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Optical color changing film which can be used in anti-counterfeiting technology
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Anti-counterfeiting technology should have the characteristics of easy identification, safe and difficult to counterfeit, and high security threshold. At the same time, anti-counterfeiting investment should meet the principle of cost-effectiveness.
The optically variable film is an optical article designed according to the principle of optical interference of a multilayer composite film. When natural light enters the multilayer film, due to the different characteristics, thickness and interface characteristics of the film dielectric materials, some light interferences will be longer, and other light interferences will be eliminated. Therefore, film reflection can be seen from different angles. The change in color.
The film structure of the multilayer optical color-changing film can be roughly classified into two types: an all-dielectric film system and a metal-media composite film system. Among them, the all-dielectric film system has good mechanical properties and chemical properties, but it needs to be subjected to multiple coating processes to achieve the desired effect, and the color purity is slightly poor, the production efficiency is low, and the cost is high, so it is not suitable for large-area products and Mass production.
The structure of the metal dielectric film system is relatively simple, only a few layers of metal medium phase, the film can achieve the desired effect, the color change effect of the film system is very good, and can be prepared in large-area batch, low cost and high efficiency, and it is likely to become an important Optical products and anti-counterfeiting samples.
In this paper, the principle of multi-beam optical interference is used, and a simple metal-based film system is used. A suitable film structure is designed according to the needs, and a color-changing film sample is produced, so that the film can obtain different reflection spectra by changing the observation angle, thereby obtaining a reflection spectrum. The color can change visually with a significant change.
Principle of optical interference film discoloration
According to Newton's optical theory, daylight is composed of many different wavelengths of light, and the colors appear differently at different wavelengths.
In daylight, a non-transparent object absorbs incident light of a certain wavelength, and another wavelength of light that is reflected is the color that the object presents.
For a transparent object, such as a transparent film, when a certain beam of light is incident on its surface and a first layer interface is encountered, part of the beam in the incident light is reflected back at an angle, and the other part is refracted into the film, encountering The second layer interface is reflected at another angle to the first layer interface and is re-refracted. Since both the reflected light and the transmitted light are from the same light wave, the two optical beams decomposed by the incident light have different optical path differences, so when they satisfy the coherent light condition, interference of light occurs, and interference fringes between light and dark appear. .
In daylight, if you want to produce interference of light through a transparent film and eliminate light of a certain wavelength, even if the intensity of the reflected light is zero, the light of other wavelengths must be strengthened, and the above two conditions must be satisfied simultaneously, that is, the formula (1) ) and formula (2).
Fig.1 Schematic diagram of thin film interference principleFig.2 Schematic diagram of metal dielectric composite film system
However, a single-layer transparent film is formed into a multilayer optical film, that is, two kinds of medium 1 and medium 2 having a large difference in refractive index are carefully selected, and n1 is interposed between n0 and n2, and at the same time, precisely controlled The membrane conditions are such that the thickness of each deposited film conforms to an odd multiple of λ/4 of the wavelength of the incident light, and the above various problems are solved.
To further verify the feasibility of this solution, we did the relevant experiments. The experiment uses a metal dielectric film structure. As shown in Fig. 2, a metal mirror with high reflectivity is used to form an Al or Ag reflective layer, and another metal mirror with a transflective half is a chromium layer, and the intermediate dielectric layer is Al2O3. Or SiO2, etc., through calculation and simulation, determine the optimal thickness of each reflective layer that can achieve different angles of color change effect. Finally, a color-changing film sample is prepared through experiments to observe the color-changing effect.