Understanding Tone Value
Part 5 in a 5-part series on color measurement fundamentals
The terms tone value, tone value increase, dot gain, and ΔTV are often confused. Tone value (TV) and tone value increase (TVI) are two of the most important metrics used by printers for color quality control. As industry acceptance of inline color measurement grows, printers are asking for education and clarity regarding color measurement systems. The correct use of these terms will increase confidence color quality control. The goal of this article is to explain the differences between TV, TVI, and related terminology.
- Tone Value (TV)/Dot Area: The percent of substrate area covered by ink.
- Nominal TV: The TV of a patch as defined by the RIP and generated on the plate.
- Tone Value Increase (TVI)/Dot Gain: The difference between printed TV and Nominal TV.
- Target TV: The TV an operator aims to achieve on press.
- Measured TV: The TV calculated from measurements of a printed halftone patch.
- Target TVI: The TVI an operator aims to achieve on press.
- Measured TVI: The TVI calculated from measurements of a printed halftone patch.
- Tone Value Difference (ΔTV): The difference between the Target TV and the Measured TV.
- Tone Value Increase Difference (ΔTVI): The difference between the Target TVI and the Measured TVI.
- 1-bit TIFF: The term TIFF is an image file format and stands for “Tagged Image File Format.” The term 1-bit refers to the fact that all pixels in the image are either black or white and mid-tones are represented by solid black halftone dots rather than pixels of different gray levels.
What is Tone Value (a.k.a. Dot Area)?
Halftones are specified by the percent of substrate area covered by ink. A solid area, being completely covered by ink has a 100% area coverage, specified as 100% TV or 100% Dot Area. TV and Dot Area mean the same thing and are used interchangeably throughout the printing industry. An unprinted substrate area, having no ink, has a 0% area coverage, specified as 0% TV or Dot Area. The amount of ink covering the substrate for a halftone lies somewhere between 0% and 100%. A halftone with half of the substrate area covered by dots has a 50% area coverage, specified as 50% TV or 50% Dot Area.
In a design file, such as a PDF or PostScript file, halftones are represented by a solid color with an appearance somewhere between that of a solid and substrate. This contrasts with the conventional perception of a halftone as a field full of dots. The illustration in the top half of Figure 1 shows a substrate patch, a gray halftone patch with a 50% TV, and a solid black patch. When printed, the halftone patch will be created by an array of dots, but the design itself does not contain any information about the size, shape, or structure of those dots, so is shown as a solid area.
The design PDF or PostScript file is passed through a Raster Image Processor (RIP) to transform the design into a series of 1-bit TIFF images, one for each printing unit. An example of a 1-bit TIFF image is illustrated in the bottom half of Figure 1 and represents the halftone using an array of solid black dots.
The 1-bit TIFF images output from the RIP are used to create the printing plates, one for each printing unit. Solid areas are represented by raised surfaces on the printing plate. The individual black dots in the 1-bit TIFF halftone plate image are represented by raised dots on the printing plate. The surface of each dot on the plate covers the same area as a dot in the 1-bit TIFF.
The TV specified in a 1-bit TIFF, and later used to create the printing plate, is called the Nominal TV. However, the printed TV is rarely the same as the Nominal TV. The reasons for the difference will be discussed in more detail in the next section. First, we will define how printed TV is measured. The calculation of printed TV requires three measurements: density of the substrate, density of a solid patch, and density of the halftone.
The substrate and solid densities serve as reference points for 0% TV and 100% TV. Equation 1 is the most common equation used to calculate tone value and is referred to as the Murray-Davies equation, or M-D for short. DP is the density of the substrate, DS is the density of the solid, and DH is the density of the halftone.
AVT uses the M-D equation to calculate TV in its color related application such as iDeal, SpectraLab, and Clarios.
What is Tone Value Increase (a.k.a. Dot Gain)?
As alluded to in the previous section, the printed TV is not the same as the Nominal TV. The rubber dots on the plate are squeezed and deformed under pressure between the plate cylinder and the impression cylinder as ink is transferred to the substrate. The ink strains as it is separated from the plate during transfer and forms a characteristic ring-like shape (microscopic images of dots are shown in Figure 3). Often, corona treatment of a film substrate causes the ink to hold a specific form as it sits on top of the film waiting to be dried. When printed on paper, the individual fibers suck up the ink and distribute it around the point of contact with the plate. In other words, there are many factors that affect the transfer of ink to the substrate and the resulting dot shape and area.
The effect of the physical properties listed above is the increase in the printed dot size compared to that on the plate. The result is that a 50% Nominal TV patch on a plate results in ink covering of more than 50% on the substrate. The increase in TV is called Tone Value Increase (TVI), also known as Dot Gain.
It is important to note that TVI is not unwanted but is an expected part of the printing process. A 50% nominal TV process colors could have TVIs (Dot Gains) around 20%, meaning the measured TV of the patch would actually be 70%. Controlling a halftone process means controlling TVI. As mentioned above, factors such as impression pressure, ink viscosity, and substrate corona treatment, can affect TVI.
TVI is often represented as a series of curves as shown in Figure 4, where TVI is plotted against the Nominal TV for CMYK process colors. Recalling that a Nominal TV of 0% represents that substrate with no ink present, TVI must be zero by definition. Similarly, at a Nominal TV of 100%, there TVI must be zero as ink is already completely covering the substrate.
The effect of TVI is illustrated in Figure 5. The patches shown in the top half of the figure illustrate the appearance of a substrate, 50% Nominal TV, and a 100% Nominal TV solid patch on a plate. When printed, the substrate and solid patches appear the same (no TVI) while the shapes of the halftone dots are distorted and enlarged, resulting in an increase in TV.
In practical terms, TVI is calculated for the printed halftone patch illustrated in Figure 5 by first measuring the density of the paper, halftone, and solid patches with our on-line spectrophotometry and not from an actual optical measurement of the dot and their structures. The measured densities are shown below the patches in Figure 6. The substrate density is 0.04D, the halftone density is 0.51D, and the solid density is 1.50D. The process for calculating TVI is shown in Figure 6. The measured densities of the substrate, halftone, and solid patches are shown on the left. Next, the TV of the halftone patch, 68%, is calculated using the M-D equation shown above. Lastly, the TVI, 18%, is calculated from the difference between the Measured TV, 68%, and the Nominal TV, 50%.
Use of Terminology
The confusion regarding TV lies in the difference between ΔTV and ΔTVI. When measuring color and density as part of the color quality control process, the operator selects a Target TV and the application provides a measured value, calculated from the spectral reflectance curve generated by the SMU. The target, whether it is in terms of L*a*b* or density, is the value the operator would like to achieve. The same definition is true for the Target TV when measuring a halftone – that is to say, the operator will often select a Target TV that is different from the Nominal TV knowing that it will not be possible to match the Nominal TV when printing as described above.
Consider the scenario illustrated in Figure 7a: The TV the operator specifies in the Job Setup window of the inline color measurement application is the TV they expect to measure, meaning the Target TV and not the Nominal TV of the patch. If they are measuring a patch with a Nominal TV of 50%, the Target TV might be 68%, as they know from experience and control to this point. They may also set yellow and red sensitivities for the TV measurement, perhaps 2.5% for yellow and 5% for red. A measured TV for this patch may be 70%, resulting in a ΔTV (the difference between Target and Measured TV) of 2%, within the green tolerance window. But note, this tolerance is NOT relative to the Nominal TV of the patch as defined in the artwork and plate.
The Target TV is often misunderstood to be the Nominal TV. Operators may think they are calculating dot gain, when, in reality, they are calculating ΔTV. This mistake can lead to color measurements being classified in the red tolerance window rather than green, as illustrated by the scenario in Figure 7b.
If instead of measuring ΔTV, the intention is to measure ΔTVI, then the operator must specify the Nominal TV and the Target TVI. The example in Figure 8 illustrates what a measurement of ΔTVI would entail. The Nominal TV is specified as 50%, for a 50% TV patch. It is understood that some TVI occurs, so the Target TVI is set to 18%. In other words, the operator expected the TV of the 50% patch to be enlarged by 18% when printed. The Measured TV is 70%, so the Measured TVI is 20%. The difference between the Target TVI and Measured TVI, ΔTVI, is 2%, within the green tolerance window.
This is the final in a series of five articles covering color measurement fundamentals. We recommend reading all five in order to fully understand all issues that impact color. Links to all the other articles can be found below.
AVT Color Measurement Fundamentals Series:
- Understanding Spectral Reflectance
- Understanding Light and Color Vision
- Understanding CIE L*a*b* and ΔE
- Understanding Density
- Understanding Tone Value