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the difference between vacuum coating and optical coating
Vacuum coating mainly uses glow discharge to impact argon (AR) ions on the surface of the target, and the atoms of the target are ejected and stacked on the surface of the substrate to form a film. The properties and uniformity of sputtered films are better than those of evaporated films, but the coating speed is much slower than that of evaporated films. Almost all new sputtering equipment use powerful magnets to spiral electrons to accelerate the ionization of argon around the target, increase the collision probability between the target and argon ions and improve the sputtering rate. Most metal coatings are DC sputtering, while non-conductive ceramic magnetic materials are RF AC sputtering. The basic principle is that argon (AR) ions are impacted on the surface of the target by glow discharge in vacuum, and the cations in the plasma will accelerate to the negative electrode surface as the splashed material, This impact will make the material of the target fly out and deposit on the substrate to form a thin film. Generally speaking, there are several characteristics of film coating by sputtering process: (1) metal, alloy or insulator can be made into film material. (2) Under appropriate setting conditions, the film with the same composition can be made from multiple and complex targets. (3) The mixture or compound of target material and gas molecules can be made by adding oxygen or other active gases in the discharge atmosphere. (4) The target input current and sputtering time can be controlled, and it is easy to obtain high-precision film thickness. (5) Compared with other processes, it is conducive to the production of large-area uniform films. (6) The sputtered particles are hardly affected by gravity, and the positions of target and substrate can be arranged freely. (7) The adhesion strength between the substrate and the film is more than 10 times that of the general evaporated film, and because the sputtered particles have high energy, they will continue to diffuse on the film-forming surface to obtain a hard and dense film. At the same time, this high energy enables the substrate to obtain a crystalline film at a lower temperature. (8) At the initial stage of film formation, the nucleation density is high, and very thin continuous films below 10 nm can be produced. (9) The target has long service life and can be produced automatically and continuously for a long time. (10) The target can be made into various shapes, which can cooperate with the special design of the machine for better control and the most efficient production.
Optical coating
1、 Wear resistant film (dura)
No matter the glasses made of inorganic or organic materials, in daily use, the friction with dust or gravel (silicon oxide) will cause lens wear and scratch on the lens surface. Compared with glass, organic materials have lower hardness and are more prone to scratches. Through the microscope, we can observe that the scratches on the surface of the lens are mainly divided into two types. First, the scratches caused by gravel are shallow and small, which are difficult for the wearer to detect; The other is the scratch caused by large gravel, which is deep and rough around. If it is in the central area, it will affect vision.
(1) Technical characteristics
1) First generation anti-wear film technology
The anti-wear film began in the early 1970s. At that time, it was considered that the glass lens was not easy to grind because of its high hardness, while the organic lens was too soft and easy to wear. Therefore, quartz material is plated on the surface of organic lens under vacuum to form a very hard anti-wear film. However, due to the mismatch between its thermal expansion coefficient and the sheet base material, it is easy to remove the film and embrittlement of the film, so the anti-wear effect is not ideal.
2) Second generation anti-wear film technology
After the 1980s, researchers found theoretically that the wear mechanism is not only related to hardness, but also has the dual characteristics of "hardness / deformation", that is, some materials have high hardness but small deformation, while some materials have low hardness but large deformation. The second generation of anti-wear film technology is to coat a material with high hardness and not easy to crack on the surface of organic lens by immersion process.
3) Third generation anti-wear film technology
The third generation of anti-wear film technology was developed after the 1990s, mainly to solve the problem of wear resistance of organic lenses coated with antireflection film. Due to the great difference between the hardness of the organic lens substrate and the hardness of the antireflection film, the new theory believes that there needs to be an anti-wear film between the two, so that the lens can play a buffer role when rubbed by gravel and is not easy to produce scratches. The hardness of the third generation anti-wear film material is between that of the antireflection film and the lens substrate, and its friction coefficient is low and not easy to crack.
4) The fourth generation anti-wear film technology
The fourth generation of anti-wear film technology uses silicon atoms. For example, the Titus hardening solution of France Yishilu company contains both organic matrix and inorganic ultrafine particles including silicon, so that the anti-wear film has toughness and improves the hardness at the same time. The most important modern anti-wear film plating technology is the immersion method, that is, the lens is immersed in the hardening solution after multi-channel cleaning, and lifted at a certain speed after a certain time. This speed is related to the viscosity of the hardening fluid and plays a decisive role in the thickness of the anti-wear film. After lifting, polymerize in an oven at about 100 ° C for 4-5 hours, and the coating thickness is about 3-5 microns.
(2) Test method
The most fundamental method to judge and test the wear resistance of anti-wear film is clinical use. Let the wearer wear it for a period of time, and then observe and compare the wear of the lens with a microscope. Of course, this is usually the method adopted before the formal promotion of this new technology. At present, the more rapid and intuitive test methods we commonly use are:
1) Frosting test
Place the lens in the promotional material containing gravel (specify the particle size and hardness of gravel) and rub it back and forth under certain control. After that, measure the light diffuse reflection before and after lens friction with a Hazemeter and compare it with the standard lens.
2) Steel velvet test
Use a specified steel wool to rub on the lens surface for a Hun of times under a certain pressure and speed, and then use a Hazemeter to test the light diffuse reflection before and after lens friction, and compare it with the standard lens. Of course, we can also operate manually, rub the two lenses with the same pressure for the same number of times, and then observe and compare with the naked eye.
The results of the above two test methods are close to the clinical results of long-term wearers.
3) Relationship between antireflection film and anti-wear film
The antireflection film on the lens surface is a very thin inorganic metal oxide material (thickness less than 1 micron), hard and brittle. When plated on the glass lens, the film layer is relatively not easy to scratch because the film base is relatively hard and the gravel is scratched on it; However, when the antireflection film is plated on the organic lens, due to the soft film base, the sand and gravel scratch on the film, and the film is easy to produce scratches.
Therefore, organic lenses must be coated with anti-wear film before antireflection film, and the hardness of the two films must match.
2、 Antireflection film
(1) Why is antireflection coating required?
1) Specular reflection
When light passes through the front and rear surfaces of the lens, it will not only refract, but also reflect. The reflected light generated on the front surface of the lens will make others see a white light on the surface of the lens when they look at the wearer's eyes. When taking pictures, this reflection will seriously affect the beauty of the wearer.
2) "Ghost shadow"
According to the theory of glasses optics, the refractive power of glasses will make the visual object form a clear image at the far point of the wearer. It can also be explained that the light of the visual object deflects through the lens and gathers on the retina to form an image point. However, due to the different curvature of the front and rear surfaces of the refractive lens, and there is a certain amount of reflected light, there will be internal reflected light between them. The internal reflected light will produce a virtual image near the far spherical surface, that is, a virtual image point near the image point of the retina. These virtual image points will affect the clarity and comfort of the visual object.
3) Glare
Like all optical systems, the eye is not perfect. The image on the retina is not a point, but a fuzzy circle. Therefore, the feeling of two adjacent points is produced by two parallel more or less overlapping fuzzy circles. As long as the distance between the two points is large enough, the imaging on the retina will produce the feeling of two points. However, if the two points are too close, the two fuzzy circles will tend to coincide with each other and be mistaken for one point.
Contrast can be used to reflect this phenomenon and express the clarity of vision. The contrast value must be greater than a certain value (detection threshold, equivalent to 1-2) to ensure that the eye can distinguish two adjacent points.
The calculation formula of contrast is: D = (a-b) / (a + b)
Where C is the contrast, the highest sensory value of two adjacent object points imaged on the retina is a, and the lowest value of adjacent parts is B. if the contrast C value is higher, it means that the visual system has higher resolution and clearer perception of the two points; if two object points are very close, and the lowest value of their adjacent parts is closer to the highest value, then the C value is low, which means that the visual system has lower perception of the two points The point is not clear or can not be clearly distinguished.
Let's simulate a scene: at night, a driver wearing glasses clearly sees two bicycles coming towards him in the distance opposite. At this time, the headlights of the car following him reflect on the rear surface of the driver's lens: the image formed by the reflected light on the retina increases the intensity of the two observed points (bicycle lights) Therefore, the length of segments a and B increases, that is, the denominator (a + b) increases, while the numerator (a-b) remains unchanged, resulting in the reduction of C value. The result of the reduction of contrast will make the driver's initial feeling of the existence of two cyclists coincide into a single image, just like the angle of distinguishing them is suddenly reduced!
4) Transmittance
The percentage of reflected light to incident light depends on the refractive index of the lens material and can be calculated by the formula of reflection amount.
Reflection formula: r = (n-1) square / (n + 1) square
R: Single side reflection of lens n: refractive index of lens material
For example, the refractive index of ordinary resin material is 1.50, and the reflected light r = (1.50-1) square / (1.50 + 1) square = 0.04 = 4%.
The lens has two surfaces. If R1 is the amount of the front surface of the lens and R2 is the reflection amount of the rear surface of the lens, the total reflection amount of the lens r = R1 + R2. (when calculating the reflection amount of R2, the incident light is 100% - R1). The light transmission amount of the lens t = 100% - R1-R2.
It can be seen that if there is no antireflection film for high refractive index lenses, the reflected light will bring strong discomfort to the wearer.
(2) Principle
The antireflection film is based on the fluctuation and interference of light. If two light waves with the same amplitude and wavelength are superimposed, the amplitude of the light wave will be enhanced; if the two light waves have the same origin and the wave path is different, if the two light waves are superimposed, they will cancel each other. The antireflection film uses this principle to coat the antireflection film on the surface of the lens to make the front and rear surfaces of the film The reflected light generated by the surface interferes with each other, so as to offset the reflected light and achieve the effect of anti reflection.
1) Amplitude condition
The refractive index of the film material must be equal to the square root of the refractive index of the lens base material.
2) Phase condition
The film thickness shall be 1 / 4 of the wavelength of the reference light. d= λ/ four λ= At 555nm, d = 555 / 4 = 139nm
For the antireflection film, many optical lens manufacturers use the light wave with high sensitivity to human eyes (the wavelength is 555nm). When the thickness of the coating is too thin (< 139nm), the reflected light will show light brownish yellow. If it is blue, it indicates that the thickness of the coating is too thick (> 139nm).
The purpose of coating reflective film is to reduce the reflection of light, but it is impossible to achieve no reflected light. There will always be residual colors on the surface of the lens, but there is no standard for which residual color is the best. At present, it is mainly based on personal preference for color, mostly green color.
We will also find that the different curvature of the residual color on the convex and concave surfaces of the lens also makes the coating speed different. Therefore, the central part of the lens is green, while the edge part is light purplish red or other colors.
3) Antireflection coating technology
The coating of organic lenses is more difficult than glass lenses. Glass materials can withstand high temperatures above 300 ° C, while organic lenses will turn yellow when they exceed 100 ° C and then decompose quickly.
The antireflection film material that can be used for glass lenses usually adopts magnesium fluoride (MgF2), but because the coating process of magnesium fluoride must be carried out in an environment higher than 200 ° C, otherwise it cannot be attached to the surface of the lens, it is not used for organic lenses.
Since the 1990s, with the development of vacuum coating technology, the combination of film and lens and the combination between film layers have been improved by using ion beam bombardment technology. Moreover, the extracted high-purity metal oxide materials such as titanium oxide and zirconia can be plated on the surface of resin lens through evaporation process to achieve good antireflection effect.
The antireflection coating technology of organic lenses is introduced below.
1) Preparation before coating
The lens must be pre cleaned before coating. This cleaning requires high molecular level. Place various cleaning fluids in the cleaning tank respectively, and use ultrasonic to enhance the cleaning effect. After the lens is cleaned, put it into the vacuum chamber. In this process, pay special attention to avoid the dust and garbage in the air from adhering to the lens surface. The final cleaning is in the vacuum chamber. In this process, special attention should be paid to avoid the dust and garbage in the air from adhering to the lens surface. The final cleaning is carried out before plating in the vacuum chamber. The ion gun placed in the vacuum chamber will bombard the surface of the lens (for example, argon ion). After this cleaning process, the antireflection film will be coated.
2) Vacuum coating
The vacuum evaporation process can ensure that the pure coating material is plated on the surface of the lens, and the chemical composition of the coating material can be strictly controlled during the evaporation process. The vacuum evaporation process can accurately control the thickness of the film, and the accuracy is up to.
3) Film firmness
For spectacle lenses, the firmness of the film is very important and is an important quality index of the lens. The quality indexes of the lens include anti-wear, anti Culture Museum, anti temperature difference, etc. Therefore, there are many targeted physical and chemical test methods to test the film fastness quality of coated lenses under the service conditions of simulated wearers. These test methods include: salt water test, steam test, deionized water test, steel wool friction test, dissolution test, adhesion test, temperature difference test, humidity test, etc.
3、 Antifouling film (top film)
(1) Principle
After the lens surface is coated with multi-layer antireflection film, the lens is particularly prone to stains, which will destroy the antireflection effect of the antireflection film. Under the microscope, we can find that the antireflection film has a porous structure, so oil is particularly easy to infiltrate into the antireflection film. The solution is to plating a top film with oil and water resistance on the antireflection film, and the film must be very thin so that it will not change the optical properties of the antireflection film.
(2) Craft
Fluoride is the main material of antifouling film. There are two processing methods, one is immersion method, the other is vacuum coating, and the most common method is vacuum coating. The most commonly used method is vacuum coating. After the antireflection film is completed, fluoride can be plated on the reflective film by evaporation process. The antifouling film can cover the porous antireflection film, and can reduce the contact area between water and oil and the lens, so that oil and water droplets are not easy to adhere to the lens surface. Therefore, it is also called waterproof film.
For organic lenses, the ideal surface system treatment should be a composite film including anti-wear film, multilayer antireflection film and top film antifouling film. Generally, the thickest anti-wear film coating is about 3-5mm, the thickness of multi-layer antireflection film is about 0.3um, and the thinnest anti fouling wax coating on the top layer is about 0.005-0.01mm. Taking the crizal composite film produced by the French Yishilu company as an example, a wear-resistant film with silicone is first plated on the lens substrate, and then it is carried out by ion bombardment using IPC technology
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