Optical properties of paper
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As the most important printing material, paper has a lot of applications in printing. There are many kinds of papers and different performances. To this end, we can fully understand the various characteristics of paper and the relationship between various characteristics and the color rendering effect of printed matter, so that we can make scientific and reasonable judgment when choosing paper, and ensure that it is ideal when printing. Printing effect. The characteristics of paper can be classified into three aspects: optical characteristics, mechanical properties and printability. Optical properties include whiteness, gloss, opacity, etc.; mechanical properties include paper wire direction, thickness, basis weight, folding resistance, tearing, stiffness, bursting resistance, etc.; printability including paper strength , air permeability, smoothness, etc. In the following, we will focus on the optical properties of paper, study the factors affecting the optical properties of paper, the test methods of optical properties and the influence of optical properties on the color of printing.
1. Whiteness
Paper whiteness is the ability of paper to be totally reflected after exposure to light, that is, the whiteness of the paper. It is a function of the total spectral reflectance of the paper, the illumination energy distribution, the viewing conditions, and the observer characteristics. When light strikes the paper, only three things occur, the light is reflected by the paper, transmitted through the fiber voids of the paper, and absorbed by the fibers and filler.
The factors affecting the whiteness of the paper are mainly the dye used in the papermaking process, the whiteness of the filler, the whiteness of the slurry and the whiteness of the coating.
1.1 whiteness test method.
At present, several instruments for measuring whiteness commonly used in the world measure the reflection ability of paper on a dominant wavelength of 457 nm. The standard measurement method of paper whiteness in China is the same as the principle of TAPPI whiteness measurement commonly used in the United States, as shown in Figure 1.
The method is to irradiate the parallel beam to the surface of the paper at an angle of 45°, measure the reflectivity of the normal direction of the paper, place a color filter in the incident light to filter out the infrared light, and place a color filter in the reflected light to provide 457 nm. Blue light is used as the measuring light. The whiteness values of several papers measured using the Derrick YQ-Z-48B whiteness meter are shown in the table below.
1.2 The effect of whiteness on the coloration of printed matter
The higher the whiteness of the paper, the more the surface of the ink can accurately express the color of the ink, because the white paper reflects the color light synthesized by the color reduction of the transparent ink layer. Therefore, the white paper with high whiteness can reflect almost all the color light, making the printed ink color bright and pleasing, and the visual effect is good. The paper with low whiteness can not express the contrast of light and dark parts because it only absorbs part of the color light. Causes color cast. The closer the whiteness values of the two types of paper are, the closer the reproducibility of the color is, the smaller the color difference is, the closer the range of the color gamut is, and the better the quality of the printed product. On the other hand, if the color difference is large and the color gamut is large, it has a bad influence on the printed product.
Therefore, as a printing company, it is necessary to select a paper of a suitable whiteness for printing.
Whiteness is one of the important characteristics of paper. Under normal circumstances, two papers with slightly different whiteness and the same surface characteristics and printing ability will be put together. People will still choose the higher whiteness and judge that the quality is higher.
2. Paper gloss
Gloss is the degree to which the specularly reflected light of the paper is close to the total specular reflection ability, that is, the degree to which the surface of the paper is reflected by the incident light at a certain angle. For printing paper, gloss is a very important parameter. The papers on the market can be divided into two types: glossy paper and matte paper. The paper can be arranged as non-coated paper and light according to the gloss from low to high. Coated paper, coated paper, coated paper and cast coated paper.
The factors affecting the gloss of the paper are mainly the smoothness of the paper, whether it is calendered, the sizing degree of the paper, and the like.
2.1 method of measuring gloss.
Gloss measurements can be made using a gloss meter that compares the paper reflectance to an object of known reflectivity. When the gloss is measured, the incident angle of the selected light is different, and the result is also different. The larger the angle of incidence, the greater the specular reflectance and the higher the gloss, and vice versa. Therefore, the gloss meter can be divided into different types according to the light incident angle, such as 25°, 45°, 75° angle gloss meter. For most papers, 75° is the best measurement angle. The gloss value of the above paper was measured using a Derrick KGZ-IB gloss meter as shown in Table 2.
2.2 The effect of gloss on the color of the printed color.
The ink is dry and colored on the surface of the paper, and the continuity and uniformity after drying and conjunctiva must be affected by the surface state of the paper, resulting in different color effects. The gloss is high. Under the same conditions, the color intensity of the ink is large, and the color shift and the gray scale are small. Otherwise, the color strength of the ink layer is low, the color shift and the gray scale are relatively high, and the color overprinting gray balance control is difficult. However, for text prints and papers for writing, in order to avoid glare that irritates the eyes due to glare, resulting in a decline in reading quality and fatigue, low-gloss paper is often used. In commercial printing based on posters, high-gloss coated paper still dominates.
3. Opacity
The opacity of the paper is the ratio of the amount of light absorbed by the amount of light reflected by the paper when it is exposed to light. The contrast ratio is also commonly used to describe the opacity of the paper, which is the ratio of the diffuse reflectance of a single sheet of paper to the black body and the diffuse reflectance of the backing of the white body. Most papers are transmissive.
The factors affecting paper opacity mainly include reflection ability, wavelength of measurement light, dye, paper quantitative, apparent density (tightness), filler, slurry, and the like. For example, increasing the amount of paper can increase the opacity of the paper. The more obvious characteristics are: paper with a basis weight of 30g/m2 to 100g/m2, the opacity is most affected by the quantitative amount, and the quantitative increase is a little to increase the opacity, and the quantitative value exceeds 105g. The opacity of /m2 is limited.
3.1 Test method for paper opacity.
Use the opacity tester to accurately measure the opacity of the paper, or use an opacity meter to roughly measure the opacity of the paper, or visually compare the opacity differences between different papers. However, when using visual methods and instruments to compare the opacity of different papers, it must be realized that when the opacity of a sheet of paper is 100% or completely opaque, the eye is more sensitive to small differences in the opacity of the paper, for example, the eyes. The difference in opacity between 93% and 94% was more pronounced than the difference in opacity between 85% and 84%.
3.2 The effect of paper opacity on color rendering.
The lower the opacity, the more scattering interface between air and fiber, filler, rubber and paint, which means that the greater the porosity of the paper, the stronger the light scattering ability of the paper, and the more easily the printing is printed. Conversely, the higher the opacity, the smaller the porosity of the paper, the less the scattering interface between the air and the fibers, the filler, the compound, and the coating. The more difficult the light is to pass through the paper, the more difficult it is for the ink to penetrate into the paper. The less likely it is that a print-through phenomenon will occur. If the printing and printing phenomenon occurs during the printing process, the front and back images of the printed matter will visually interfere with each other, resulting in chaotic printing graphics, serious deterioration of the quality of the printed matter, light and defective products, and serious economic losses.
In summary, the optical properties of the paper will affect the color rendering effect of the printed matter, and the various properties of the paper are related to each other. In some cases, the mutual complementation is made up, and in other cases, the conflict may be offset. Therefore, the paper is selected. When printing, it is necessary to consider the various properties of the paper in order to select the appropriate paper in order to achieve the desired printing effect.