The difference between optical glass and other glass is that it must meet the requirements of optical imaging as an integral part of the optical system. Therefore, the determination of the quality of optical glass also includes some special and more stringent indicators. There are the following requirements for optical glass:

First, the specific optical constants and the same batch of glass optical constants are consistent

Each type of optical glass has a specified standard refractive index value for different wavelengths of light, 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 the allowable deviation of these values, otherwise the actual image quality will be inconsistent with the expected result of the design and affect the quality of the optical instrument. At the same time as the same batch of equipment is often the same batch of optical glass manufacturing, in order to facilitate the uniform calibration of the instrument, the same batch of glass refractive index tolerance than their deviation from the standard value more stringent.

Second, a high degree of transparency

The brightness of the optical system is proportional to the glass transparency. 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 absorbed by the interface 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, which is very large for the high refractive index glass, such as about 6% of the surface light reflection loss of the heavy flint glass. Thus, for an optical system comprising 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 a surface-enhanced film layer. For large-sized optical components such as astronomical telescope of the objective lens, because of its large thickness, optical system transmittance mainly depends on the glass itself, the optical absorption coefficient. By increasing the purity of the glass raw material and preventing any coloring impurities from mixing during the entire process from the furnish to the smelting, it is generally possible to make the glass having a light absorption coefficient of less than 0.01 (i.e., a glass having a thickness of 1 cm for a light transmittance of more than 99%)