Until the 1950s, there were no industry standards for the colors of inks used in printing.
At the time, each manufacturer of inks followed its own standards and did not share them with competitors. That was the cause of much frustration.
If inks were purchased from multiple vendors, the colors would not match; that was the origin of the Pantone Matching System (PMS). Its introduction changed the situation, thanks to the utilization of a standardized ink color book.
Today, by contrast, there are industry standards for color formulations across different printing technologies. There are also standardized color spaces, for different printing conditions to give a reference or a target at which to aim. Plus, there are different methods to follow.
Color management, which was a random process in the 1950s, has evolved into a very organized process for the graphic industry; from input—scanning, photography and desktop files—to output—printed graphics; whether offset, flexographic or digital printing. Final pieces of print might be generated by a simple desktop printer, or a high-end flexographic press.
Regardless of how a file is created or produced, customers demand accurate colors. Let’s explore the three traditional methodologies. Independently of the method used, the objective is the same: The proof to the printed image should have a common visual appearance. The three basic methods are:
- Dot gain/TVI curves based on tone scales
- Tonal curves based on Near Neutral Calibration (gray balance or G7)
- CIE Lab color management system
All of them have different benefits and levels of complexity. All require standardization—Choosing a target for colors and dot gain is essential. If the same reference is selected, results should be very similar.
Let’s explore the differences and similarities.
Dot Gain/TVI Curves
In this method, the first step consists of defining a printing reference and selecting the curves we want to aim for (see Image 1). We also need to know what inks (CMYK) will be used.
Then, we need to print tonal scales (see Image 2) during the press fingerprint trial and measure results to determine the dot gain. For example, let’s say we define the International Organization for Standardization (ISO)-B curve as our desired aim. After printing, we measure the dot gain using a densitometer (Image 3—press dot gain for the cyan color). The next step is to calculate how much we need to cut back, in this case, to match the desired ISO-B curve—and we will call this curve the adjustment curve.
Prepress applies the adjustment curve (or set of curves) to the output file for each color separation (CMYK), since each color is treated independently, and is expected to have a different curve for each color. This method has been utilized by the industry for many years and is well understood.
The shortcoming of this approach is that it does not consider gray balance, overprints, or differences in colorants and gamuts between the press and the proofing device, or as differences in substrate hue. In fact, it only uses a densitometer, so color itself is not really considered. It is a basic method and the result will depend on how close your condition is from one of the standard conditions.
Near Neutral Calibration
Mostly known as G7 Methodology—Near Neutral Calibration (NNC) is based on an optimized tonal curve, the neutral print density curve (NPDC), calculated from three-color overprints (CMY) instead of single-color tone scales.
The NPDC is a more evenly spaced tonal curve that represents ink on paper and a printing press. It is important to mention that this methodology is not specific for different ink manufactures. But the methodology will give a better gray balance and will make the images look better overall.
An individual NNC curve is applied to each separation (CMY). For the black separation, a traditional dot gain/TVI curve is applied. Matching a common tonal curve, the NPDC corrects for dot gain/TVI, ink trap and gray balance simultaneously.
To briefly explain the steps, first it is important to define the ink set to be used and try to make it match some of the industry references (see FIRST Section 20.2.2/3) and try to match the L*a*b* value of the color instead of density.
After that, we will fingerprint the press using the target known as Idealliance P2P (Image 4) instead of the tonal scale (seen in Image 2). Measuring with a spectrophotometer, we will evaluate multiple samples. The Committee for Graphic Arts Technology Standards (CGATS) recommends measuring at least six random samples. Data derived from averaging the results of multiple samples is used to create the NNC print curves. Here again, it is very likely that the curves are different for different color separations. The final step is to apply this NNC curve set to the files to make the plates.
The advantage of this methodology, compared to the TVI method, is that it considers color and gray balance. If we use ISO inks and print ISO conditions, the result will be very close to one of the ISO references. If we don’t have standardized inks, at least the result will reflect gray balanced images.
The third method is known as the CIE Lab color management system. It is a collection of software/hardware tools that quantifies and reconciles the color differences between various color output devices—monitors, scanners, proofers and printing presses—to help ensure consistent color throughout the color reproduction process.
The way it works is that, by knowing the color spaces of both devices, it is possible to reduce the bigger color space inside the smaller one and convert an L*a*b* (lightness + red/green value + blue/yellow value in a CMYK formula. By doing this, the L*a*b* value will be the same for both devices, but the CMYK formulas will be different. Colors that are outside the smaller color space won’t be reproduced.
CIE Lab-based color management uses the CIE Lab color space to quantify color, independent of the device used to produce that color. An algorithm does the conversion between them to give the same L*a*b* color (see Image 5).
Usually, when we implement a CMS system, instead of performing a fingerprint run, we turn to a press characterization run where different targets can be used, depending on the software utilized to create the press profile (see Image 6).
Once we have the press profile, or target profile, that we will input, we can use it to simulate different outputs. Monitors and proofers predict how a color will be printed at the press. The big advantage of this system is that it can be used with any printing technology and the results afford a closer representation of what will actually be printed.
To explain the difference of the three different methods, let’s say we have a label supplied by the customer as a reference to match (as seen in Image 7). Specifically:
- If we are printing with a different ink set and we only apply the TVI method, the dot gain will match, but the color might still look very different, and the image will be affected (see Image 8)
- If we apply the G7 methodology, even if the colors are not the same, the image will be balanced (like in Image 9)
- The CMS process offers the best color match (see Image 10)