The main production process and requirements of optical glass

There are many varieties of glass and a wide range of uses. In addition to the common quartz glass and tempered glass, there are also various types of optical glass, color-changing glass, laminated glass and so on. As there are many types of glass, it is more important to choose the right type. In order to find suitable glass products, we need to understand the main production process of optical glass and know what quality requirements are required for optical glass.

1. The main production process of optical glass

The raw materials for the production of optical glass are some oxides, hydroxides, nitrates and carbonates, and phosphates or fluorides are introduced according to the requirements of the formulation. In order to ensure the transparency of glass, the content of colored impurities, such as iron, chromium, copper, manganese, cobalt, and nickel, must be strictly controlled. When ingredients are required, accurate weighing and uniform mixing are required. The main production processes are smelting, forming, annealing and inspection.

①Smelting: There are single crucible batch smelting method and tank kiln (see kiln) continuous smelting method. Single crucible melting method can be divided into clay crucible melting method and platinum crucible melting method. No matter what kind of smelting method is used, it needs to be stirred with a stirrer, and the temperature and stirring are strictly controlled to make the glass liquid reach a high uniformity. Clay crucible can smelt most crown glass and flint glass, with low cost, and is used when the melting temperature of the glass exceeds the service temperature of platinum. The platinum crucible can smelt high-quality glass that has a serious corrosive effect on the clay crucible, such as heavy crown, heavy barium flint, rare earth glass and fluorophosphorus glass. Platinum crucibles are heated by electricity, generally silicon-carbon rods or silicon-molybdenum rod electric furnaces are used. However, high-frequency heating can be used to manufacture glass that has a high tendency to crystallize, requires rapid cooling, and has certain requirements on the atmosphere.

Since the 1960s, various countries have successively adopted continuous tank smelting lined with platinum, which greatly improved the output and quality of optical glass. This is the main trend in the development of optical glass production technology.

②The molding methods for forming optical glass include classical crucible method, rolling method and pouring method, but leakage molding (using a single crucible or continuous melting liquid) is more and more widely used, which can directly draw rods or drop materials Pressing or leaking to form large-size blanks can improve the utilization rate of gobs and the yield rate.

③ Annealing In order to eliminate the internal stress of the glass to a greater extent and improve the optical uniformity, it is necessary to formulate a strict annealing system and perform precision annealing.

④Inspection The indicators measured are: optical constant, optical uniformity, stress birefringence, fringes, bubbles, etc.

Second, what are the quality requirements of optical glass

1. The specific optical constants and the consistency of the optical constants of the same batch of glass

Each type of optical glass has a prescribed standard refractive index value for different wavelengths of light, which serves as the basis for optical designers to design optical systems. Therefore, the optical constants of the optical glass produced by the factory must be within a certain allowable deviation range of these values, otherwise the actual imaging quality will not match the expected result during the design and the quality of the optical instrument will be affected. At the same time, since the same batch of instruments are often made of the same batch of optical glass, in order to facilitate the unified calibration of the instruments, the allowable deviation of the refractive index of the same batch of glasses is more stringent than their deviation from the standard value.

2. High degree of transparency

The image brightness of the optical system is proportional to the glass transparency. The transparency of optical glass to light of a certain wavelength is expressed by the light absorption coefficient Kλ. After the light passes through a series of prisms and lenses, part of its energy is lost by the interface reflection of the optical parts and the other part is absorbed by the medium (glass) itself. The former increases with the increase of the refractive index of the glass. For high-refractive-index glass, this value is very large. For example, the light reflection loss of a surface of the counterweight flint glass is about 6%. Therefore, for an optical system containing multiple thin lenses, the main way to increase the transmittance is to reduce the reflection loss of the lens surface, such as coating the surface with an antireflection coating. For large-size 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 improving the purity of the glass raw materials and preventing any coloring impurities from mixing in the entire process from batching to smelting, the light absorption coefficient of glass can generally be made less than 0.01 (that is, the light transmittance of glass with a thickness of 1 cm is greater than 99%) .