Tone reproduction curves

It is important to adjust for dot gain on press to achieve the best print quality, with maximum tonal separation and maximum amount of detail, particularly in midtones and shadows. You should create tone reproduction curves (TRCs) to conform to industry- or shop-quality standards for printed dot gain.
Switching to a digital flexo platemaking process from a film-based workflow means that all TRCs, for all media and line screens, must be fingerprinted again and recreated. To do this, you must have target dot gain percentages to work with.
As a flexo printer, you may not have a formal standard for press gain control and be unable to provide target dot gain percentages. You may have relied on trial and error to produce something that satisfies your customers, without having specific numbers to work with. You may have curves that represent your film targets, but may not know what you actually want to achieve on plate. The best possible solution in this case is to locate test targets that represent "best print" conditions from plates created using your previous film workflow. This will provide the Kodak flexo specialist with data that can be measured and used to create a spreadsheet with target dot gain values.
You also may not have recorded dot-gain data and pre-printed test targets that can be used to derive this data. You may have a spreadsheet that indicates how you were adjusting the exposure on a film imagesetter in order to produce suitable plates. Unfortunately, these spreadsheets are of no use for digital flexo implementation. Because of oxygen sharpening, what is exposed on the digital carbon mask is not the same as what results on plate.
Also, because of inherent dot gain in traditional film processes, what you have exposed on film is not exactly the same as what was developed on plate. There is no way to correlate previous film exposures to new ThermoFlex mask exposures.
In the absence of this data, you can derive TRCs by using an existing industry standard such as ISO 12647-6 for flexo, or a well-understood standard such as SWOP. The ISO standard is specifically directed towards flexo and represents an industry-specific standard. (FTA FIRST, Third Edition, gives ink density specifications, but no information on dot gain targets.) Although SWOP is designed for the web offset market rather than flexo, the reason for using SWOP is that SWOP is seen on a wide range of printed products on a daily basis, and more closely represents the normal visual appearance of printed products. Also, one packaging product often contains an assembly of different print products: cartons, instructional brochures, tags and labels, pouches, and so on. It could be considered desirable to have all these products display the same visual appearance for common images and content. In this case, SWOP could be a good choice.

As an example, SWOP and ISO specify the following numbers:

Gain is an absolute number applied to the nominal value. For example, a gain target of 23 means that a 50% dot will print at 73%.
In order to establish a managed dot gain workflow, you must create a spreadsheet that records values measured on mask, plate, and final press results.
You begin by exposing a test target using the bump curve created in the previous section. You confirm the mask exposure values by measuring the mask with a standard transmission densitometer and then record the values in your spreadsheet.
Alternately, you can output a step wedge to VPS and measure the output tint percentages.
With the most recent versions of Prinergy, you have the option of creating a RIP Output Values file, which captures the actual values being output into a comma-separated values (.csv) file that can be opened in Microsoft Excel software. For more on this method, see Creating a final RIP Output Values file in Prinergy.

Measuring output tint percentage in VPS 

Only a few values are recorded in this example, but you should record more detail in the highlights and shadows (for example, every 1% from 1–15% and 85–100%) and every 5% through the rest of the tone scale. The Kodak test target StepWedge_v3b, with 1% highlight and shadow detail and 5% values from quarter tones to three-quarter tones, provides a lot of detail for setting TRCs. It will also be very useful in later implementation stages, when you are optimizing Maxtone, HyperFlex, and DigiCap.

StepWedge_v3b Step Test file

Ideally, a managed plate workflow would also record the actual values received on plate—after the mask is exposed to UV light and the plate is developed. In actual practice, this is unusual, because very few printers have video capture devices capable of reading a flexo plate, such as X-RiteDot, CCDot, Betaflex, VipFlex, FlexoCam, or QEA. Even with these devices, it can be difficult to get accurate readings in highlights and shadows.
If you have a plate measuring device, after the bumped plates are washed out, dried, and post-processed, you can measure them and add the data to the spreadsheet.
It is valuable to capture this data, because if print results suddenly change, it makes it easy to determine at which point a problem is occurring. Benchmark data that contains both mask and plate values can help you to determine if a problem is occurring at the UV exposure or plate washout stage.

The most important step in determining TRCs is to print the plates on a specific substrate. The values measured here are what really count in controlling dot gain.
Add the print values resulting from the test plate to the quality control spreadsheet.

With these values you can compare what was printed on press to the shop or industry-standard dot gain targets that you have decided to use. The fingerprint data that you have gathered provides a look-up table that can be used to calculate exactly what mask exposure values produce the desired press results. It is important to note that flexo printers commonly talk about using cutback curves when outputting film to make flexo plates. With the built-in oxygen sharpening of digital flexo, many printers find it unnecessary to make cutbacks in their TRCs, and in some cases may actually have to make a push curve, where they increase the mask exposure values to attain their gain targets.
The relative positive or minus amounts are calculated, and a new TRC is created in Harmony. The same three-curve system of current, target and calibration curve is used, but the same current curve (linear plate) used previously can be used again to create the TRC (calibration curve). It is important that the TRC be anchored at the 1% value by creating a node and entering 1% as the value. This is necessary to ensure that the bump curve value is not changed in any way. You can then proceed to add or remove the relative amounts necessary to achieve the target gain results. In the following example, 3% is subtracted at the 25% point, 3% added at the 50% point, and 2% added at the 75% point.
After the curve is created, you specify it in the output process template using the Print Calibration Curve menu.

Harmony, created TRC target curve
Note: The 1% value is unchanged to prevent affecting the bump.

Prinergy, setting TRC in calibration curve
Output the test target again, with both the bump and TRC applied, and print it again on press. Record the mask, plate, and press values to confirm that the plates are being produced to the desired press standard.

Note: Fingerprint and TRC data, showing what the new mask exposure should be.

See Also

• Creating a separate tone reproduction curve
• Combining bump curves and tone reproduction curves