The optical constants of laser lenses and the optical constants of the same batch of glasses should be consistent. The first type of filter has the specified standard refractive index values for different wavelengths of light, which is the basis for optical designers to design optical systems.
Therefore, the optical constants of the filters produced in the factory must be within the allowable deviation range of these values, otherwise, the actual image quality will be inconsistent with the expected results in the design, which will affect the quality of the optical instruments. At the same time, because the same batch of instruments are often made of the same batch of optical glass, in order to facilitate the unified calibration of the instrument, the allowable deviation of refractive index of the same batch of glass is more strict than their deviation from the standard value.
Secondly, the infrared cut-off filter needs to be highly transparent, and the brightness of the image of the optical system is proportional to the transparency of the glass. The transparency of the filter to a certain wavelength of light is expressed by the light absorption coefficient K λ. When light passes through a series of prisms and lenses, part of its energy is lost in the interface reflection of optical parts, while the other part is absorbed by the medium (glass). The former increases with the increase of glass refractive index, which is very large for high refractive index glass. For example, the reflection loss of a surface light of heavy flint glass is about 6%.
Two main factors that affect the performance of color filter are temperature and moisture absorption.
The relationship between the optical properties and the factors such as aggregation density, temperature change and moisture absorption was analyzed. According to the thickness error of each layer of narrow-band filter caused by moisture absorption and temperature, the models of spectral drift before and after moisture absorption and before and after temperature change are given. Based on the analysis of the optical stability of the narrow-band filter and the preliminary experiment, the measures to improve the optical performance of the narrow-band filter are put forward.
When all dielectric narrow-band filters are exposed to air, macroscopical water absorption will appear soon. First, there are many water absorption spots on the surface of the film, then gradually expand and connect into a piece, and finally form a uniform water absorption layer. When the water absorption of the film is stable, it will be found that not only the peak wavelength moves to the long wave, but also the transmission of the peak wavelength increases. Causes: there are a certain number of pores in the film. Before water absorption, the pores in the film are filled by air with a refractive index of 1. After water absorption, the pores are filled by water with a refractive index of 1.33. Because of the change of the refractive index in the pores, the average refractive index of the film increases, and the optical thickness of the film increases with the increase of the refractive index, so the peak wavelength of the filter moves to the long wave phenomenon.