Analysis of surface glossiness of prints (1)

First, the basic concept

1. The physical significance of gloss. Gloss as the surface properties of an object depends on the ability of the surface to reflect light. The so-called specular reflection is a reflection phenomenon in which the reflection angle is equal to the incident angle. If the surface of the object is an optically smooth surface, that is, the surface recessed gap is less than 1/16 of the incident wavelength, when the incident light is a parallel beam, the specular reflection light is also a parallel beam, and is completely independent of the color of the object itself, and the incident light is white light. The specular reflection light is still white. In theory, gloss is defined as the closeness of the mirror reflection ability of the object surface to the full specular reflection ability. For a mirror, incident light is reflected almost entirely in the mirror direction. For “matt” surfaces, the incident light is reflected at any angle the same, and so-called diffuse reflection occurs. Most prints are neither completely mirrored nor completely matte, but are somewhere in between.

In addition, gloss also has its characteristics in psychology.

2. The relationship between gloss and surface smoothness. Gloss and smoothness are the surface properties of the object, both of which depend on the microstructure of the surface of the object. As early as 1919, the physicist Chinmayanandam studied the relationship between the quantity of light reflected by the surface of an object and the surface roughness. Later, many scholars have conducted more extensive research in this area. They got the same conclusion: When the light with the wavelength λ illuminates the surface of the object at the incident angle θ, the surface roughness (also known as the root mean square roughness) expressed by the standard deviation of the peak-to-valley height distribution on the paper is σ. If the incident light intensity is I, then the intensity I′ of the specular surface reflectance of the object surface can be obtained by the following equation:

I'=Iexp[-(4πσ·cosσ/λ)]

Cate et al. specifically studied the relationship between paper gloss and root mean square roughness, and pointed out that there is a good correlation between the square of the average roughness σ2 measured by the appearance profile method and the optical method and TAPPI gloss ( The correlation coefficient is 0.91), as shown in Figure 4-28. As can be seen from Figure 4-28, the larger the σ2 value (ie, the greater the surface roughness), the smaller the TAPPI gloss. When measuring the smoothness of other methods, since the measurements are made under a certain pressure, the measured value and the gloss are between The correlation is poor. Table 4-6 shows the results of a correlation study conducted by Mitsubishi Paper Corporation. The values ​​in the table are the correlation coefficients between the 75° angle gloss value and several smoothness measurement methods. From the table, the correlation between them is poor. This reflects that the gloss is only related to the apparent smoothness of the surface of the object, and there is not much correlation with the smoothness measured in the general pressure state.

Table 4-6

Smoothness Index Bekk Smoothness Print Smoothness Surface Roughness
0.05 0.14 -0.53 -0.35

3. The relationship between gloss and color. Is there some form of connection between gloss and color? This is a concern for printers. From the available information, gloss has no effect on hue, but it has a direct impact on lightness. Brightness is one of the three elements of color, which is evaluated by the diffuse reflectance of light on the surface of the object. Thus, the diffuse reflectance can indirectly measure the lightness of the object.

Fetsko et al., by printing the same black ink on different papers, found a diminishing relationship between the lightness and gloss of the color, as shown in Figure 4-29. The data in the figure was measured using the same black ink printed on 30 different cardboard and coated papers. As can be seen from Figure 4-29, the higher the diffuse reflectance of the print, the higher the lightness, the lower the gloss. They also printed on these papers with color inks of different ink film thicknesses, and they also got the same situation, but because the inks are different, the shapes of the curves are also different.

Hammel et al. used a variety of black inks to print on the same type of paper and found some differences from the above. Figure 4-30 is a plot of the measured data printed on 28 same black inks using 28 black inks. The 28 inks use 4 types of pigments, a large particle pigment with a diameter of about 83 nm, and the remaining three diameters are between 31 and 38.4 nm. From Figure 4-30, the correlation between lightness and gloss depends on the size of the ink pigment particle diameter. Large particles of pigment, the gloss will not affect the brightness. This means that in the same brightness, the gloss of the printed matter may be high or low. Therefore, in this case, the correlation between brightness and gloss is minimal. With a small particle size, the gloss decreases with increasing diffuse reflectance. Since 31 to 38.4 nm diameter pigment particles are used in actual printing inks, the trend reflects the actual situation of printing.

In summary, there is a certain relationship between luster and lightness. This relationship can cause problems for the quality of prints. That is, color matching will lead to changes in luster of printed materials. At the same time, the two must be satisfied, and it is extremely difficult for printers to speak. of.

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