What should I pay attention to when coating optical glass lenses?

Coating optical glass lenses is a key step to improve the performance of lenses, and there are many precautions in this process:

Preparation before coating

Lens cleaning

Cleanliness: Before coating, the lens surface must be absolutely clean. Any tiny dust, grease, grinding or polishing residue will have a serious impact on the coating quality. For example, a small dust particle may cause the coating to protrude during the coating process, making the lens surface uneven, thus affecting the light propagation and imaging quality.

Selection of cleaning method: Multi-step cleaning method is usually adopted. First, organic solvents (such as acetone or ethanol) are used to remove grease pollutants, and then deionized water is used for washing to remove water-soluble impurities and residual organic solvents. After cleaning, you can check the cleanliness of the lens surface through a microscope.

environmental control

Cleanliness requirements: coating operation should be carried out in a clean room environment to minimize dust particles in the air. Generally, the cleanliness level of the clean room is required to reach 1000 or even higher to ensure that the lens surface will not be polluted by new dust during the coating process.

Temperature and humidity adjustment: Temperature and humidity will also affect the coating. Appropriate temperature and humidity contribute to the uniform deposition of coating materials. For example, if the temperature is too high, the coating material will evaporate too quickly, making it difficult to control the coating thickness. Excessive humidity may damp the coating material and affect its optical properties.

Coating process control

Selection of coating materials

Matching materials according to requirements: Select suitable coating materials according to the specific use of lenses. If it is to increase the light transmittance of the lens and reduce the reflection, low refractive index materials such as magnesium fluoride (MgF₂) are usually selected. In order to increase the wear resistance of lenses or realize special optical functions, high refractive index materials such as titanium dioxide (TiO₂) or multilayer composite coating materials may be selected.

Material purity guarantee: the purity of coating material is very important. Impure materials may introduce impurities and change the optical properties of the coating. For example, the coating material containing metal impurities may absorb light with a specific wavelength and reduce the light transmittance of the lens.

Coating process parameters

Deposition rate control: Deposition rate refers to the deposition rate of coating materials on the lens surface. This speed needs to be accurately controlled according to the nature of the coating material, the size of the lens and the required coating thickness. If the deposition rate is too fast, it may lead to uneven coating and thickness difference; If it is too slow, it will affect production efficiency.

Coating thickness control: the coating thickness directly affects the optical performance of the lens. For example, the optimal thickness of antireflection film is usually calculated according to the wavelength of light and the refractive index of the material. Controlling the coating thickness can be achieved by accurately controlling the parameters such as coating time and evaporation rate of materials.

Vacuum maintenance: Many coating methods, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), need to be carried out in a vacuum environment. Proper vacuum degree can ensure that the coating material is uniformly evaporated and deposited on the lens surface. If the vacuum degree is not enough, gas molecules in the air may react with the coating material or interfere with the deposition process of the material.

Inspection and treatment after coating

quality detection

Optical performance detection: use spectrophotometer to detect the optical performance indexes such as transmittance and reflectivity of the lens after coating. Check whether the optical characteristics at different wavelengths meet the design requirements. For example, optical lenses for photographic lenses need to have high transmittance and low reflectivity in the visible light band (400-700nm).

Adhesion inspection of film: Check the adhesion between film and lens surface by scratch test or tape sticking test. If the adhesion of the film is poor, the film may fall off during use, which will affect the performance and life of the lens.

aftertreatment

Annealing treatment (if necessary): For some coating processes, in order to eliminate the stress inside the film, annealing treatment may be needed. Proper annealing temperature and time can improve the stability and durability of the film, but the annealing process needs to be carefully controlled to avoid damaging the film or affecting the optical properties of the lens.

Protective packaging: the coated lens should be immediately protected and packaged to avoid scratches, pollution or moisture on the film. Generally, special optical lens packaging materials will be used, such as packaging with anti-static, dustproof and cushioning functions.