Optical glass can change the direction of light propagation and change the relative spectral distribution of ultraviolet, visible, or infrared light.
Optical glass in the narrow sense refers to colorless optical glass; optical glass in the broad sense also includes colored optical glass, laser glass, quartz optical glass, anti-radiation glass, ultraviolet and infrared optical glass, fiber optic glass, acousto-optic glass, magneto-optical glass, and photochromic Glass.
Optical glass can be used to manufacture lenses, prisms, mirrors, and windows in optical instruments. Components made of optical glass are key elements in optical instruments.
This article will describe how optical glass is manufactured and will review the process steps and types of optical glass commonly used.
Optical Glass Properties
The special properties of optical glasses stem from the need for specific optical performance characteristics relevant to their application, and optical properties are very important when you consider the intended use of optical glass.
Photographic lenses that exhibit chromatic aberration produce color fringes along the border of light intensities from light to dark, which is certainly not considered a desirable feature by photographers or a sign of high performance. Therefore, the manufacturing goal of optical glass must be to ensure a specific level of performance in terms of how light is transmitted, which is not considered by other types of glass.
The nature and optical properties of light
Light is a form of electromagnetic energy that can exhibit wave-like or particle-like behavior. When considering the design of optical glass, the wave nature of light is often considered the primary model for understanding the performance of optical glass and its effect on the energy of incident light.
When light passes from one medium to another (for example, from air to glass), the incident wave undergoes reflection and refraction. Reflection means that a portion of the incident light is reflected off the surface it’s pointed at (in this case, a glass surface) and back into the air. If the incident angle of the measuring light relative to the glass surface is θi and the reflection angle is specified as θr, then
θi=θr
This means that reflected light can bounce off the glass surface at the same angle as it was incident on the glass.
The remaining light of the incident wave is transmitted into the glass and refracted. In simple terms, refraction is the bending or changing direction of light that occurs when it enters a medium of different density. The speed of light propagating in a given medium varies with the refractive index of that medium. If c denotes the speed of light in free space, then the speed of light in different media of refractive index n is equal to:
Thus, the refractive index (n) of a medium or substrate can be defined as the ratio of the speed of light in free space (vacuum) to its speed in that medium.
Incident light transmitted into a medium such as optical glass will be refracted such that
nisinθi=ntsinθt
ni=refractive index of incident medium, θi=incident angle of light energy, nt=refractive index of transmission medium, θt=refractive angle of transmitted light energy
Optical media can be either homogeneous (also called isotropic), meaning that the properties of the medium are the same in all directions, or anisotropic, in which case the behavior of light traveling through the medium will Depends on the direction or path followed through that medium.
Another optical property to be aware of is dispersion.
Dispersion refers to the phenomenon that the phase velocity of light energy passing through a medium is related to frequency. This means that light of different frequencies will travel through the medium at different speeds. Therefore, the refractive index of light depends on the frequency of the light.
Dispersion is a common manifestation of this concept, and the amount of dispersion can be measured with a value called Abbe’s number (Vd). The Abbe number for a given medium or substrate is calculated by measuring the refractive index of that medium at a specific wavelength. These correspond to wavelengths of light energy as follows:
Fraunhofer D = 589.3 nm
Fraunhofer F = 486.1 nm
Fraunhofer C = 656.3 nm
If we designate these measured values of refractive indices as nD, nF, and nC respectively, the Abbe number of the optical medium is calculated as follows:
When applied to the manufacture of optical glasses, the refractive index and Abbe number prove to be useful parameters when designing optical systems.
The index of refraction measures how fast light travels through a medium, while Abbe’s number measures the amount of dispersion (or change in the index of refraction) that a particular glass formulation may experience.
Glass with a higher index of refraction bends light more easily, so the lens needs less curvature, but light also travels slower in a medium with a higher index of refraction.
A glass with a higher index of refraction will also exhibit less spherical aberration, and having a high Abbe number means less dispersion and aberration.
In addition to the optical properties of glass, designers also focus on the mechanical, chemical, and thermal properties of optical glass. These properties include the material’s density, coefficient of thermal expansion, and the formulation’s maximum operating temperature.
Optical Glass Manufacturing Process
The raw materials for producing optical glass are some oxides, hydroxides, nitrates, and carbonates, and according to the requirements of the formula, phosphate or fluoride are introduced. In order to ensure the transparency of the glass, the content of colored impurities such as iron, chromium, copper, manganese, cobalt, nickel, etc. must be strictly controlled. Accurate weighing and uniform mixing are required for ingredients. The main production processes are smelting, forming, annealing, and inspection.
1. Smelting
There are single crucible batch smelting methods and pool kiln (see kiln) continuous smelting methods.
The single crucible melting method can be divided into the clay crucible melting method and the platinum crucible melting method. No matter what kind of smelting method is used, it needs to be stirred by a stirrer, and the temperature and stirring are strictly controlled to make the glass liquid highly uniform.
Clay crucibles can melt most crown glass and flint glass at low cost, and are used when the melting temperature of glass exceeds the service temperature of platinum.
Platinum crucibles can be used to melt high-quality glass that has a severe corrosion effect on clay crucibles, such as heavy crowns, heavy barium flint, rare earth glass, and fluoro phosphorus glass.
The platinum crucible is heated by electricity, generally using a silicon carbon rod or silicon molybdenum rod electric furnace. However, high-frequency heating can be used to manufacture glasses that have a large tendency to devitrify, require rapid cooling, and have certain requirements for the atmosphere.
Since the 1960s, countries have successively adopted continuous tank furnaces lined with platinum for smelting, which has greatly increased the output and quality of optical glass. This is the main trend in the development of optical glass production technology.
2. Molding
The molding methods of optical glass include the classical broken crucible method, rolling method, and pouring method, but at present more and more widely used leakage molding (using a single crucible or continuous melting to flow out the material liquid), which can directly pull the rod or drop the material to press the shape Or leaking material to form a large-sized blank, improving the utilization rate of gobs and the yield of finished products.
3. Annealing
In order to eliminate the internal stress of the glass to the greatest extent and improve the optical uniformity, it is necessary to formulate a strict annealing system and carry out precise annealing.
4. Inspection
The indicators measured in the inspection process include optical constants, optical uniformity, stress birefringence, fringes, bubbles, etc.
Special requirements for optical glass
The difference between optical glass and other glasses is that it, as an integral part of the optical system, must meet the requirements of optical imaging. Therefore, the judgment of optical glass quality also includes some special and stricter indicators. There are the following requirements for optical glass:
1. Specific optical constants and consistency of optical constants of the same batch of glass
Each type of optical glass has a 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 design results and affect the quality of optical instruments.
At the same time, because the same batch of instruments is 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 imaging brightness of the optical system is proportional to the transparency of the glass. The transparency of optical glass to a certain wavelength of light 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 in the interface reflection of 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, and the ratio is very large for high refractive index glass, such as the light reflection loss of a surface of heavy 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 on the lens surface, such as by coating the surface with an anti-reflection coating.
For large-scale optical components such as the objective lens of astronomical telescopes, due to their large thickness, the transmittance of the optical system is mainly determined by the light absorption coefficient of the glass itself.
By improving the purity of glass raw materials and preventing any coloring impurities from mixing in the whole 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%).
Precautions for optical glass
Selecting an optical glass requires consideration of the optical, mechanical, and thermal properties of the glass and how these factors will affect the application.
The considerations are as follows:
Clarity – depends on the degree of control of impurities in the glass formulation
Homogeneity – process dependent, proper mixing and slow cooling required for minimal birefringence
Refractive Index – A high refractive index is useful for lens applications as it allows weaker curvatures to be used in the lens
Dispersion – can be controlled by choosing a low dispersion glass option such as crown glass
Density – an important consideration when lightweight optics are required
Hardness – high hardness provides robustness and durability but has the disadvantage that the glass is more difficult to cut and polish
Melting point – optical glasses with higher melting temperatures allow for high-temperature manipulation but are also more difficult to manufacture