Analysis of surface glossiness of prints (2)

Second, the measurement of gloss

1. Glossy representation. Although the gloss is related to the specular reflection on the surface of the object, it also interacts with the viewer's physiological and psychological state, and thus cannot be simply characterized by the physical measurement of the specular reflection. Hunter proposes six representation methods, of which mirror gloss and reflection gloss are suitable for printing and papermaking industries and have been widely adopted.

Specular gloss (Gs) is the ratio of the amount of specular reflection light (S) to the amount of incident light (I) on the surface of the object, as shown in Figure 4-31a:

Gs=S/I

When the angle of incidence is 85°, the measurement is called gloss and is generally used to describe low-gloss surfaces.

Contrast Gloss (Gc), also known as Contrast Gloss, is the ratio of the amount of specular reflection light (S) to the total amount of reflected light (D).

Gc=S/D

Figure 4-31(b) shows the contrast gloss describing the degree to which the surface of the object deviates from matte, and is particularly suitable for describing a low-gloss surface.

2. Glossy measuring angle. Measuring Gloss Using the Glossmeter, most Glossmeters measure the surface reflectivity of an object, which is specular gloss. The angle of the selected incident angle is different when measuring, and the result is also different. The larger the incident angle, the greater the specular reflectance and the higher the gloss; vice versa. This shows that the level of gloss depends not only on the surface properties of the object, but also on the measurement angle. There is no universal standard for what kind of angle measurement is appropriate. Generally, a high angle is adopted for a high-gloss surface, such as a 45° angle; for a low-gloss surface, larger angles such as 60° and 75° are measured. Many current gloss meter angles are adjustable for arbitrary selection. In addition, Hammel also found that the gloss depends not only on the measurement angle but also on the range of angles used for illumination and observation. Figure 4-32 shows the relative amount of light reflected by the print at different angles for illumination at a 45° angle. The two curves on the graph represent the same ink on coated paper and non-coated paper, respectively. As can be seen from Figure 4-32, the surface of the coated paper print is smooth and even, and when viewed at an angle of 45° (mirror direction), there is a high and narrow peak in the relative reflected light quantity. Any slight change in the angle will have a significant effect on the gloss value, so it must be very accurate. The reflectance distribution curve of the non-coated paper print is quite flat. As can be seen in Figure 4-32, the maximum reflectance of non-coated paper prints is not at an angle of 45°, but offset to about 55° in the left and right directions. This is because the surface of the paper is rough, and many irregular sloped tops are formed on the print surface. When parallel light beams are irradiated, the incident angles are not equal at each point, some are greater than 45°, and some are less than 45°. If the probabilities of these two conditions are equal, then the maximum reflectance shifts to a direction greater than 45° due to the large amount of reflected light at angles greater than 45°.

In summary, the best angle for measuring gloss is due to the paper used. The ink is different and different. Therefore, the corresponding angle should be selected according to different conditions during the measurement.

Third, the impact of print gloss factors

In the printing process, the ink film is transferred to the surface of the paper and fills the uneven surface of the paper, making the surface of the print very smooth. The specular reflection of light on the ink film surface of the printed matter determines the size of the printed gloss. The ink film surface is related to printing conditions, paper, ink properties, and the cooperation of the two.

1. Printing conditions.

1 ink film thickness. Ink film thickness is the main factor affecting the gloss of prints. After the paper absorbs the ink binder to the maximum extent, the remaining binder remains in the ink film, which can effectively improve the gloss of the print. The thicker the ink film, the more the remaining linking material, the more conducive to improving the gloss of the print. Studies have found that the tendency of gloss to increase with ink film thickness varies with different paper and different inks.

Figure 4-33 shows the relationship between gloss and ink film thickness for the same ink printed on four different papers. From the figure, it can be seen that although the inks are the same, the tendency of the gloss of prints formed by different papers to change with the ink film is different. When the ink film is thin, the glossiness of the printed matter of the high-gloss coating paper decreases as the thickness of the ink film increases. This is because the ink film masks the original higher glossiness of the paper itself, and the ink film itself forms a glossiness due to the paper. Absorbed and lower. As the ink film thickness increases, the original higher gloss of the paper itself is more difficult to reflect through the ink film. When the ink film thickness reaches 1.5 μm, the absorption of the web by the paper is substantially saturated. After that, as the thickness of the ink film continues to increase, the more of the remaining binder material, the gloss will continue to increase. The gloss of coated cardboard prints increases rapidly with the increase of the ink film thickness. After the ink film thickness increases to 3.8 μm, the gloss does not increase with the increase of ink film thickness. However, regardless of the type of paper, the amount of gloss increase with the increase of ink film thickness has a certain limit.

Figure 4-34 shows the relationship between the gloss of the prints printed on the same coated paper and the ink film thickness for a group of black inks. This group of inks uses two pigments, one having a particle diameter of about 31 [mu]m and the other about 83 nm. They were used to formulate 20%, 15%, 10%, and 5% inks, respectively. From Fig. 4-34, it can be seen that when the ink with high pigment content is used, the gloss of the printed matter does not increase as the ink film thickness increases, as does the use of the ink with a low pigment content. From Fig. 4-34, it can also be found that when the ink film thickness is 3.5 μm, the gloss of the prints formed by printing inks with different pigment contents is very different. However, when the thickness of the ink film is 6.0 μm, the gloss of prints formed using inks with different pigment contents is almost equal. In contrast to inks with large pigment particle diameters, when the ink film thickness is 3.5 μm, the prints formed with inks with different pigment content are almost equal in gloss, whereas when the ink film thickness is thick, inks with different pigment contents are used. The gloss of prints formed by printing varies greatly. Obviously, this difference is caused by the difference in the pigment diameter of the ink.

From the above discussion, it can be seen that the tendency of the gloss of the printed matter formed by printing with wood and paper and ink to vary with the thickness of the ink film is not the same, and that only a single test does not give them a general relationship of change. To accurately obtain the tendency of the gloss of the printed matter to change with the thickness of the ink film, it is necessary to compare the sizes of the glossiness formed under different ink film thicknesses.