We all know that light will reflect and refract when passing through different media. However, the structure of modern mobile phone lens is more complex, and the number of lenses is more, so the number of times of reflection and refraction will be more after the light enters the lens. This will lead to two problems: the light passing through the lens will have a greater loss; the second is that the light will be reflected and refracted many times in the lens, which will produce what we call stray light and ghost shadow; and the coating technology can effectively improve these problems.
Optical coating is based on the fluctuation of light and interference phenomenon. A thin layer of material, such as magnesium fluoride, silicon dioxide, aluminum fluoride, is coated on the lens surface to improve the transmittance of the lens and reduce the reflectivity of the lens. In short, the thickness of the optical film is thin, which can be compared with the wavelength of the incident light. Secondly, it will produce certain optical effects and cause light interference.
In the mobile phone field, in addition to the image quality, the lens transmittance plays a very important role in improving the image quality. At present, resin is usually used as the base material of the lens in the mobile phone industry. In order to reduce the reflection of the lens and improve the transmittance, AR antireflection film (antireflection film) is coated on the surface of the lens. It is a kind of hard heat-resistant oxide film, which can reduce the reflectivity of the surface of components within a specific wavelength range. In the case of uncoated optical element, about 4% energy loss will be produced due to reflection on each surface of optical element.
Another application in the mobile phone field is infrared cut-off filter, also known as IR cut filter. According to the principle, there are two kinds of reflective filters and absorption filters, which use different glass substrates. The principle of reflective filter is to alternately coat multiple layers of high refractive index optical film on ordinary optical glass to achieve high transmittance (400-630nm) in visible light band, cut-off of light in near-infrared band (700-1100nm), and cut off part of near-infrared light that interferes with imaging quality. Because infrared light is cut off by reflection, it is easy to produce secondary reflection and produce stray light and ghost. The absorption filter mainly uses blue glass as the base material to filter the infrared light through the light absorbing material in the blue glass. At the same time, the infrared cutoff film and antireflective film are coated on one side to improve the infrared cutoff performance and visible light transmittance at the same time.