In optical elements, the light energy is lost due to the reflection of the element surface. In order to reduce the reflection loss of the element surface, transparent dielectric film is often coated on the surface of optical elements, which is called antireflective film.
The principle of increasing transmission of antireflective film can be qualitatively analyzed from the perspective of energy conservation, and the principle of increasing transmission of antireflective film can be quantitatively explained according to Fresnel formula and refraction law, and the principle of increasing transmission of antireflective film can be theoretically explained using electromagnetic theory of electrodynamics.
What should we pay attention to in optical coating processing? When light enters into different transmission materials (such as glass from air), about 5% will be reflected off. There are many lenses and refractors in the optical collimator, which together can cause 30% to 40% of the incident light loss. Modern optical lenses are usually coated with single-layer or multi-layer antireflection film of magnesium fluoride. The single-layer antireflection film can reduce the reflection to 1.5%, and the multi-layer antireflection film can reduce the reflection to 0.25%. Therefore, if the entire sight is properly coated, the light transmittance can reach 95%. The lenses coated with single-layer antireflection film are usually blue-violet or red, while the lenses coated with multi-layer antireflection film are light green or dark purple.
For the antireflective film used on glass and plastic substrates, take a 35mm autozoom camera composed of 18 lenses. Assume that the interface between each glass and air has 4% reflection, the light transmittance of the lens without antireflection is 27%, the light transmittance of the lens coated with a film (the remaining reflection is 1.3%) is 66%, and the light transmittance of the lens coated with a multilayer film (the remaining reflection is 0.5%) is 85%. In many optical systems, a very important component is the coating on the lens that can reduce reflection. In many applications, antireflective film is indispensable, otherwise it cannot meet the application requirements.
The essence of antireflective film to increase the intensity of transmitted light is that in the process of propagation of light waves as electromagnetic waves, on the interface of different media, due to the different boundary conditions, the energy distribution is changed. For single-layer thin films, when the media on both sides of the antireflective film are different, the thickness of the film is odd times of 1/4 wavelength and the refractive index of the film is n=(n1 * n2) ^ (1/2) (respectively the refractive index of media 1 and 2), the incident light can be completely transmitted through the media. Generally, optical lens is used in the air. For optical glass with general refractive index of about 1.5, in order to achieve 100% antireflection effect of single film, n1=1.23, or close to 1.23; Also make the thickness of antireflective film=(2k+1) times a quarter wavelength. Single-layer film can only enhance the transmission of electromagnetic wave of a specific wavelength. In order to achieve the transmission enhancement in a larger range and more wavelengths, people use multilayer coating to achieve it.
People have a lot of experience in the use of antireflective films, and found many materials that can be used as antireflective films; At the same time, we have mastered many advanced coating technologies, so the application of antireflective film involves all walks of life, such as medicine, space exploration, and has made a significant contribution to the progress of human science and technology.
