There are different levels of process control that are used for print. While visual comparisons can be used to provide a rudimentary judgment of a match, they can be very subjective, and thus not very accurate, or repeatable.
Using a densitometer can provide quantitative actionable feedback for the press operator. This can include measurements of solid ink density, tone value increase (also known as dot gain), Ink Trap, and other print characteristics. These are known as mechanical print characteristics. That means that we are measuring changes as a result of a mechanical change on press. For example, a change in ink density can be a direct result of the change of ink film thickness or volume that is applied. A change in dot gain can be a result of a change in impression pressure or the blanket and packing condition. Ink trap is a measure of how well an ink transfers to a substrate with a previous ink applied.
Monitoring all of these mechanical print characteristics is a good foundation to process control and especially critical for the consistent reproduction of continuous tone images and graphics.
What is Density Measurement?
First, it is important to understand that density is not color. Density only represents how light or dark the color is, but not if the color is correct. For example, if an ink is contaminated, and the color has shifted, you may still be able to achieve the correct density value, but the color will appear different. If you need to confirm that the color is correct, such as to match a specified L*a*b* value, you will need to use a spectrophotometer.
Let’s take a look at the types of measurements that you can use to control density on press with a densitometer. First, we will look at four-color process control, then spot color ink control.
1 - Four-Color Process Control
Density is the most basic measurement of the mechanical characteristics of print. Reflective density is calculated from the amount of light that is reflected from the substrate (paper, film, etc.) and the ink. It is a simple way to evaluate changes in the ink film thickness or ink concentration that is being laid down. As ink thickness or concentration increases, more light is absorbed, and less light is reflected, so the instrument reports the darker appearance as higher density.
On an offset press, the ink pigment load is very consistent, so a change in density is directly correlated with a change in ink film thickness. On a flexographic press, the ink load may change with concentration of the ink. The addition of extender or water to flexographic ink will result in less ink pigment transferring to the substrate, and this will be lighter, with a lower density value. With evaporation of the ink carriers, the flexographic ink concentration will increase, and measure as a higher density. Using the density values of the Cyan, Magenta, Yellow and Black solid inks to monitor and correct the ink film thickness or concentration is key to press control.
Tone Value is also known as dot area. This is the measurement of printed screen tint values. It is conveyed as a percentage of the solid ink density. Tone value increase (TVI) is also commonly known as dot gain. TVI represents the difference between the printed tone value, and the tone value in the original file. For instance, if a control patch for the 50% tint is measured, and the result is a tone value of 68%, then the tone value increase is 18%. Depending on your print process and press calibration method, typical normal CMYK TVI values range from 16-22%.
Some would argue that for images, the tone value is more important than the solid ink density. Images and many graphics are made up from screen tints. In these cases, there is no solid ink within these areas. If the TVI is too high in all of the process colors the images will be too dark. Even worse, if the TVI is too high in some of the process colors and lower in others, the image will have an undesirable shift in color balance.
Ink Trap (more accurately apparent trap) is a measure of how well a wet ink will transfer to the substrate and the previous ink in the printing sequence. Higher trap percentages are better. While there is no specific standard for this, it is generally desirable to achieve 80% trap or better. This may be easier to achieve on a press with dryers in between the ink stations, like many flexographic presses, and more difficult on a press that does not, like most offset presses. Measuring trap is a way to predict a color shift in your overprint colors. Consider an image with a red car on a green lawn and a blue sky. Poor trapping characteristics will make it difficult to achieve and maintain a color match of both the blue sky (Cyan and Magenta) and green grass (Cyan and Yellow).
However, the press conditions may limit the ability to achieve good ink trapping. Several press variables can influence ink trap, some are controllable by the operator, such as press speed, or dryer settings on presses that have dryers in between press units. The ink tack may be affected by the offset press fountain solution or ink film thickness.
Some variables require changes that are beyond the operator’s control, such as the ink tack specified in the offset ink. In this case, the ink vendor may be able modify the ink with additives or even switch to a different ink system that has a different tack behavior. There are other variables that are more difficult or impossible to change, at press time, like the temperature at the press, and the substrate.
Shadow detail carries important information in many images. Print contrast values correlate well to the subjective evaluations of print quality, such as “flat” (low print contrast values) versus “jumps off the page” (high print contrast values). Compared to mid-tone dot gain values, which change with ink film thickness and therefore Density, high Print Contrast values require both high density and sharp printing to maintain shadow detail.
This can be a valuable tool for press fingerprinting to establish a density target that does not result in reduced or unstable print contrast. It can be impacted by printing pressure, blankets, and ink additives. During production, it is often used as a guide for maintaining proper ink/water balance, and a deterioration of contrast over the run may indicate a need for blanket washing.
2 - Spot Ink Process Control
Spot color density can be tricky. A traditional densitometer has built in filters that are optimized for each of the process ink colors. These filters are not optimal for all of the spot ink colors that you may run. In fact, when measuring spot colors with this traditional filter set, the densitometer may attempt to select the filter automatically. Plus, the CMY filter choice could possibly change from one measurement to the next, of the same ink. This will provide wildly different numeric results.
So, if you need to measure the density of a spot ink color, you will need a device that can measure a spot color at a spectral wavelength that is optimal for that ink. This will report the density based on the wavelength of light that is absorbed the most. This wavelength will be identified with the standard symbol for wavelength, λ, which is called lambda.
Spot Color Tone Value (SCTV) is a relatively new method for calculating the tone value of your spot color inks (ISO 20654:2017). This method uses the spectral reflectance values in order to produce tone values between the paper (0%) and solid ink (100%). A traditional densitometer cannot provide this metric, for exactly the same reasons discussed earlier that it cannot provide spot color density. SCTV tone values are also different than CMYK tone values. The math that is used for spot color tone values results in linear target values, so a 50% tint patch should measure as a 50% tone value, or 0% TVI.
Do I Need a Densitometer?
What are the options from X-Rite? The eXact Basic will provide the capability of the traditional densitometer for the basic four-color process density process control. For more advanced density functions and for spot color density and tone value, you will need the eXact Basic Plus. In the future, if you need to add more control of color, in L*a*b*, or need to align your production to a print standard, like GRACoL, SWOP, Fogra etc., then your eXact densitometer can be upgraded to an eXact Standard or eXact Advanced spectrophotometer.
Get in touch to speak with a Color Expert about your specific needs.