Common optical coatings can be divided into three types: anti reflection coatings, also known as AR coatings or anti reflection coatings; Ultra low reflection coating; Chemical coating. The first two are used to reduce the surface reflectivity, while the third is used to change the surface characteristics of the lens.
AR coating
The first type is a conventional anti reflection film layer, aimed at reducing surface reflection and thus reducing stray light in the lens.
The basic principle of anti reflection film is that the phase of the reflected light at the two interfaces of the film layer is exactly opposite, causing them to cancel out each other, thereby reducing the intensity of the reflected light. This is also known as "destructive interference" that we encounter when learning the refractive index. Of course, a single layer of coating can only achieve effective anti reflection at one wavelength position, but typically the working wavelength of a lens has a certain range, which is why actual anti reflection coatings may have several to dozens of layers.
The design of anti reflection films is similar to optical design, with performance indicators being the reflectivity at various wavelengths and incidence angles, and design degrees of freedom being the refractive index and thickness of the material. There are also specialized film design software available to optimize the composition of the film system.
Schematic diagram of interference cancellation
The coating method of AR film is usually vacuum electron beam evaporation. That is to place the lens in a vacuum chamber, and then heat the coating material with an electron beam. The coating material evaporates after being heated, and the target molecules are evaporated onto the surface of the lens. In fact, this process is much like when steaming steamed buns, the hot water in the pot will condense on the lid of the pot, but the water here is replaced with a coating material. These materials are usually dielectric materials with different refractive indices, such as silicon dioxide, magnesium fluoride, etc.
If you want to control the thickness of the film more accurately or pursue better film uniformity, ALD (atomic layer deposition) can also be used. This method can be considered as only laying one layer of atoms in the membrane layer for each cycle, which is also the origin of the name "atomic layer deposition". The advantages of ALD are uniformity and good encapsulation, but the disadvantage is that each cycle only covers one layer of atoms, resulting in lower production efficiency than electron beam evaporation.
Furthermore, it can be understood from here that ALD refers more to different preparation methods and does not necessarily mean that the design of the membrane layer is more excellent.
Schematic diagram of one cycle of ALD 9f1b53d769ddb44bdaa381ec0b660c41_v2-4a03db55f0f95b1ca5733230bae1dea4'r.jpg
Ultra low reflection coating
The second type is unconventional (but actually very common now) ultra-low reflection film, represented by various gradient refractive index film layers (or subwavelength structured coatings).
The meaning of gradient refractive index is that the refractive index of the film layer gradually changes between the air and the lens to match the two.
Although coatings based on gradient refractive index materials can effectively reduce reflectivity, some stubborn stray light is significantly reduced. However, the disadvantage is that the mechanical properties of this type of coating are often worse, in simple terms, the hardness is not high, so it is often only used on lenses with severe stray light inside the lens.
Canon's introduction to its own SWC coating
Chemical coating
The third type is some chemical coatings, which are used to change the physical or chemical properties of the lens surface. The reason why it is called a chemical coating is because it mainly achieves specific functions by forming a film on the surface of the lens through compounds.
A typical example is the use of a hard film layer in resin lenses. By using compounds with higher hardness to form a film on the surface of the lens, resin lenses with lower hardness and less wear resistance can have better scratch resistance. In addition, some chemical coatings can make the surface of the lens waterproof, oil proof, anti fog and other functions, such as the fluorine coating on the surface of the camera lens.
The coating method of chemical coating includes liquid immersion or gas diffusion to form a film (placing the lens and compound into a vacuum vessel simultaneously, so that the diffused compound forms a film on the lens).
Even eyeglass lenses may have complex coatings
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