Optical glass differs from other glass in that it is an integral part of the optical system and must meet the requirements of optical imaging. Therefore, the determination of the quality of optical glass also includes some special and more stringent indicators. The following requirements for optical glass:

Optical glass - specific optical constants and consistency of optical constants of the same batch of glass

Each type of optical glass has a specified standard refractive index value for different wavelengths of light, which serves as the basis for the optical designer to design the optical system. Therefore, the optical constant of the optical glass produced by the factory must be within a certain tolerance range of these values. Otherwise, the actual image quality will be inconsistent with the expected result at the design and affect the quality of the optical instrument. 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 uniform calibration of the instrument, the refractive index tolerances of the same batch of glass are more strict than their deviation from the standard value.

Second, a high degree of transparency

The brightness of the optical system imaging is proportional to the transparency of the glass. The transparency of the optical glass to a certain wavelength of light is expressed by the light absorption coefficient Kλ. After passing through a series of prisms and lenses, the energy is partially lost to the interface reflection of the optical part and the other part is absorbed by the medium (glass) itself. The former increases with the increase of the refractive index of the glass, and the value of the high refractive index glass is very large, for example, the surface light reflection loss of the heavy glass is about 6%. Therefore, for an optical system including a plurality of thin lenses, the main way to increase the transmittance is to reduce the reflection loss of the lens surface, such as coating the surface antireflection film layer. For large-sized optical parts such as the objective lens of an astronomical telescope, the transmittance of the optical system is mainly determined by the light absorption coefficient of the glass itself due to its large thickness. By increasing the purity of the glass raw material and preventing any incorporation of any coloring impurities during the entire process from batching to smelting, it is generally possible to make the glass have a light absorption coefficient of less than 0.01 (i.e., a glass having a thickness of 1 cm has a light transmittance of more than 99%). .

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