Ink color and color difference data measurement (1)

The paper discusses the feasibility of quantitative description (measurement) of the data and the user-provided color code and the screen-printed color reproductions printed according to the color code, and the quantitative description of the ink color and color difference to make screen printing. Manufacturers can easily, quickly, accurately, and automatically monitor, measure, and determine the feasibility of the quality (color quality) of screen-printed color reproductions. It is of practical value to the color management of ink screens used by manufacturers engaged in color screen printing, and to the production process control of the workshop team and to the quality management of color printing.

Key words screen printing ink color measurement quality management

Abstratct This article discusses the capability for quantitative measurement of ink color, control patch and color print.The numerical measurement of ink color and color tolerance makes montor and judge the quality of color reproduction in screen printing even even more, speedly,presisely and automatically.It Is of use value for color matching printing process control and quality management in color printing houses.

Key words screen printing;ink;color;measruement;quality control

One, color measurement standards can be visually

In the process of color reproduction of screen printing (for example, membrane switches, thin film panels, large-scale outdoor advertising), if we only use color to describe colors, people will always feel that they have indescribable, unclear and incompetent feelings; When the quality of screen printing color reproduction products is defined, we judge whether the color of the ink is qualified or whether the color of the ink of the color reproduction is consistent with the color of the color sample (color mark, color card or physical object) provided by the customer. You will also encounter embarrassing and unpleasant situations. At this time, perhaps the user thinks that the color of the ink you produce is unqualified and the manufacturer's opinion is exactly the opposite of the user's judgment. When the supply and demand sides have the contradiction of “public opinion axiom and her husband saying justice”, the judgment is made. Is the product quality (color quality) qualified or not subject to the subjective judgment (self-perception) of the parties? As a result, can the ink color be quantified or digitized? Is it possible to judge the ink color of the product with the data standard? Is it possible to measure the color difference between a color reproduction and a user-supplied color sample and give a reasonable tolerance (color tolerance) to the color difference? Of course, as far as the actual color reproduction of screen printing is concerned, many manufacturers have been “virtual” since the beginning because there is no “bottom” in mind. They do not know that the color of their inks is different from the one provided by the user. How to measure; the so-called "monitoring" in the production process is just an empty phrase; they do not know whether the results of printing can be accepted by the customers (because they are relying on the visual and emotional sense of the colors of the formulated inks); they cannot guarantee the Several batches of product inks have the same color as the inks of the first batch or other batches of product; they do not know that the user-supplied color samples or even the PANTONE color samples have the possibility of fading (changing) colors. As shown in Figure 1) and the serious consequences of using faded color samples. Because of this, they also lack sufficient self-confidence, and it is impossible to justify the user's guarantee that the ink color of their products is qualified. They often have to look at the user’s face. When they are inconsistent with the quality of the product's certified ink color and cannot convince the customer, they can only smile at the user, say good things, or even have to sacrifice their own interests in order to solve the problem. The solution. The reason for this contradiction between users and manufacturers lies in the lack of a scientific, objective, and mutually acceptable standard for the supply and demand sides for the judgment of the ink color or whether the ink color is qualified or not.

Figure 1 Schematic diagram of the color fading case (the picture above is printed six months ago. The picture below shows the current printing)

The ancients said: "No rules can not be a radius." So, can the measurement of ink color also set a "rule"? Is the color of the ink digitally quantifiable? Is it possible to quantitatively describe the color difference (or color tolerance) of two inks with different colors or similar colors?

Second, the basic characteristics of color

The color data measurement is based on the CIE1976LAB uniform color space and its color difference formula (CIE for the International Commission on Illumination); the most basic feature of color is the theoretical foundation of color, and only a clear understanding of the basic characteristics of the color can deepen it. Understand the correct direction of ink color deployment until the desired color is recalled.

1. Hue

Hue refers to the basic appearance of color. It is the most important and basic feature that distinguishes colors from each other. It represents the difference in color quality. Therefore, hue is an important feature that indicates the different color psychological responses caused by different wavelengths of light stimulation. For example, red, green, yellow, and blue are all different hues (see Figure 2).



Figure 2 Hue

2. Brightness

Brightness is a characteristic quantity that expresses shades of color. Is the second attribute of color.

The same color of the hue, due to the different degree of reflection or transmission of incident light, the color will have a light and dark points. For example, dark green, dark green, bright green, light green and a series of green, the difference is mainly in brightness (as shown in Figure 3). For the seven colors of the spectrum, yellow is the brightest, red and green are centered, and blue and purple are dark (see Figure 2).

From the point of view of the characteristics of the color pigment itself, the color brightness of the more vivid color is moderate, and the brightness of the color is reduced or increased as the black and white components are added. When the lightness is reduced to the minimum or increased to the maximum, the color becomes black or white. For example, green, with the addition of black components, green gradually dark: dark green - dark green - black; with the addition of white components, green gradually becomes light: light green - white. It can be seen that: bright colors with achromatism (achromatism is not a matter of hue, only the difference in brightness. The white is the brightest, the black is the darkest, there is a series of gray, dark gray and light gray difference between black and white. It is the addition of components that reflect or transmit varying degrees of incident light, which produces a series of changes in brightness.

In the color classification, quantifying the lightness is using an experimental method to distinguish the lightness level (in the CIELAB color space that we will talk about later, the lightness is divided from 0 to 100; with L representing the lightness and L=100 being the most obvious That is white, L=0 is the darkest or black).

With the lightness level, a few colors can be distinguished into many different shades of color. Figure 4 shows the change in brightness due to color contrast.



Figure 3 Different brightness colors

3. saturation

Saturation refers to the degree of vividness of color. For the two colors with the same hue and lightness, there are also differences in the degree of bright colors. If a certain color has a high reflection (transmission) rate for a narrow band of light, and the reflection (transmission) rate of light for other wavelengths is very low, it means that it has a high spectral selectivity, and the color saturation is high. As shown in Figure 5, the A curve has a higher saturation of the color than the B curve.



Figure 4 Brightness Changes Due to Color Contrast



Figure 5 A, B color saturation comparison

The level of color saturation is expressed in the ratio of the color components to the achromatic components contained in the color. The smaller the achromatic component, the more pure the color, the brighter the color, and the higher the saturation of the color; conversely, the more the achromatic component, the darker the color, the lower the saturation of the color. For example, gradually adding white ink to the red ink, we will see the red ink gradually fades, this is due to the increase in the proportion of decolorization, red saturation decreases. See Figure 6 for the relationship of color saturation changes.

If a color can reflect a certain color and reflect some white light at the same time, the saturation of the color is small. The spectral colors are all monochromatic and the color is the purest. Therefore, the most vivid color is the spectral color.

4. Relationship between color three attributes

The color three attributes are three basic features that represent color.

For ease of understanding, the schematic diagram of the three-dimensional space is commonly used to explain the respective meanings of the three color attributes (see FIG. 7).

The horizontal section in FIG. 7 indicates the hue. On the circumference of the horizontal section, different hue can be surrounded by a hue circle in a certain order; the vertical axis indicates the lightness, and the upper white and the lower black are divided into different brightness levels; the horizontal axis indicates saturation, For several grades, the central longitudinal axis is the achromatic axis, and the color saturation gradually increases from the central axis. In this color space, any one color can find its corresponding spatial position according to its three attributes.

(1) Prime phase and lightness

The brightness of the same hue is determined by the ratio of black and white achromatic components. Contains more white components, higher brightness; contains more black components, lower brightness. Reflected white light has a large proportion of color, brightness is also higher. The yellow, magenta, and cyan wavelengths that reflect the spectrum 2/3 are higher than the red, green, and blue colors that reflect the 1/3 band of the spectrum. See Table 1.



Figure 6 Differences in color saturation

Table 1 Spectral light efficiency function values ​​of human eye to spectral color



Light Color Wavelength (nm) Bright Vision V(λ) Dark Vision V(λ)
Red 7000.00410.00001780
Orange 6200.38100.00737
Yellow 5800.87000.1212
Green 5500.99500.481
Qing 5000.32300.982
Blue 4710.90100.676
Purple 4200.00400.0966

The data in the table shows that the sensitivity of the human eye to spectral colors decreases in the order of green, yellow, orange, cyan, blue, red, and violet. The sensitivity of the three-colored cells of the retina to the spectral sensitivity can also be clearly seen: the human eye is most sensitive to green light, followed by red light, followed by blue light (see Figure 8). Therefore, in the case where the reflectance is almost the same, the color containing green light is always brighter, and the color containing more blue light component is darker. For example, yellow, cyan, and orange all contain green light components that look bright, while blue, violet, and red colors contain blue and red light components, whereas lightness is less.

(2) Hue and saturation

According to the theory of three primary colors, the saturation of each color is determined by the relative value between the strongest and weakest stimulation of the three color cells of the human eye. According to the experiments of Kratyakov and others, it can be known that in the spectral colors, the saturation of blue and red is very large, while the yellow and blue are