Density Linearization

Density Linearization

The Density Linearization determines how much ink for each color the printer applies and how much of it the material can handle. It is not concerned with the balance between the colors. Rather, it examines every color on its own to evaluate the amount of ink that is applied by the printer at different intensities and how much density that ink achieves on a material. This is necessary because, due to the ink drop size of the printer, the viscosity of the inks, and the absorption of the material, print results can vary wildly between different combinations of ink/material/printer.
The Density Linearization also needs to ensure a linear progression from 0% ink to 100% ink. A common problem is that areas of medium density (E.g., 50% black)  have too much coverage applied. This is something the Density Linearization can correct by adjusting the output to avoid such behavior.
To create a Density Linearization, the RIP has to analyze the «As is» behavior of the printer. Based on that, it can apply adjustments to the print data that counteract unwanted behavior and ensure a linear progression of density, and that the ink doesn’t bleed or soak through the material.

To achieve this, a chart is printed that contains patches of increasing density for each color. This chart is then measured using a spectrophotometer and saved in the Print Environment as the density file.

Preparation

To begin with the Density Linearization, you’ll need:
  1. A Color Measuring Device (Spectrophotometer)
  1. A Print Environment set up to connect to your printer with a port and PrintQueue
  1. The material and inks you want to calibrate are loaded in your Printer
To use the spectrophotometer, we’ll first need to detect it in the software. Go to Tools > Application Defaults > Application > Spectrophotometer. From the Spectrophotometer dropdown menu at the top of the dialog, select the measuring device you are using and click the Detect button. The RIP will establish a connection to your measuring device. If the connection is successful, it will display the type and serial number of your device. The other settings in the Spectrophotometer dialog can be left to default in most cases unless you specifically want to use a different measurement standard or if you are using a special application such as white underprinting, primer, etc. In that case, adjust the settings as required. Confirm with Ok to exit the Application Defaults window.
Note that while working with the Density Linearization Wizard, you can skip back and forth at any time. Should you need to interrupt your work on the density, you can skip to the end of the wizard and click Finish to save your current progress and settings (Though keep in mind that this will overwrite your previously saved density). Whenever you want to pick up again, simply start the Density Linearization Wizard again and skip ahead to the place you were. The exception here is in-progress measurements, as they are not saved to the density until completed. If you need to quit during a measurement, you can save it separately in the measure dialog, though.

First Linearization

Printing the Chart

When you are ready to begin the first linearization, click the Density Linearization Icon in the Toolbar or go to Tools > Linearization & Profile Tools > Density Linearization to start the Density Linearization wizard.
The first window lets you configure your Density Linearization chart.


Initial Values lets you set an initial default density adjustment to optimize your first measurement. Selecting No Estimate will not make any assumptions about the printer behavior and will let you manage from scratch. When using 540dpi or less, we recommend using Standard Adjustment for Low resolution, while for 720dpi or more, we recommend Standard Adjustment for high resolution.
Now we’ll need to set the right chart Geometry for our measuring device. Different measuring devices require different types of charts so they can understand them, and many have their own specific chart layout. From the dropdown menu, select the geometry for your device by name.
Set the number of patches in the No of Patches per Channel. This determines how many color patches are printed per color. So, e.g., setting 20 means that the chart will contain 20 Cyan patches, 20 Magenta patches, etc.
When setting the number of patches, the recommended number depends on how many colors and the strengths of individual colors you have. If you are using light inks, printing more patches can be helpful for the software to really get a good grasp on how the inks behave together.
If you use only one «strength» per ink (E.g., CMYK), we recommend setting it to about 30 patches. If one or more colors appear in two strengths (E.g., CMYK Light Cyan, Light Magenta), we recommend setting about 40 patches. If one or more colors appear in more than two strengths (E.g., CMYK Light Black 1+2), we recommend setting about 60 patches.
To avoid wasting material on misprints of individual patches or areas of the chart, you can use Print multiple Charts across Job Width and Print additional Charts rotated 90 degrees across Job width. This prints the same chart across your material multiple times to increase chances that each color patch is properly printed at least once, so it can be measured if you have a difficult printer on your hands.
Print mixed patches for scattering prediction adds additional mixed color patches to the density chart that are measured along with the regular density calibration patches. The spectral readings from these patches can be used during color profiling in the ColorGPS *Ink Mixing Model* feature to calculate the recommended light scattering factor for textured or matte substrates.
When you are ready, you can use the Test Chart Size button to have the RIP calculate if a chart with your settings would fit the material, how many pages it would require, etc.
Next, click Print to generate the Density Chart and send it to the printer. The chart behaves like any other in that it will go straight to the PrintQueue and wait to be sent out from there. If it isn’t already, set your PrintQueue to Online to send the Job on its way.
After sending the Job, you’ll notice a Chart ID appearing next to the Measure button. This is used to identify the chart you just printed and is also printed at the top of the density chart. Let the density chart printout finish. Double-check that the Chart IDs match and click Measure.
Note on Resolve Problems with Calibration Chart: If you notice problems in the printout for the first chart, such as overinking, making it impossible to read the chart properly, Resolve Problems with Calibration Chart has some tools that can
help adjust the first chart to make it readable. For more information, check the Troubleshooting section at the end of this document.
 

Measure Calibration Chart

Now that the Chart is printed, take it out of the printer and get your measuring device ready. Click the Measure button in the Density Linearization window to reach the Manage Measurements dialog.


On the top left, you’ll see your Color Chart Pages list. If your chart is broken up into multiple pages, you’ll see all of them here. This is also where you select which chart you want to measure, so make sure to select the first one before beginning your measurement.
On the bottom left, you’ll see the Measurements section, which lists all the measurements you conducted for this page in this session so far. At first, you’ll only see the expected listing as there are no actual measurements yet. When you have more measurements, you can toggle them to be visible or not visible in the preview in the center of the dialog.
At the center, you’ll see the Color Patches. Initially, they will only show the expected values for each color patch. But as you conduct measurements, they will be added to the view so you can compare the measured values with the expected ones and quickly spot outliers and measurement errors.
At the top right, you’ll see the section dedicated to showing information about the currently selected patch. When you have measurements available, it will show what values were measured and how your measurements compare to the expected values by numbers.
The Spectrophotometer button brings up the measuring device detection again. This way, you can swap measuring devices when needed, redetect it if the connection is lost, or just detect your Spectro if you missed connecting to it in Application Defaults > Tools.

Click Measure... to begin measuring. Since we have already detected the spectrophotometer, it should automatically be recognised and ready (Though it might ask for a white calibration).
The Next Measurement Section shows you which line is to be measured next. You can switch between Single Measurement (Measure every patch individually) or Scan measurement using the buttons to the right.
 The Measurement Values section shows you the last measured line and both the measured values (Top) and the expected values (Bottom). On the bottom right, you’ll also see the DeltaE difference between measured and expected for the currently selected pair.
Once you have measured all the lines, you can click Ok to save the measurements and go back to the Manage Measurements dialog. From here, we strongly recommend starting another measurement for the same chart page. Repeat this for every chart page you have until you have at least two measurement passes for each page. This allows the RIP to create an average between the different measurements and eliminate accidental discrepancies and measurement errors.

Since Patch Release 17.6.2.0282, the Measurement Management Wizard has been greatly improved. When you need to manage three (or more) measurements per color patch to increase and stabilize the measurements before passing them through the respective color tool feature (e.g. Density Correction, ColorGPS, SwatchMatcher), instead of comparing the measurements to the first measurement, an average of all measurements is used to identify potential errors (based on a customizable dE limit compared to the average). The Delete Marked Button now supports quickly cleaning up outliers. An additional algorithm called Intelligent Detection (recommended to use) makes sure that there are always at least two measurements left to build a more stable average.


When you are finished, click Ok in the Manage Measurements dialog to have the RIP calculate the average between all the measurements and take your measurement results back to the Linearization wizard.
Click Next in the Wizard until you reach the Save dialog. We recommend not making any changes to the measurement results curve or the Target Density at this point, as we will do another pass through the density to refine the results.
In Save, fill out the fields as you see fit and click Finish to complete the first measurement. The Density Linearization will now automatically be saved into your Print Environment as an embedded density.
But we’re not quite done yet!

Second Linearization

Now that we have the first Density Linearization, we’ll refine it further by doing a second one that builds on the first.
Start the Density Linearization dialog again while still having the same Print Environment selected. If a density is already present, the Linearization wizard will automatically take it into account when starting Density Linearization again.
This time, we won’t need to set an Initial Value since we have the first linearization to go from. Generate a new Chart using the same Geometry as before, print it, and repeat the steps from the first linearization until you return to the Density Linearization wizard after the measurement and reach the window with the Measurement Results curve.

Measurement Results and Limiting Inks

On the left side of the dialog, you’ll see the measurement results displayed as a rising curve where each dot on the curve represents a measured patch. The horizontal Axis represents the number of printed pixels, while the vertical axis represents the optical density. Ideally, we want this to be an even curve rising steadily to the maximum value without too many dips or variations in between. By default, the chart will show the values in optical density, but you can also switch the display to relative dot size using the radio buttons to the right.
On the right side, you’ll see the numerical values for the individual measurements. The identifier is the ID of the patch, so you can find the corresponding patch on the chart. The density represents the measured material coverage, and the dot size is the measured size of the dot. Chroma represents the measured Chroma value for this patch.
Additionally, you can use the Show values dropdown to change the displayed values for the patches from density (default) to Lab and LCh.
By clicking on one of the dots in the left curve, you also directly select its value in this list.

What we want to do here is to eliminate outlier values that are likely caused by measurement errors or misprints that aren’t representative of the actual printer behavior. To do this, look for any zigzags in the curves or dots in the curve that look obviously out of place. These are most likely measurement errors and should be deleted by selecting them and clicking Delete Point.
While we mentioned above that the ideal would be a steadily rising curve, that is often not quite the case, and we don’t want to change the curve into one. The purpose of the measurements is to examine how the printer actually behaves so it can be corrected, so avoid the temptation to remove points just to make the curve linear. Dips and rises are fine as long as they are organic and reflect the printout.
You can also use the Auto Correction button to have the RIP automatically remove any measurement values it thinks are measurement errors. This is, of course, not as flexible as checking yourself and may accidentally remove legitimate measurement values, so manual is usually the recommended way.
Now that we have a curve that’s representative of what the printer's output looks like, we’ll want to ensure that the rise in density is a steady progression. In many cases, you’ll find that the density curve will rise until the highest point toward the end, and then either go flat or go down for the last couple of percentages. Since we want the density to be a steady progression, we’ll want to get rid of that last part so the more printed pixels always correspond with more density. Setting limitations to maximum density like this optimizes ink usage and helps prevent overinking in mixed colors.
Switch between the individual measured colors using the dropdown menu at the top, and set limitations where the optical density falls off, stays relatively the same towards the end of the curve. To do this, select the last patch before the curve drops or evens out and press the Limit at selected Patch button. This will add a red line to the curve showing where the limitation is. The density will now cap ink usage at this point, setting it as the new 100% value for this ink channel. Repeat this for all channels.
You can also use the Auto-Limit button to let the density wizard do a limitation on all channels for you, but keep in mind that this will likely not be as optimal as doing it by hand.
To further evaluate your measured density, you can use the Show Gamut button to open the Gamut Viewer. This lets you compare the measured density with other color spaces such as ICC profiles, Print Environments, Swatchbooks, and images in 3D space.
When you are satisfied, click Next to move on.

Dot Gain

Now we’ll deal with the Target Density and Dot Gain. This tells the RIP how you want the density to behave ideally. The RIP will then use the measurements we just conducted to calculate the difference between actual and target printing behavior and then tweak the output data accordingly to get as close to the target as possible.
Generally, we recommend going with the default setting of 20 for the Dot Gain curve, as they reflect the standards of offset printing, making it optimally suited for CMYK printing.
Should you wish to make changes to the Dot Gain curve, you can do so using the ruler at the bottom of the dialog, or by activating Advanced mode at the top and then manipulating the curve directly.
If you find yourself using the same custom Target Density often, you can use the Import/Export buttons to save the settings as an external file and quickly apply them by importing them again as needed.

When you are satisfied, click Next to proceed. We’re almost there! On the last screen of the linearization wizard, we can edit the description of the Density Linearization.
You can also use the Visual Test Printout function to print a test chart to visually evaluate the quality of your Density Linearization and make sure you are happy with the density progression. Each patch is distributed uniformly across the available density range from 0 to 100% to assess the linearization correction curves visually.
In the Visual Test Printout section, click Test Print… and in the opening dialog, set how many patches per color you want to print. Confirm with Ok to send the test print to the printer.

Click Finish to complete the Density Linearization. The Density Linearization in your Print Environment will then be overwritten with the new, refined one.

Troubleshooting

High Density in the middle areas

If you find your output to be too strong in mid-tone areas, you can work against this by adjusting the Dot Gain before the first measurement using the Resolve Problems function in the first window of the Density Linearization. This lets you use the Dot Gain ruler to adjust your expected Dot Gain for each color individually to counteract excessive ink in midtones. Note that this feature can only be used before you have done your first measurement.

Overinking or Flaking before the first Linearization

If you notice your ink bleeding or flaking so you cannot measure the chart at all, you can limit the Ink Application using the Resolve Problems function in the first window of the Density Linearization. This lets you set limits for Ink Application in the individual channels before printing the chart. This way, you can restrict how much ink each channel can apply to get to a measurable result. Note that this is not a replacement for the ink limitation after measuring; it should only be used to get the chart to a measurable amount of ink application.

Measurement Problems due to varying Print Quality

Most printers show a variance in their printing behavior due to passes, different amounts of ink application in different areas, etc. Usually, this is not an issue for printouts as they are viewed in their entirety. However, they can sometimes cause issues when measuring, since spectrophotometers measure with pinpoint accuracy, so different applications within a patch can invalidate a measurement entirely. Luckily, we have some tools to work around this. If you notice high variance in your measurement results and a lot of outlier measurements, try the following:
Measure the chart multiple times. We already recommend measuring every chart at least twice. When you notice measuring issues like above, try doing more measurements of each line and hitting different spots in the patches every time. This lets the software get a larger number of measurements to average from and can help alleviate such issues.
Use Print Multiple Charts across Job Width and Print additional Charts rotated 90 Degrees across Job Width in the first dialog of the Density Linearization wizard. This will print additional charts in different orientations, letting you select the best one to measure. You can also measure the best lines from different charts to get the best values. You can also measure the same line on multiple charts to get average values between different printouts of the same chart.

Using Non-Supported Measuring Devices

Unfortunately, the Ergosoft RIP can not support all measuring devices available on the market. However, even non-supported measuring devices can be used together with the Ergosoft RIP if they are able to measure spectral values using their own software and export the measurements into an ASCII file.

ASCII Syntax for Spectral Data

The ASCII file containing the measurement results needs to follow a specific syntax, so some tweaking might be required to bring the data into the proper format, depending on how your measuring device stores the spectral data. The supported syntax is as follows.
The first line of the file is reserved to contain comments or designations and is not processed by the Ergosoft RIP.
All following lines need to begin with “Spec_400_10_700“, followed by the spectral values of 400 until 700 (incl.) in percent and including two decimals after the comma, with a step size of 10 (31 values). The data has to be separated by spaces or tab-stops; empty lines will be ignored. The patches have to be listed in the order that the ErgoSoft RIP expects them: color by color, beginning with field A1.
As an example, here's what the first two lines of such a file would look like:

Density values
Spec_400_10_700  22.04  27.58  29.44  28.59  27.29  25.29  24.26  29.33  44.71    62.95  74.24  78.47  79.60  80.22  80.89  81.27  81.63  81.88  82.55  83.27  83.72     83.83  84.23  84.91  85.98  86.86  87.24  87.37  87.29  87.04  87.43

ASCII Syntax for Densitometer Data

Should your measuring device only be able to measure density data rather than spectral data, you can also use the densitometer format in the ASCII file to import the data. Though note that this will only work for the density, not for creating color profiles in ColorGPS.
The first line of the file is reserved to contain comments or designations and is not processed by the Ergosoft RIP.
All the following lines begin with the letter “D“, followed by the density value, which is separated from “D“ through a space or tab-stop. The patches have to be listed in the order in which they are expected from the ErgoSoft RIP: Column-wise from the upper left to the lower right. Only the density values of one color are used.

As an example, here's what the first 10 lines of such a file would look like: 
Density Values
D   0.000
D   0.070
D   0.279
D   0.488
D   0.662
D   0.800
D   0.995
D   1.441
D   1.593

Importing the ASCII file

To import an ASCII file containing numeric measurement values, set up your Measuring Device as From File either in the Measurements Management dialog or in Tools > Application Defaults > Application > Spectrophotometer. This will tell the RIP to fetch a file when measuring starts. When From File is the active measuring device, pressing Measure in any dialog will bring up a file browser, letting you select your ASCII file. Confirm the file to import it into the Ergosoft RIP. If your Syntax is correct, it should then populate your list with the measurement results from your file.