Vacuum coating machine optical coating and related knowledge
There are many reasons for the optical coating of vacuum coating machine and related knowledge to affect the transmittance of a flat lens. The roughness of the mirror surface will cause the diffusion of incident light and reduce the transmittance of the lens. In addition, the material's optical rotation will cause some of the frequencies of some incident light sources to dissipate particularly seriously. For example, materials that absorb red light appear green. However, these poor processing factors can be removed as much as possible. It is a pity that there are defects in nature. When incident light passes through different media, problems of reflection and refraction must occur. If it is our normal incidence material, we can define the reflectance and transmittance.
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Many people are curious to ask: What is the transmittance of a perfect and uncoated glass? Since the transmittance of uncoated glass is not good, the transmittance should be worse after adding several layers of coating? The refractive index of the coating is actually the same as these two issues. As long as you can understand the first problem, the other nature will be solved.
According to the basic theory of electromagnetics, transmission and reflection of different media are mentioned. If the medium n1 is perpendicularly incident to n2, the reflectance = [(n2-n1) / (n1 + n2)] 2, the transmittance = 4n1n2 / (n1 + n2) 2, if the refractive index of air is 1.0, the refractive index of the coating film nc (for example: 1.5), the refractive index n of the glass (for example: 1.8) (1) direct penetration of glass by air = 4 × 1.0 × 1.8 / (1 + 1.8) 2 = 91.84% (2) entry by air Re-enter glass after coating = [4 × 1.0 × 1.5 / (1 + 1.5) 2] × [4 × 1.5 × 1.8 / (1.5 + 1.8) 2] = 95.2% Visible glass with coating will increase light transmission degree. In addition, from this formula, we can calculate that light penetrates both sides of the lens, and found that even a perfect lens (refractive index 1.8) has a transmittance of about 85%. If a layer of coating (refractive index 1.5) is added, the transmittance can reach 91%. This shows the importance of optical coatings.
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Coating thickness of vacuum coating equipment Finally, we will discuss the difference in coating thickness of vacuum coating equipment, what will be the impact? We already know that the transmittance is related to the refractive index of the vacuum coating machine coating, but it has nothing to do with its thickness. However, if we can work hard on the thickness of the coating of the vacuum coating machine, we will find that the optical path difference between the reflected light A and the reflected light B is nc × 2D. If nc × 2D = (N + 1/2) λ where N = 0,1,2,3,4,5 ..... λ is the wavelength of light in the air, which will cause the reflected light of that specific wavelength to have a phase Effect, so the color of the reflected light will change.