Unlocking anti-counterfeiting refers to a latent image anti-counterfeiting technology that reproduces a specific image hidden in the printed unlocking base map by adjusting the angle and position of the unlocking diaphragm by means of a specific unlocking diaphragm, which is implicit anti-counterfeiting. There are many ways to unlock anti-counterfeiting in China, such as unlocking, Moir effect graphics, invisible graphics, and disturbing texts. At present, unlocking and anti-counterfeiting has been widely used in China. China's 2005 edition of 100 yuan has adopted this technology.
In this paper, the principle of unlocking anti-counterfeiting is magnified, and the concealing effect of the unlocking latent image and the factors that affect the effect of the unlocking and the two major problems that need to be paid attention to in the application of anti-counterfeiting are mentioned in detail.
The principle of unlocking anti-counterfeiting
The principle of unlocking anti-counterfeiting is relatively simple, that is, the lines on the unlocking diaphragm are used to block the reflected light, thereby changing the color depth of the latent image area in the unlocking base map. The schematic diagram of the unlocking process is shown in Figure 1. In the prepress design process, the K color version of the unlocking base map is designed with an unlocking latent image, and its partial enlarged view is shown in Fig. 2(a). The unlocking latent image is actually a embossed latent image produced by the shift of the line. Due to the high frequency of the lines on the unlocked base map, the change of the line cannot be distinguished under normal vision, so the unlocking latent image is not visible.
The unlocking diaphragm is similar to the grating sheet, and opaque lines are arranged in parallel, and the distribution frequency of the lines is the same as the frequency of the lines on the unlocking base map. When the unlocking diaphragm is superimposed with the unlocking basemap, the lines of the two are either coincident or staggered. In the coincident area, the color of the line does not change much; the staggered area and the width of the line increase, giving the impression that the line color becomes darker. The unlocking latent image is divided into two types: dark latent image and light-colored latent image. The unlocking principle of the two is shown in Figure 2(b) and (c) respectively.
The anti-counterfeiting effect of unlocking is closely related to the design and printing process of the unlocking base map. The key point is the hidden effect of the latent image when unlocked and the effect of the latent image when unlocking.
Analysis of the hidden effect of latent image without unlocking
The hidden effect of the latent image when unlocked is mainly determined by the distribution frequency and edge effect of the line.
1. Distribution frequency of lines
Unlocking basemap field lines and the lines are divided into two kinds of cable screening, the principle of both lock and unlock the same way, but the effect of unlocking the unlocking effect of the field lines than the screen of the network cable. Here, the field line is taken as an example for analysis.
When the number of screen lines is 100 lines/inch, the human eye cannot distinguish the network points and the network lines under normal viewing distance, and the same is true for the lines on the unlocking base map. When the distribution frequency of the line is higher than a certain value, the human eye cannot distinguish the specific line. The minimum spacing of the parallel lines that the human eye can distinguish is about 0.1 mm, that is, when the two lines are spaced apart by 0.1 mm, the human eye cannot distinguish two lines.
Assume that the line width on the unlocked base map is equal to the interval between the lines, and the correspondence between the distribution frequency of the lines and the line width is as shown in Table 1. As can be seen from Table 1, when the distribution frequency of the lines is 120 lines/inch, the interval between the lines is 0.106 mm (about 0.1 mm). At this time, it is difficult for the human eye to distinguish the specific lines. Feel the change in color depth. The higher the distribution frequency of the lines, the smaller the interval between the lines, and the better the hidden effect of the latent image.
2. The edge effect of lines
In order to improve the concealing effect of the latent image when unlocking, in addition to increasing the distribution frequency of the lines, it is also necessary to reduce the edge effect generated when the lines are broken. When the line breaks and shifts, the edge forms a distinct outline. In order to eliminate this edge effect, the embossing form is often used to shift the lines, so that the lines are bent and deformed, but they are displaced without breaking, and the line transition is natural, which reduces the edge effect.
If the line distribution frequency of the unlocked base map is the same as the number of screen lines, when the breakage occurs, if the dot of the edge of the latent image is broken, the edge effect will be very obvious and needs to be corrected.
In order to improve the hidden effect of the latent image when unlocking, it is usually necessary to overprint other color plates on the latent image color plate. The arrangement angle of each color plate needs to refer to the printing screen angle to prevent the potential image from being generated by the moire fringe between the color plates. Visible or reduced unlocking effect.
Analysis of the effect of unlocking latent image
When the lock is unlocked, the greater the difference in color between the latent image area and the non-latent image area, the more obvious the unlocking effect is. The two major factors affecting the color difference are the ink density and the area percentage of the unlocked base map of the ink (hereinafter referred to as "ink area percentage"). Since the dark latent image and the light-colored latent image are unlocked in the same principle, a dark latent image is taken as an example for specific analysis.
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