What Are The Main Applications Of Optical Coating? (Middle)

Jan. 10, 2020

Optical Coating Supplier, High Precision Optical Thin Film, Oem Optical Thin Film Supplier

Optical coatings are ubiquitous in our lives. From precision and optical equipment, display equipment to optical coating applications in daily life, specific optical coating suppliers will analyze and introduce them in detail.

 High Reflective Metal Coatings

High Reflective Metal Coatings

Second, the principle of thin film interference

1.Wavability of light

In the 1860s, American physicist Maxwell developed the theory of electromagnetics, pointing out that light is an electromagnetic wave, which made the theory of waves to a perfect level.

From the wave-particle duality of light, we can see that light is the same as radio waves, X-rays, and electromagnetic waves, but their frequencies are different. The relationship between the wavelength λ, the frequency u, and the propagation rate V of the electromagnetic wave is:

V = λu

Because electromagnetic waves of various frequencies travel at equal speeds in Germany, electromagnetic waves of different frequencies have different wavelengths. The high-frequency wavelength is short, and the low-frequency wavelength is long. In order to facilitate comparison, you can arrange radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays into a spectrum in order. This spectrum is called the electromagnetic spectrum.

In the electromagnetic spectrum, the longest wavelength is radio waves, and radio waves are divided into long waves, medium waves, short waves, ultrashort waves, and microwaves due to different wavelengths. Followed by infrared, visible and ultraviolet, these three parts are collectively called optical radiation. Of all electromagnetic waves, only visible light can be seen by the human eye. Visible light has a wavelength between about 0.76 micrometers and 0.40 micrometers, accounting for only a small part of the electromagnetic spectrum. X-rays again. The shortest electromagnetic wave is y-ray.

Since light is an electromagnetic wave, in the process of propagation, it should manifest its characteristics-interference, diffraction, polarization and other phenomena.

Thin film interference

High precision optical thin film can be a transparent solid, liquid or a thin layer of gas sandwiched between two pieces of glass. The incident light is reflected by the upper surface of the film to obtain the first light, the refracted light is reflected by the lower surface of the film, and the second surface is refracted by the upper surface to obtain a second light. Is coherent light and belongs to sub-amplitude interference. If the light source is an extended light source (area light source), interference can be observed only in a specific overlapping area of two coherent beams, so it is a localized interference. For planar films with two surfaces parallel to each other, the interference fringe is localized at infinity, usually observed in the focal plane of the image with the help of a converging lens; for wedge-shaped films, the interference fringe is localized near the film.

Experiments and theories have proven that interference fringes can only occur if the two columns of light waves have a certain relationship, and these relationships are called coherent conditions. The coherence conditions of the film include three points: the two light waves have the same frequency; the vibration directions of the light waves are the same; and the phase difference between the two light waves remains constant.

The optical path difference formula of the thin film interference two-coherent light is:

Δ = ntcos (α) ± λ / 2

Where n is the refractive index of the thin film; t is the thickness of the film at the point of incidence; α is the angle of refraction within the film; λ / 2 is due to two coherent beams of light at two interfaces with different properties (one is from the sparse medium to the light Medium, the other is the additional optical path difference caused by the reflection of the light dense medium to the light sparse medium). The principle of thin film interference is widely used in the inspection of optical surfaces, the precise measurement of minute angles or linearity, the preparation of anti-reflection films and interference filters.

Light is radiated from changes in the motion of atoms or molecules in a light source. Each atom or molecule emits light waves in only a short column, with a duration of about 1 billion seconds. For two independent light sources, The three conditions that cause interference, especially the condition that the phase is the same or the phase difference is constant, are not easy to meet, so two independent general light sources cannot constitute a coherent light source. Not only that, even if the light emitted from different parts of the same light source, because they are emitted by different atoms or molecules, they generally do not interfere.

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