Kunshan Noli Optical Introduction: optical glass manufacturing method and process steps

Optical glass can be described by defining it as a set of glass formulations specialized for the manufacture of optical components such as lenses, prisms, beam splitters, and optical Windows. The difference between optical glass and ordinary glass such as sodium-calcium glass is that optical glass needs to have specific performance characteristics related to its ability to transmit light and does not introduce aberrations that could harm the system using glass. The key to manufacturing optical glass is to carefully control the chemical composition that makes up the glass formula, as the presence of these additives will ultimately determine the optical properties of the glass and thus its properties.

This article will introduce the manufacturing method of optical glass, and review the process steps and types of commonly used optical glass. To learn more about other types of glass, please see our guide to types, properties and manufacture of glass, what is the difference between sodium-calcium glass and borosilicate glass? What is borosilicate glass and its uses?

Optical glass properties

The special properties of optical glasses stem from the need for specific optical performance characteristics related to their application. If you consider ordinary glass that can be used to produce containers, such as water cans used to hold drinks, then the expected performance of glass in that application may be related to the material's ability to withstand the cold temperatures of refrigerators. But there is little need to define the specific optical properties of the glass used in this way. No one cares about the transmittance of light through the glass of the pitcher, and no one cares whether and to what extent the light is dispersed.

However, when you consider the intended uses of optical glass, such as photographic lenses, telescope lenses, and laser beam splitters, optical properties become very important. Poor performance photography lenses produce streaks of color along light intensity boundaries from light to dark, which is certainly not what photographers consider a desirable feature or a sign of high performance. Therefore, the manufacturing goal of optical glass must be to ensure a specific level of performance from the perspective of light transmission, while other types of glass do not take this into account.

Before describing the manufacturing method of optical glass, it is useful to review some terms related to light behavior and optical properties and explain what these terms mean.

Properties of light and optical properties

We know that light is a form of electromagnetic energy that can behave like a wave or a particle. When considering the design of optical glass, fluctuations in light are often regarded as the primary model for understanding the properties of optical glass and its effect on the energy of the incident light.

Optical glass manufacturing process

Optical glass is manufactured using a multi-step process in which raw material chemical elements are first selected and then these chemical elements are measured and combined to form a glass formula. The process may vary depending on the specific type of optical glass being manufactured and its intended application, but from a general process perspective, the steps used to manufacture optical glass can be summarized as the following main operations:

Raw ingredients

melt

Refining/degassing

homogenization

molding

anneal

The initial raw material steps include selecting, measuring, and combining the chemicals required for a particular optical glass formulation. Attention must be paid to the purity of the chemical elements, as common contaminants such as iron can affect the optical properties of the finished product. For example, this may manifest as unwanted absorption of light at a particular wavelength of interest, a situation that leads to unacceptable performance, especially in applications involving monochromatic light sources such as lasers. The correct mixing of chemicals is also crucial, as the inhomogeneity of the formulation can cause changes or fluctuations in the refractive index of the glass.

Once the chemicals are batched and mixed, the melting process begins and the temperature of the mixture rises, allowing the chemicals to be mixed into the initial glass melt. As the temperature rises, convection will form, which will facilitate a certain degree of mixing of the liquid glass melt.

During the refining stage, at the highest temperature of the melting process, the liquid glass melt exhibits low viscosity, which allows any bubbles to rise to the surface. At this stage refining agents can be added to the glass, which breaks down and produces oxygen bubbles. Oxygen bubbles will assist in the degassing process by combining with other dissolved gases in the melt, resulting in larger bubbles that will rise faster to the surface of the melt and thus be easier to remove. High-quality optical glass must be free of bubbles, so the refining stage is crucial in the process of removing residual gases.

During the homogenization stage, the temperature is lowered and the melt is continuously stirred so that all components are evenly distributed throughout the glass melt batch. The goal here is to eliminate streaks, which represent local regions within the material that have a slightly different refractive index than the surrounding glass. As mentioned earlier, optical glass properties are measured according to the refractive index, and its change is a key parameter that controls the optical properties required for the finished glass. At the end of the homogenization process, the temperature of the glass melt is slowly reduced to achieve the right viscosity, resulting in the formation of a glass blank. Blank glass represents the original glass shape that needs additional processing, finishing, and molding,

The forming of the blank can take place in what is called a continuous or discontinuous process. Continuous molding has all the steps necessary to perform molding glass at different locations in the process flow. In contrast, discontinuous processes, also known as pot melting, involve all processes that occur in the same physical space but at different points in time.

In the annealing step, the formed glass is brought to room temperature so that subsequent production operations can be carried out on the blank. These operations include cutting, grinding, polishing and forming. For optical glass, a fine annealing process can be used, which requires the use of an annealing furnace to first raise the temperature of the glass and release any thermal stress that may have been created during molding. The glass is then slowly cooled to room temperature at a very slow rate, perhaps as low as a few tenths of a degree per hour. Fine annealed glass will exhibit low residual stress birefringence. Birefringence is a property of glasses and crystals in which the refractive index of the glass has different values depending on the direction of the glass relative to the incident light energy.

After the glass is cooled in the annealing step, the blank will be used to manufacture the finished product of the optical system, including lenses, beam splitters, viewing Windows, and other similar products. Optical glass is usually characterized as flint glass or crown glass. The difference between the two kinds of glass has to do with their characteristics. Flint glass is produced from lead and tends to be denser than crown glass. Crown glass is made using a higher concentration of potassium oxide and therefore has a lower density than flint glass.

Crown glasses are those that exhibit low dispersion, with an Abbe number usually greater than 55. They also have a low refractive index. Flint glass is a glass with Abbe number below 50, relatively high refractive index and high dispersion.

Precautions for optical glass

Choosing an optical glass requires consideration of the optical, mechanical, and thermal properties of the glass and how these factors will affect the application.

Factors to consider are as follows:

Transparency - depends on the degree of control of impurities in the glass formulation

Uniformity - Depending on the process, proper mixing and slow cooling are required to achieve minimal birefringence

Refractive index - A high refractive index is useful for lens applications because it allows weaker curvature to be used in lenses

Dispersion - This can be controlled by selecting a low dispersion glass option, such as crown glass

Density - an important consideration when lightweight optics are needed

Hardness - High hardness provides sturdiness and durability, but the disadvantage is that the glass is more difficult to cut and polish

Melting point - Optical glasses with higher melting temperatures can be operated at high temperatures, but are also more difficult to manufacture