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Film Coating In Visual Optics

Dec 10, 2018

Film coating in Visual optics


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1.Wear resistant film (hard film)

No matter it is made of inorganic material or organic material, in daily use, the friction with dust or gravel (silica) will cause the wear of the lens, resulting in scratches on the lens surface. Compared with glass sheets, organic materials are less rigid and are more likely to produce scratches. Through the microscope, we can observe the lens surface scratches are mainly divided into two types; One is because the grit produced scratches, shallow and small, wearing glasses is not easy to detect; The other is scratches made by larger grains of sand, which are deep and rough around the edges and can affect vision in the central area.

(1) technical feature

1) The first generation of anti-wear film technology

Anti-wear films began in the early 1970s, when it was thought that glass lenses were hard to grind because of their hardness, while organic lenses were too soft to wear. Therefore, the quartz material is plated on the surface of the organic lens under vacuum, forming a very hard anti-wear film. However, due to the mismatch between its thermal expansion coefficient and the substrate material, the film is easily removed and the film layer is brittle, so the anti-wear effect is not ideal.

2) Second generation anti-wear film technology

Since the 1980s, researchers have found in theory that the mechanism of wear and tear is not only related to hardness, but the film material has the dual characteristics of "hardness/deformation", that is, some materials have higher hardness but smaller deformation, while some materials have lower hardness but larger deformation. The second generation of anti-wear film technology is to dip the surface of the organic lens coated with a high hardness and not brittle material

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 anti-reflection film. Because the hardness of the organic lens substrate is very different from that of the anti-reflection film layer, the new theory believes that there needs to be an anti-wear film layer between the two, so that the lens can play a buffering role in the sand and gravel friction, and it is not easy to produce scratches. The hardness of the third-generation anti-wear film layer is between the hardness of the anti-reflection film and the base of the lens, and its friction coefficient is low and it is not easy to crack.

4) The fourth generation of anti-wear film technology

The fourth generation of anti-film technology USES silicon atoms. For example, TITUS plus hard liquor of France etv company contains both organic matrix and inorganic ultra-fine particles containing silicon elements, which makes the anti-wear film have toughness and improve the hardness at the same time. Modern anti-wear film plating technology is the most important is to use immersion method, that is, after the lens through multiple cleaning, immersed in the hard fluid, a certain time, with a certain speed. This speed is related to the viscosity of the hard solution and the thickness of the film against wear. Mentions the oven of around 100 ° C in polymerization 4-5 hours, coating thickness of about 3 to 5 microns.

(2) Testing method

The most fundamental way to judge and test the wear resistance of the anti-wear film is to use it clinically, let the wearer wear it for a period of time, and then observe and compare the wear of the lens with the microscope. Of course, this is usually the method used before the new technology is officially promoted. At present, the more rapid and intuitive test method we commonly use is:

1) Grinding test

Put the lens in the propaganda material containing gravel (stipulate the grain size and hardness of gravel), and rub back and forth under certain control. At the end, the diffuse reflectance of light before and after the lens friction was measured with fogometer and compared with the standard lens.

2) Steel velvet test

The number of times a specified steel velvet was used to rub the houhoun on the surface of the lens at a certain pressure and speed. The number of times the houhoun was used to test the diffuse reflectance of the light before and after the lens friction with a fogometer and compared with the standard lens.Of course, we can also manually rub the two lenses with the same pressure for the same number of times, and then observe and compare them with the naked eye.

The results of these two methods are close to those of long-term wearing glasses.

(3) Relationship between antireflection film and antiwear film

The anti-reflection film layer on the lens surface is a very thin inorganic metal oxide material (thickness less than 1 micron), hard and brittle. When coated on the glass lens, the film layer is relatively easy to produce scratches because the film base is relatively hard and gravel runs across it. However, when the antireflection film is coated on the organic lens, as the film base is soft, sand and gravel run across the film layer, and the film layer is easy to produce scratches. Therefore, the organic lens must be coated with anti-wear film before the anti-reflection film, and the hardness of the two film layers must match.

2. Antireflection film

(1) Why do we need a reflective film?

1) Specular reflection

When light passes through the front and rear surfaces of a lens, it not only refracts, but also reflects. The reflected light on the front surface of the lens will cause others to look at the wearer's eyes and see a piece of white light on the surface. When taking photos, this reflection can seriously affect the beauty of the wearer.

2) "Ghost"

According to the optical theory of glasses, the refractive power of the lens will make the observed object form a clear image at the far point of the wearer. It can also be explained as the light of the observed object is deflected through the lens and concentrated on the retina to form the image point. However, because the curvature of the front and rear surfaces of the refractive mirror is different, and there is a certain amount of reflected light, internal reflected light will be generated between them. The inner reflected light will produce a virtual image near the far penalty spot, that is, near the image point of the retina. These virtual points affect the clarity and comfort of the object.

3) Glare

Like all optical systems, the eye is not perfect, and the image on the retina is not a dot but a blur. Therefore, the sense of two adjacent points is generated by two parallel 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 two-point sensation, but if the two points are too close, then the two fuzzy circles will tend to coincide and be mistaken for a point.

Contrast can be used to reflect this phenomenon and express the clarity of vision. The pair ratio must be greater than a certain threshold (the detection threshold, equivalent to 1-2) to ensure that the eye recognizes two adjacent points.

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

Where C is the contrast, the highest perceived value of the two adjacent object points on the retina is a, and the lowest perceived value of the adjacent part is b. If the contrast C value is higher, the higher the visual system's resolution of the two points is, the clearer the perception will be. If two object points are very close, and the lowest value of their adjacent parts is close to the highest value, then the value of C is low, indicating that the visual system does not feel clear about the two points or cannot clearly distinguish them.

Let's simulate such a scenario:

At night, a bespectacled motorist could clearly see two bikes coming towards him from a distance. At this point, the headlights of the cars following them reflect off the surface behind the driver's lenses: the reflected light forms an image on the retina that increases the intensity of the two points being observed (bicycle lights). So if the length of a and b increases, then the denominator (a*b) increases, and the numerator (a -b) stays the same, then the C value decreases. The result of this reduction in contrast is that the driver's initial perception of the existence of two cyclists overlaps into a single image, as if the Angle at which they are distinguished were suddenly reduced!

4) Transit dose 

The percentage of reflected light in the incident light depends on the refractive index of the lens material, which can be calculated by the formula of reflected light :R=(n-1) squared /(n-1) squared

R: single-side reflectance of the lens n: refractive index of the lens material

For example, the refractive index of common resin material is 1.50, and 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 front surface of the lens and R2 is the amount of reflection on the rear surface of the lens, then the total amount of reflection of the lens R = R1+R2. (when calculating the reflection of R2, the incident light is 100% -r1). The light transmittance of the lens is equal to 100% minus R1 minus R2.

It can be seen that if the lens with high refractive index does not have the anti-reflection film, the reflected light will bring about a strong sense of 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 the same wavelength are superimposed, the amplitude of the light wave will be enhanced. If the two waves have the same origin and different path, if they are superimposed, they cancel each other out. Antireflection film is to use this principle, the surface of the lens coated with antireflection film, so that the film before and after the surface of the reflected light interference, so as to offset the reflected light, to achieve the effect of antireflection.

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 conditions

For antireflection coatings, many lens manufacturers use highly sensitive light waves (555nm). When the coating is too thin (139nm), the reflected light appears light brown or yellow. If it is blue, it is likely to be too thick (139nm).

The purpose of coating reflective film is to reduce the reflection of light, but it is impossible to achieve no reflection of light. The surface of lens also always has the color that remains, but which kind of color that remains is best, do not have a standard actually, basically be given priority to with the individual be fond of to color at present, more it is green color department.

We can also find that the curvature of the residual color on the convex and concave surface of the lens also makes the coating speed different, so the central part of the lens is green, and the edge part is mauve red or other colors.

(3)Antireflective coating 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 the 1990s, with the development of vacuum coating technology, ion beam bombardment technology was used to improve the combination between film layer and lens and between film layers. And the extracted high-purity metal oxide materials such as titanium oxide and zirconia can be coated on the surface of the resin lens by evaporation process to achieve good anti-reflection effect.

 

The following is an introduction of antireflection film coating technology for organic lens.

(1) Preparation before coating

Lenses must be pre-cleaned before coating, which requires a high degree of molecular cleaning. Various cleaning fluids are placed in the cleaning tank, 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 lens surface again. The final cleaning is done in the vacuum chamber, during which special attention should be paid to avoid dust and garbage in the air sticking to the lens surface again. The final cleaning is done before the 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 the cleaning process, the anti-reflection film will be coated.

(2)  Vacuum coating

Vacuum evaporation process can ensure that the pure coating material coated on the surface of the lens, and in the evaporation process, the chemical composition of the coating material can be strictly controlled. The vacuum evaporation process can control the thickness of the film accurately and achieve the precision.

(3)The firmly of film

For the lens, the firmness of the film layer is very important, which is an important quality index of the lens. The quality index of lens includes anti-wear, anti-culture hall and anti-temperature difference. Therefore, now there are many targeted physical and chemical testing methods, in the simulation of the user's use of the mirror, the coated lens coating quality of the test. These test methods include: brine test, steam test, deionized water test, steel velvet friction test, dissolution test, adhesion test, temperature difference test and humidity test.

3. Antifouling film (apical film)

(1) Principle

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

(2)Process

Anti-fouling film materials are mainly fluoride, there are two processing methods, one is soaking method, one is vacuum coating, and the most common method is vacuum coating. The most common method is vacuum coating. When the anti-reflection film is finished, fluoride can be plated on the film by evaporation process. Anti-fouling film can cover the porous anti-reflection film layer, 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, so it is also called waterproof film.

For organic lenses, the ideal surface system treatment should be a composite film that includes anti-wear film, multi-layer anti-reflection film and anti-fouling film. Generally, anti-wear film coating is the thickest, about 3-5mm, the thickness of multi-layer anti-reflection film is about 0.3um, the top layer of anti-fouling wax coating is the thinest, about 0.005-0.01mm. Take the diamond crystal (crizal) of France etv company as an example, the composite film is firstly coated with silicone wear-resistant film on the substrate of the lens. Then, IPC technology was used to pre-clean the anti-reflection film before plating by ion bombardment. After cleaning, high hardness zirconia (ZrO2) and other materials were used for vacuum plating of multilayer anti-reflection film. Finally, the top film with 110 contact angles is plated. It shows that the surface treatment technology of organic lens has reached a new height.

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