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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 colorâ€) 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 colorâ€.
Optical interference pigments are thin-film optics based on the principle of multi-beam interference of parallel plates based on the masterpiece of mankind imitating nature, and are the theoretical basis of artificial "interference colors". Thin film optics realizes the desire of humans to control and select the surface reflectance spectrum and optical properties of objects. FIG. 1 is a schematic diagram of a multilayer optical film structure (film thickness is in nanometers) (only reflected light and related physical quantities related to us 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 at the interface of the incident light on each layer of the thin film are: r1, r2e-2iδ1,..., rk+1e-2i (δ1+δ2+...+δk), where r1, r2,..., r k+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 defined (designed as needed) membrane structure, the material N and thickness d of the film and the incident medium N0 are known constants, and the surface reflectance R is only the incident light wavelength λ and incidence The function of the 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. (To be continued)
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.