Table of Contents
Glass Matters in Optics
Glass is a material that is not only transparent and clear but also extremely strong and hard. Therefore, it is of indispensable importance in daily production and life. Many types of optical glasses exhibit isotropic properties due to their random structure. This article will introduce the optical properties of optical glass, different types of optical glass, and the applications of glass optical components in various fields, helping you to fully understand all aspects of optical glass.
What is Optical Glass?
Optical glass, a versatile optical material, possesses the ability to alter the direction, transmission, intensity, and spectral distribution of light through refraction, reflection, and absorption. In a narrower context, it specifically denotes colorless optical glass, extensively used for crafting lenses, prisms, reflectors, and windows in optical instruments.
The optical characteristics of optical glass are intricately linked to its chemical composition. Modern optical glass spans the entire periodic table, requiring specific oxides of elements like Si, B, P, Pb, K, Na, Ba, As, and Al, among others. This fusion forms a molten solution at high temperatures, solidifying it into a supercooled amorphous melt after cooling. Predominantly, optical glass comprises SiO2 as the primary component, classified as silicate glass. Alternately, B2O3 and P2O5 lead to borate and phosphate glasses, respectively.
Categorized by optical properties, the optical glass can be divided into multiple types such as colorless optical glass, colored optical glass, radiation-resistant optical glass, ultraviolet and infrared optical glass, etc. to meet the needs of different applications.
Optical Glass Properties
Optical glass, renowned for its indispensable role in optical systems, boasts several key properties shaping its functionality:
.1 Optical Properties of Optical Glass
1）Refractive Index and Abbe Number
The two main optical properties of optical glass are refractive index and Abbe number.
The refractive index dictates the speed of light within the material, influencing bending. Simultaneously, the Abbe number quantifies dispersion across wavelengths. A balance is sought; a high refractive index bends light significantly, and a suitable Abbe number mitigates chromatic aberration. Typically, optical glass exhibits refractive indices of 1.5 to 1.9, with Abbe numbers ranging from 24 to 64.
2)Optical Uniformity:
Characterized by isotropy, optical glass ensures consistent internal properties in all directions. This uniformity, encompassing refractive index, hardness, and thermal expansion, is pivotal in upholding the quality and performance of optical components.
3)Dispersion:
Playing a pivotal role in optical systems, optical glass leverages its dispersive properties. Dispersion, the bending of light at varying angles with different wavelengths, is carefully managed in optical system design to minimize chromatic aberration and enhance imaging quality.
4)High Transparency:
Surpassing ordinary glass in transparency, optical glass efficiently transmits light, minimizing reflection and absorption. This exceptional transparency makes it a cornerstone in the production of optical components, including lenses and prisms.
.2 Chemical Properties of Optical Glass
1) Additives
Optical glass is often enriched with additives to fine-tune its chemical properties. This may include boron oxide, zinc oxide, phosphorus pentoxide, fluorite, and barium oxide. These additives serve to adjust the optical and chemical characteristics of the glass. For instance, borosilicate glass is a notable type that benefits from the inclusion of boron oxide.
2) Corrosion resistance
Optical glass boasts remarkable corrosion resistance, making it resilient against staining even under relatively harsh conditions, such as elevated temperatures (up to 50°C) and high humidity levels (up to 80%). This resistance extends to alkalis and various environmental factors, ensuring the longevity and reliability of optical glass in diverse applications.
.3 Mechanical Properties of Optical Glass
1) Density
Optical glass exhibits a diverse range of densities, spanning from 2.39 g/cm³ to 6.19 g/cm³. This extensive range enables the customization of optical glass for various application scenarios, ensuring versatility in meeting specific requirements.
2)Thermal expansion coefficient
The typical thermal expansion coefficient of optical glass falls within the range of 7.00 to 9.00 x 10^–6/°C. This coefficient, although relatively high, is a critical factor influencing the response of optical glass to temperature variations. Understanding this behavior is essential for its optimal performance across different thermal conditions.
.4 Other Properties
1) Stability
Optical glass has a high viscosity at room temperature, making it in a relatively stable state and not prone to the transition from a glassy state to a crystalline state.
2)Continuous reversible transformation
The process of glass transformation from molten state to solid state is continuous and reversible. There is no solidification point, only a solidification temperature range, which makes glass have excellent heat treatment process performance.
These properties make optical glasses ideal for numerous optical applications, where their isotropy, uniformity, and excellent optical properties allow them to play a key role in lenses, prisms, and other optical components.
Types of Optical Glass
Optical glass can be divided into three common types according to its color, transparency and special properties: colorless optical glass, colored optical glass and special glass.
- Colorless optical glass: This is a kind of transparent glass that usually does not filter the color of light. Colorless optical glass is the most widely used optical material, and its characteristic properties are represented by refractive index, dispersion coefficient (Abbe’s number) and partial dispersion coefficient. This glass is often used to make transparent optical components such as lenses and windows.
- Colored optical glass:This is a type of glass that is given a specific color by adding specific metal oxides or other compounds during the manufacturing process. Sometimes the color is meant to block or transmit specific wavelengths of light for a specific application, such as sunglasses.
- Special glass: This type of glass has special properties, which may include higher refractive index, low dispersion, radiation resistance, etc. They are designed for special optical applications such as lasers, fiber optic communications, etc.
.1 Colorless optical glass:
Colorless optical glass is a type of glass that is almost completely transparent in the visible and near-infrared broad bands. It is one of the most widely used optical materials. Its characteristics are described by parameters such as refractive index and dispersion coefficient (Abbe number). This glass is mainly used to make transparent optical components such as lenses and windows.
1). Composition of colorless optical glass
Colorless optical glass is composed of a variety of oxides and is divided into two categories. One type can generate a glass network structure, such as SiO2, B2O3, P2O5, etc.; the other type cannot generate a network structure, such as Na2O, K2O, CaO, BaO, PbO, etc., which are inserted into the network structure. These two types of oxides are called network bodies and network exosomes. Commonly used as lenses, prisms, reflectors, etc. for telescopes, microscopes, cameras, etc.
2) .Colorless optical glass classification
There are more than 160 kinds of colorless optical glass, which are divided into two categories according to the refractive index and dispersion. Crown glass is represented by K, which has a small refractive index and a large dispersion coefficient, such as fluorine crown (FK) and phosphorus crown. (PK), etc.; Flint glass is represented by F, which has a large refractive index and a large dispersion coefficient, such as crown flint (KF), light flint (QF), etc.
3) Common colorless optical glass
K9 glass
K9 glass is glass made of K9 material and is mainly used in optical coating and other fields. Its composition is SiO2=69.13%, B2O3=10.75%, BaO=3.07%, Na2O=10.40%, K2O=6.29%, As2O3=0.36%. The optical constants are refractive index = 1.51630, dispersion = 0.00806, and Abbe number = 64.06.
4) Quality indicators of colorless optical glass
The quality indicators of colorless optical glass include the allowable difference between the refractive index, dispersion coefficient, and the standard value, the consistency of the refractive index and dispersion coefficient in the same batch, optical uniformity, stress birefringence, streakiness, bubble degree, and light absorption coefficient E. Radiation resistance (N series glass), etc.
.2 Colored Glass
Colored glass, also known as optical filter glass, selectively absorbs or transmits specific wavelengths of visible, ultraviolet, or infrared light. With over a hundred varieties, it serves diverse functions in optical instruments, from observation to photography and ultraviolet applications. Its capabilities include selectively transmitting light of specific wavelengths, passing a certain spectrum range in the continuous spectrum, and uniformly weakening the spectrum of white light.
1) Classification of Colored Optical Glass:
Colored optical glass is categorized based on its coloring principle and mechanism. This includes colloidal colored glass and ion-colored colored glass, with colorants existing in colloidal or ionic states. The coloring mechanisms comprise ionic coloring, metal colloid coloring, and sulfur selenide coloring, mainly used in manufacturing optical filters.
Colored glass is classified into three types: selective absorption, cut-off, and neutral gray based on spectral characteristics. Selective absorption filters specific colors, neutral filters absorb all wavelengths uniformly, and interference filters reflect unwanted colors.
2) Types of Colored Optical Glass:
There are many types of colored optical glass, each designed for specific applications such as filters, lenses, etc. Here are some common types and uses of colored optical glass:
- UV Clear Glass (ZWB): Primarily transmits UV rays and is used in UV optical systems and applications such as UV lamps and UV photography.
- Purple glass (ZB): Absorbs most visible light, transmits ultraviolet and blue-violet light, and is used in photography and some optical experiments.
- Cyan (blue) glass (QB): Absorbs most infrared and red light, transmits blue and ultraviolet light, and is used in photography and optical filters.
- Green glass (LB): absorbs infrared light and red light, transmits green light, and is widely used in photography and some optical instruments.
- Infrared transparent glass (HWB): Transmits infrared light and is commonly used in infrared optical systems and applications.
- Protective Glass (FB): Mainly used to filter certain harmful light radiation, such as protective glasses for UV protection.
- Infrared transparent white glass (BB): Transmits both infrared and white light and is used in some special infrared optical applications.
Designed for specific applications, these colored optical glasses contribute significantly to diverse fields, each leveraging its unique transmission and absorption properties in different wavelength ranges.
.3 Specialized Glasses
Apart from the conventional optical glass, there exist several specialized glasses, encompassing radiation-resistant glass, quartz glass, insulating glass, crystallized glass, heat-resistant glass, hard glass, and optical glasses glass.
1) Radiation-proof optical glass:
It exhibits a superior ability to absorb high-energy radiation. Two types include high-lead glass for preventing gamma rays and X-ray irradiation, and the CaO-B2O2 system glass for absorbing slow and thermal neutrons. Mainly used in nuclear and medical industries for shielding and observation windows.
2) Radiation-resistant optical glass:
Possesses all the properties of colorless optical glass and remains stable under radioactive irradiation. It is used in optical instruments subjected to gamma irradiation, with chemical composition similar to colorless optical glass but containing a small amount of ceria to minimize color changes post-irradiation.
3) Quartz glass:
Derived from pure crystal, it is fused quartz with over 99.9% SiO2 content. Featuring exceptional properties such as high spectral transmittance, minimal thermal expansion, remarkable thermal stability, excellent heat resistance, chemical stability, and mechanical strength. Ideal for various applications due to its broad optical and physical advantages.
4) UV and Infrared Optical Glass:
Tailored for specific optical constants, excelling in transmittance within either the ultraviolet or infrared bands. Primarily used in ultraviolet or infrared optical instruments and as window materials for these applications.
Applications of Optical Glass
Optical glass is one of the most common materials in the field of optics, with a wide range of applications ranging from cameras and microscopes to medical and scientific research instruments.
.1 Optical lenses:
Optical glass is most widely used in manufacturing lenses. This includes concave lenses, convex lenses, aspheric lenses, etc., as well as lenses that combine the refraction of light with other properties of optical glass. These optical lenses are key components of optical equipment such as cameras, microscopes, telescopes, etc. and are used to produce clear images.
.2 Filter:
Optical glass filters selectively transmit or block specific wavelengths or colors of light. They are widely used in medical equipment, scientific experimental instruments, and are an important part of imaging and observation instruments such as spectroscopy and microscopes.
.3 Optical window:
Optical glass windows are engineered to provide minimal light distortion while reducing absorption and reflection. They are mainly used in systems involving light transmission, such as cameras, spectrometers, laser systems or optical sensors where precision components need to be protected.
Optical Glass Selection
In optical design, it is key to check the parameters of each glass, including availability, price, projection properties, thermal properties, contamination resistance, etc., to ensure optimal glass selection.
（1）Availability:
Glass is available in Preferred Glass, Standard Glass and Custom Glass. Preferred glass refers to existing stock, standard glass is the variety listed in the glass company’s catalog, and custom glass can be ordered upon request.
（2）Projectivity:
Most optical glasses have good projectivity in the visible and near-infrared light regions. However, in the near-UV region, most glasses absorb to some extent. For systems that need to project UV light, commonly used materials include fused silica and fused silica.
（3）Birefringence properties:
Generally optical glass is isotropic, but may become anisotropic under the influence of mechanical and thermal stress. This is critical for systems that transmit polarized light and requires careful selection of materials.
（4）Chemical stability:
Including properties such as weather resistance, pollution resistance, acid resistance and alkali resistance to ensure stability in different environments.
（5）Thermal Properties:
Optical glass has a positive coefficient of thermal expansion, which needs to be consistent with the properties of the lens structure. The matching of thermal expansion and contraction properties is crucial to the stability of the system, especially in optical systems that require athermalization.
Careful consideration of these parameters helps ensure the performance and stability of the optical system.
Conclusion
Optical glass stands as a crucial component in various light-based technologies, finding its utility in the production of photographic lenses, optical prisms, and filters. This article aimed to provide insights into the nature and functionality of optical glass, covering its diverse types and properties. By exploring this information, it is our aspiration that readers have gained a comprehensive understanding of optical glass and its applications in the realm of optics and technology.
With a decade-long presence in the optical industry, Noni is a trusted supplier that excels in delivering high-precision optical lenses tailored to customers’ specific requirements and designs. Our vast experience in the field enables us to provide optimal solutions based on customers’ samples and drawings.