of fairly small patches. For this purpose, accuracy is balanced with smoothness.
Some targets are instead going to be used in a quality control setting. These are smaller targets that you run regularly through your proofer or digital press as a control strip or a heat map. These targets will monitor daily fluctuations in printing and measurements of these targets will require a high degree of accuracy. Considering the variability and accuracy of your instrument, you will want to design your target to have patches large enough to minimize any noise in the measurements. Pass / fail results from a proofer or a heat map analysis can depend on the smallest fraction of a delta E being within or outside your shop tolerances. So it is important that your instruments and your target design are up to the task.
For example, our testing has shown that significant noise is found in iSis measurements of patches less than 1/2 inch square. A common IDEAlliance control strip had patches that were smaller than recommended by X-rite for their i1Pro device.
How many pages?
A lot of time, the exact number of patches you end up using is dependent on how many pages your target consists of. When your measurement instrument has a limit to how large a page it can measure, you may need to spread your patches across several pages. At this point you're playing a juggling game between number of patches, patch size, page count and all the other factors we have discussed so far. If your last page only has a few patches on it, consider reducing the patch size or enlarging the page margins slightly so that those patches can be reordered to not need the last page.
If you are receiving targets from a separate facility or from someone who - how do I say this gracefully? - um, who's consistency is "suspect" - then you should consider containing all your patches in one sheet. Remote profiling has enough blind spots that can cause your color to go off. When you find yourself tracking down the reason - at least you can rule out possible causes like using different paper for different sheets, or "The ink ran out and we switched ink on the second sheet", or the ever-popular: "We went to lunch between printing the first sheet and the second sheet" and so forth.
The above applies to profiling targets particularly. Control strip targets are by nature small and fit onto a portion of a sheet. Heat map targets generally fill a single page with patches to enable the cross-sheet variation to be calculated.
Once you figure out how many pages to fill, and figure out a nice balance between patch count and patch size, you oftentimes end up with a few spots left to fill. Sure, it's okay to have some extra paper white patches at the end of the target, but why not make use of that space and add some of the key brand colors your favorite customers use? You could also add an extra 300% or 400% patch so you can know where they are if you want to make spot measurements.
Proper target design is a combination of several factors, only one of which is the number of patches used. Putting them all together in a form that does it all is a satisfying little puzzle to solve. And a good target will be of service to your organization for years to come.
Thanks for reading,