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  • How to coating optical films by PVD vacuum coating machine
    Oct 10, 2018

    Optical coating

    1.Wear resistant film (hard film)

    The lens, whether made of inorganic or organic materials, will be worn and scratched on the surface of the lens due to friction with dust or gravel (silica) in daily use.Compared to glass,the hardness of organic materials is relatively low, more likely to produce scratches.Through the microscope, we can observe that the scratches on the surface of the lens are mainly divided into two types. One is the scratches caused by sand and gravel, which are shallow and small.The other is a scratch created by larger grains of sand, deep and rough around the edges, which in the center can affect vision.

    (1) technical features

    1) first-generation anti-wear film technology

    Anti-wear film began in the early 1970s, when glass lenses were thought to be hard to grind because of their high hardness and organic lenses were too soft to wear.Therefore, the quartz material is plated on the surface of organic lens under vacuum conditions to form a very hard anti-wear film. However, due to the mismatch between thermal expansion coefficient and substrate material, it is easy to take off the film and brittle the film layer, so the anti-wear effect is not ideal.

    2) second-generation anti-wear film technology

    After the 1980s, the researchers found in theory that the mechanism of wear is not only related to hardness, but also that the film layer material has double characteristics of "hardness/deformation", that is, some materials have higher hardness but less deformation, while some materials have lower hardness but larger deformation.The second generation of anti-wear film technology is through the immersion process on the surface of the organic lens plating a high hardness and not brittle crack materials.

    3) third-generation anti-wear film technology

    The third-generation anti-wear film technology was developed after 1990s, mainly to solve the problem of wear resistance of organic lens coated with anti-reflection film.Because the hardness of the base of the organic lens differs greatly from that of the reflective film, the new theory suggests that a layer of anti-wear film is needed between the two, so that the lens can be buffered when subjected to gravel friction without scratch.The hardness of the third generation anti-wear film layer is between that of the anti-reflection film and the base of lens.

    4) fourth -generation anti-wear film technology

    The fourth- generation of anti-film technology is based on silicon atoms. For example, TITUS of the French company ilus has added inorganic ultrafine containing both organic matrix and silicon elements to the hard liquid, which makes the anti-wear film toughness and improves the hardness.Modern anti-wear coating technology is the most important use of immersion method, that is, after the lens after multiple cleaning, immersed in the addition of hard liquid, after a certain time, at a certain speed.This velocity is related to the viscosity of the hard fluid and to the thickness of the wear film.Mentions the oven of around 100 ° C in polymerization 4-5 hours, coating thickness of about 3 to 5 microns.

    (2) test method

    The most fundamental method for judging and testing wear resistance of anti-wear film is clinical use, which allows the wearer to wear it for a period of time, and then the wear condition of the lens is observed and compared with that of the lens.Of course, this is usually the method adopted before the formal promotion of this new technology. At present, the more rapid and intuitive testing method we commonly use is as follows:

    1) grinding test

    The lens is placed in the propaganda material containing sand and gravel (the size and hardness of the sand and gravel are specified), and it is rubbed back and forth under certain control.The diffuse reflection of light before and after the lens friction is measured with a fogometer and compared with the standard lens.

    2) steel wool test

    With a specified type of steel wool, the number of times to grind on the lens surface at a certain pressure and speed, and then test the light diffusing reflection before and after the lens friction with the mist meter, and compare with the standard lens.Of course, we can also operate by hand, rubbing the two lenses with the same pressure for the same number of times, and then observing and comparing with the naked eye.

    The results of the two methods are close to the clinical results of long-term wearing of the patients.

    3) the relationship between anti-wear film and anti-reflection film

    The antireflection film on the lens surface is a very thin inorganic metal oxide (less than 1 micron thick), hard and brittle.When coated on glass lenses, the film layer is relatively less likely to scratch because the substrate is hard and the sand and gravel across it.However, when the antireflection film is coated on the organic lens, because the substrate is soft, sand and gravel across the film layer, the film layer is easy to scratch.

    Therefore, the anti-wear film must be coated before the anti-reflection film is coated, and the hardness of the two layers must be matched.

     

    2. Anti reflection film

    (1) why is anti-reflection film plating necessary?

    1) mirror reflection

    When light passes through the front and rear surfaces of the lens, it not only refracts but also reflects.This reflected light on the front of the lens causes the wearer's eyes to see a white light.When taking photos, this kind of reflection can also seriously affect the beauty of the wearer.

    2) "ghost"

    The optical theory of glasses holds that the refractive force of the lens makes the object visible at the far point of the wearer a clear image, which can also be interpreted as the light of the object deflected through the lens and gathered on the retina to form the image point.However, due to the different curvature of the front and rear surfaces of the diopters and the existence of a certain amount of reflected light, internal reflected light can be generated between them.The internal reflected light will produce a virtual image near the far penalty plane, that is, near the retinal image point.These virtual dots can affect the clarity and comfort of the object.

    3) the glare

    Like all optical systems, the eye is not perfect, and the image on the retina is not a dot, but a fuzzy circle.Therefore, the sense of two adjacent points is produced by two parallel or more or less overlapping fuzzy circles.As long as the distance between the two points is large enough, the image on the retina will produce a sense of two points, but if the two points are too close, the two fuzzy circles will tend to overlap and be mistaken for a point.

    Contrast can be used to reflect this phenomenon, expressing visual clarity.The ratio must be greater than a certain value (detection threshold, equivalent to 1-2) to ensure the eye recognizes two neighboring points.

    The calculation formula of contrast is: D = (a-b)/(a+b).

    Where C is the contrast, the maximum imaging sensation of two adjacent points on the retina is a, and the minimum value of adjacent parts is b.The higher the contrast C value, the higher the resolution of the visual system to the two points, and the clearer the feeling;If two object points are very close, the lowest value of their adjacent parts is closer to the highest value, then the C value is low, indicating that the visual system feels unclear to the two points or cannot clearly distinguish them.

    At night, a bespectacled motorist could see clearly two bicycles coming towards him from a distance.At this point, the rear headlights of the car reflect on the rear surface of the driver's lens: the reflected light forms on the retina, increasing the intensity of the two observation points (the bicycle lights).So, the length of a and b goes up, and the denominator (a+b) goes up, while the numerator (a -b) stays the same, which causes the C value to go down.The reduced contrast results in the driver's initial perception of the existence of two cyclists overlapping into a single image, as if the Angle of the distinction were suddenly reduced!

    4) transmittance

    The proportion of reflected light to incident light depends on the refractive index of the lens material, which can be calculated by the formula of reflecting amount.

    Reflection formula: R = (n-1) squared /(n+1) squared

    R: one side reflection of the lens n: refractive index of the lens material

    For example, the refractive index of common resin materials is 1.50, and the reflected light R = (1.50-1) squared/(1.50+1) squared = 0.04 = 4%.

    The lens has two surfaces. If R1 is the amount of the front surface of the lens and R2 is the amount of reflection on the back surface of the lens, then the total reflection amount of the lens is R = R1+R2.(when calculating the reflection of R2, the incident light is 100%-r1).The transmittance of the lens T = 100%-R1-R2.

    Therefore, it can be seen that if the lens with high refractive index does not have anti-reflection film, the reflected light will bring more discomfort to the wearer.

    (2) principle

    The antireflection film is based on the wave and interference of light.If two light waves with the same amplitude and the same wavelength are superimposed, then the amplitude of the light wave is enhanced.If the two light waves are of the same origin and different wave lengths, if the two light waves are superimposed, then they cancel each other out.Using this principle, the anti-reflection film is coated on the surface of the lens, so that the reflected light generated on the front and back of the film will interfere with each other, thus cancelling the reflected light and achieving the anti-reflection effect.

    1) amplitude condition

    The refractive index of the film material must be equal to the square root of the refractive index of the lens substrate material.

    2) phase condition

    The film thickness should be 1/4 wavelength of the reference light.D = 155/4 =139nm

    For anti-reflective coatings, many eyeglass manufacturers use light waves with higher sensitivity to the human eye (with a wavelength of 555nm).When the thickness of the coating is too thin (< 139nm), the reflected light will appear light brown and yellow. If it is blue, the thickness of the coating is too thick (> 139nm).

    The purpose of the coating is to reduce the reflection of light, but it is not possible to do without reflecting light.The surface of lens also always has the color of remain, but remain color which is best, do not have a standard actually, basically be with the individual to be fond of color at present give priority to, more for green color department.

    We also find that the curvature difference of the residual color on the convex and concave surfaces of the lens also makes the coating speed different. Therefore, the center part of the lens is green, while the edge part is light purple red or other colors.

    3) plating anti-reflection film technology

    Organic lens coating is more difficult than glass lenses.Glass material to withstand high temperature above 300 ° C, and the organic lenses will be yellow when more than 100 ° C, then quickly break down.

    Can be used for glass lenses minus reflection membrane material used magnesium fluoride (MgF2), but as a result of magnesium fluoride coating process must be under the environment of higher than 200 ° C, otherwise can not attached on the surface of the lens, so the organic lenses don't use it.

    After 1990s, with the development of vacuum coating technology, the combination of film layer and lens was improved by using ion beam bombardment technology.High purity metal oxide materials such as titanium oxide and zirconia can be prepared by evaporation process and coated on the surface of the resin lens.

    The following is an introduction of anti-reflection coating technology for organic lens.

    1) preparation before coating

    The lens must be pre-cleaned before receiving the coating, which is very demanding and up to the molecular level.In the cleaning tank, various cleaning solutions are placed separately, and ultrasonic wave is used to enhance the cleaning effect. When the lens is cleaned, it is put into the vacuum chamber. In this process, special attention should be paid to avoid dust and garbage in the air to adhere to the surface of the lens.The final cleaning is done in the vacuum chamber, and special care should be taken to avoid dust and rubbish from the air before sticking 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, with argon ions). After finishing the cleaning process, the anti-reflection film will be coated.

    2) vacuum coating

    The vacuum evaporation process can ensure that the pure coating material is coated 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 control the thickness of the film accurately and reach the precision.

    3) film firmness

    For eyeglasses, the firmness of film layer is very important.The quality index of lens includes anti-wear, anti-culture and anti-temperature.Therefore, there are many specific physical and chemical testing methods to test the film fastness quality of coated lens under the use condition of the wearer.These test methods include: salt water test, steam test, deionized water test, steel velvet friction test, dissolving test, adhesive test, temperature difference test and humidity test, etc.

    Iii.

    3. Anti-fouling film (top film)

    (1) principle

    After the lens surface is coated with multi-layer anti-reflection film, the lens is especially prone to stains, which will damage the anti-reflection effect of anti-reflection film.Under the microscope, it can be found that the antireflection film layer has a porous structure, so oil is particularly easy to infiltrate into the antireflection film layer.The solution is to coat the anti-oil and water-resistant top film on the anti-reflection film layer, which must be very thin so that it does not change the optical properties of the anti-reflection film.

    (2) technology

    The anti-fouling film materials are mainly fluoride, there are two processing methods, one is immersion method, one is vacuum coating, and the most common method is vacuum coating.The most commonly used method is vacuum coating.When the anti-reflection film is finished, fluoride can be plated on the reflecting film by evaporation process.Anti-fouling film can cover the porous anti-reflection film layer, and can reduce the contact area of water and oil with the lens, so that oil and water droplets do not stick to the surface of the lens, so it is also called waterproof film.

    For organic lenses, the ideal surface system treatment should be a composite film including anti-wear film, multi-layer anti-reflection film and top film anti-fouling film.Generally, the thickness of anti-wear film coating is the thickest, about 3-5mm, the thickness of multi-layer anti-reflection film is about 0.3um, and the thinnest anti-pollution wax plating on the top layer is about 0.005-0.01mm.Take crizal, a composite film of French company, for example, the anti-wear film with organosilicon is first plated on the film base of lens.Then, IPC technology is used to pre-clean the reflective coating with ion bombardment.After cleaning, high-hardness zirconia (ZrO2) and other materials were used for vacuum plating of multilayer anti-reflection film layer.Finally, the top film with 110 contact Angle is plated.The development of the composite film technology shows that the surface treatment technology of organic lens has reached a new height.

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