Light

Optical interference color-changing inks (abbreviated as optical variable inks, also known as optical color-changing inks) are novel inks with dynamic color-changing effects that were only introduced in the 1990s. With its printed graphic, from different perspectives can change the different colors. Especially when viewed around 0° and when viewed around 60°, two very distinct and distinct colors will appear. This discoloration effect comes from the light interference pigment in the ink.
What is light interference pigment? This starts with the colors of nature. It is well known that color is the sensitometric response of the eye and brain to the visible spectrum (wavelength range 400-700 nm). When white light (such as natural light) shines on an object, we only feel the light (“reflection color”) reflected by the surface of the object, and the rest of the wavelength is absorbed by the object and cannot be perceived. The absorption and reflection of light waves by different objects are very different, and everything in the world therefore has its own characteristics. The "reflection color" is determined by the intrinsic characteristics of the object. The color of the object remains unchanged regardless of the angle of observation and the direction of the light source. In contrast, "interference color" does not belong to the characteristics of the object itself, but is the color that appears due to the physical effects of light interference. For example, a soap bubble does not absorb light and is colorless and transparent. However, when it is thick and thin, it satisfies the conditions of light interference, and it shows a rainbow of colors. The “interference color” can change color with the incident angle of light and the observation angle, which is not achieved by the “reflection color”.
Light interference pigments are masterpieces of human imitating nature.
The thin-film optics based on the parallel plate multi-beam interference principle is the theoretical basis of the artificial "interference color". Thin-film optics realizes the desire of humans to control and select the surface reflectance spectrum and optical properties of objects. Figure 1 shows a schematic of a multi-layered optical film structure (film thickness in nanometers) (only reflected light related to us and related physical quantities are discussed here, and the complicated film calculation expression is omitted). From the theory of thin-film optics, it is known that the amplitude vectors of the reflected light of the incident light at the interface of each layer of thin film are r1, r2 e −2i δ1,..., r k+1 e −2i (δ1+δ2+...+δk), where r1 and r2. ..., rk+1 is a function of the refractive indices N0 and N1, N1 and N2,..., Nk+1 and Nk on both sides of the interface, and is called a reflection coefficient. δ is the phase difference between the reflected light:
Δ1 = 2π/λ·N1d1cosθ1,..., δk = 2π/λ·Nkdkcosθk
Where: λ is the wavelength of incident light, d1, dk are the thicknesses of the first layer and the kth layer respectively, θ1, θk are the incident angles of the light in the first layer and the kth layer (θ1, θ2,... Θk is determined by θ0 and N0, N1, ..., Nk).
The reflectance R of the multilayer optical film is a function of the respective reflection coefficients r (ie, N1,..., Nk) and the phase difference δ (ie, λ, θ0 and d1,..., Dk). Based on this, we analyze the following:
1. For a fixed (designed as required) film structure, the material N and thickness d of the film and the incident medium N0 are known numbers, and the surface reflectivity R is only the incident light wavelength λ and The function of the incident angle θ0. Given a θ0 , the reflection spectrum of the film at wavelength λ0 can be obtained. That is, each defined film system has a specific reflection spectrum. The reflection spectrum will change with the incident angle θ0. This is the reason why the "interference color" produced by the artificial film system can change with the incident angle of light and the observation angle.
2. We can design the membrane system as needed, reasonably calculate the optional film material and film thickness, and change the N and d values ​​to achieve the predetermined reflectance spectrum and optical performance index. Realize the desire of people to control and select the reflective spectrum and optical properties of the object surface.
Optical interference pigments are produced by a series of pigmentation processes such as crushing, grading, and surface treatment of optical films with interference structures. It successfully used the properties of optical films to produce a variety of dynamic interference colors. This unique flake pigment maintains all optical properties of the optical film. As long as the angle of observation and the angle of incidence of the light are different, each piece of pigment will reflect a colorful change of color toward the mirror surface. The accurate use of optical interference technology allows precise control of color changes.
The optical film is formed according to the requirements of the film structure, and the materials with different refractive indexes are alternately deposited on the same carrier under high vacuum conditions. The material is evaporated into the carrier in the form of molecules or atoms. In order to ensure the uniformity and compactness of the film, the consistency of the evaporation rate must be strictly controlled; and in order to control the color index of the film system, the thickness of the film must be changed. It is controlled in the range of Angstroms (1 Angstrom = 10-1 nm = 10-7 mm). This shows the precision and specificity of optical film production.
Membrane design to meet the spectral requirements of the premise, the number of membrane layers should be as small as possible to reduce production costs. At present, the ideal light interference pigment film is a 5-layer symmetric structure as shown in FIG. 2 , with a film thickness of about 1 μm, and an opaque reflection layer in the middle (which can be regarded as a common base for two upper and lower interference chambers). It is a symmetrical transparent colorless dielectric layer and a translucent metal layer, so that both the upper and lower sides of the diaphragm have the same interference color. By changing the thickness of the transparent dielectric layer, the color of the film system can be changed to obtain various colors of light interference pigments. Since the film is made under a high vacuum, all the materials used are colorless or transparent inorganic materials, and the generated light interference pigments have stable performance, dense structure, no fading, strong weather resistance, and good shear resistance. The requirements for the particle size of the ink are arbitrary.
At present, there are interference pigments produced by chemical coating on the basis of natural mica flakes and metallic aluminum flakes. Interference pigments obtained by using the characteristics of liquid crystal flakes are also available. However, neither color nor physical properties can compare with light interference pigments. They are mainly used in the coatings industry and it is difficult to meet the demanding requirements of printing inks for flake pigments.
The dynamic color-changing effects of printed products of light-variable inks cannot be duplicated with high-definition scanners, color copiers, and other equipment;
In addition, its design and production technology are highly professional; the production process is complex;
The investment is huge; it is difficult to copy, so it is used by more than 70 countries in the world for currency security. Since the use of 100 RMB in the 99 edition, the security mark has not been copied on counterfeit money, and it has become an important publicity mark for newspapers and magazines to identify serious fake RMB. In addition can also be widely used in various types of securities, famous trademarks, documents and documents, product packaging and other printed materials security.
At present, only the United States can produce thin-film light interference pigments, and there are only a handful of companies that use U.S. pigments to produce optically variable inks. Domestic Huizhou City Sunlight Science and Technology Co., Ltd. has successfully developed and mass-produced light interference pigments and optical variable inks in batches. The products have international advanced level. Has been used in a number of famous cigarette packaging and important documents of security printing.

Huizhou Huasun Sunshine Technology Co., Ltd. 0752-2246114 Hu Jia