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Principle and application of optical chameleon effect ink



When the new version of RMB was issued on 01/10/1999, the phenomenon of optical discoloration was little known. Today, people are getting to know it. If you take a 100 yuan "red note" to buy something, you will usually see a salesperson turn around at random, which is to see the light effect to identify the authenticity. In the new version of RMB 100 yuan and RMB 50 yuan, optical color change ink is used for anti-counterfeiting for the first time in China, which reflects the advanced anti-counterfeiting trend in the world. This kind of ink seems to have a kind of inherent noble quality, and favored by the governments of various countries. In fact, it's really expensive.


1、 Principle of discoloration
The optical color changing effect of the optical color changing ink is produced by the photochromic pigment in the ink. The main component of the photochromic pigment is the photochromic film fragments with specific spectral characteristics. The optical color changing film is designed according to the light interference principle of multi-layer film. It is formed by depositing a variety of materials with different refractive index on the same carrier by using high vacuum coating technology and precisely controlling the thickness, ratio and sequence of each film layer according to the specific film structure design requirements. Its reflection spectrum changes with the change of incident angle; that is, with the change of observation angle, the light variable pigment will show different colors. Figure 1 shows the structure of a multilayer optical thin film, which is used to discuss the reflected light and related physical quantities. 

According to the theory of thin film optics, the amplitude vectors of the reflected light at the interface of each layer of film are r1、r2 e -2iδ1、…、r k+1 e -2i(δ1+δ2+…+δk), Where r1、r2、…、r k+1 are the refractive indices of N0 and N1, N1 and N2, respectively The function of, Nk+1and Nk is called the 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 and dk are the thickness of the first and k-layer thin films respectively (the film thickness is nanometer), and θ1 and θkare the incident angles ((θ1、θ2、…、θk) of light on the first and k-layer films respectively Andθ0 is composed of ( N0、N1、…、Nk).

The reflectivity of multilayer optical thin films is the reflection coefficient R (i.e. N1 And phase difference δ (λ、θ0andd1、…、dk). Based on this, our analysis is as follows:

The thickness of the incident film is defined as a function of the incident wavelength and the thickness of the incident film. Figure 2 incident angle and reflected light color. Given a θ0, we can get the schematic diagram of the film system under the condition of θ0

The reflection spectrum curve of cloth. In other words, each determined film system has a specific reflection spectrum. The reflection spectrum will change with the change of incident angle θ0. This is the reason why the "interference color" produced by the artificial film system can change color with the change of incident angle and observation angle of light (as shown in Fig. 2).


II. We can design the film system according to the needs, reasonably calculate and select the coating material and film thickness, and change the N and D values to achieve the predetermined reflection spectrum and optical performance index. To realize people's desire to control and select the reflectance spectrum and optical properties of the object surface.

The optical color changing film is produced by high precision vacuum coating process. After a series of physical and chemical treatment such as grinding, special connecting materials and additives are added to make ink. When the ink is printed on the substrate, the photochromic film particles will be arranged in parallel and on the surface of the floating ink layer, thus maintaining the color changing characteristics of the optical color changing film. This is the key of optical color change ink used for anti-counterfeiting.

2、 Product category
Optical color change ink printing has metallic luster, the color block generally presents a pair of colors. For example:
Purple to green ●dark green to purplish red

Green to red     ● green to dark blue

Red to gold      ● red to green

Gold to silver   ● yellow to green

Some colors present as many as three colors. For example: green purple yellow.

Due to the characteristics of flake and large particle size of optical discoloration pigment powder, there are some limitations in the application of printing ink. At present, the main technical mature products are: screen ink, ordinary gravure ink and engraving gravure ink.

3、 Application status
Optical color change ink has the characteristics of complex production process, highly professional design and production technology, and huge investment. Its dynamic color change effect cannot be copied by high-definition scanners, color copiers and other equipment. It has strong anti-counterfeiting reliability. Therefore, it has been used by many countries in the world for the most demanding and difficult currencies, certificates and securities Anti counterfeiting. This anti-counterfeiting technology is used in the banknotes of $50 in 1988, US $100 in 1996 and US $20 in 1998, and RMB 100 and RMB 50 in 1999. Up to now, there are six versions of the new version of counterfeit RMB coins that have been seized in China, and no optical color change ink has been found. At present, in terms of anti-counterfeiting, in addition to government use, this once "mysterious" ink has been gradually used in packaging and printing of some civil goods such as cigarettes, wine, medicine, fertilizer, etc. At the same time, because of its unique optical effect, optical color changing pigments are also used in the appearance decoration of high-end products, such as automobiles, mobile phones, etc.