The Ultimate Guide For Optical Lens Materials and Glasses Lens Manufacturing Processes

The Ultimate Guide For Optical Lens Materials and Glasses Lens Manufacturing Processes

Lenses are a crucial component of eyeglasses, providing necessary vision correction and eye protection. To fully understand the production process of lenses, we need to explore various materials, characteristics, and manufacturing processes. This article will start with the basic properties of lenses and gradually explain the classification, characteristics, and production processes of lenses, helping readers gain a comprehensive understanding of the world of lenses.

The article will be developed from the following perspectives:

1. How to make high-quality lenses
2. Common lens materials
3. Lens classification
4. Features of each type of lens
5. Lens manufacturing processes and techniques
6. Design of aspherical lenses
7. Standards for lens appearance

I: How to Make High-Quality Lenses

To make high-quality lenses, we must have a comprehensive understanding of the basic properties and parameters of lenses. The optical purpose of eyeglass lenses is to restore clear vision to eyes with refractive errors through corrective lenses. When selecting lens materials, the following considerations are important:

1. Geometric properties of the material: radius of curvature, surface shape
2. Physical and chemical properties of the material: refractive index, Abbe number, etc.
3. Optical properties: calculating diopter effect and controlling optical performance
4. Mechanical and thermal properties
5. Electrical properties of the material
6. Chemical properties: understanding material changes through potential external chemical contact

Let’s discuss each characteristic in detail.

Lens Curvature Design

1. Choose the appropriate curvature for optimal optical performance.
2. Design the flattest curvature possible while ensuring the best visual effect. Curvature can be spherical or aspherical.

Optical Performance Light refraction and reflection at the lens surface, material absorption, and scattering and diffraction phenomena. Parameters involved include refractive index (N), specific gravity (g/cm²), transmittance (T%), Abbe number (V), and UV absorption rate.

Light Refraction Light passing through a lens will refract or deviate at the front and rear surfaces of the lens. The degree of deviation is determined by the refractive index of the material and the angle of incidence of the light on the lens surface. Here, we mention two parameters: refractive index and dispersion coefficient.

Light Absorption Light absorption in eyeglass lenses refers to the absorption of light within the material, expressed as the percentage of light absorbed by the front and rear surfaces of the lens.

Light Reflection While light refracts at the lens surface, it also reflects. Light reflection can affect lens clarity and create interfering reflective light on the lens surface. A higher refractive index results in more light loss due to reflection.

Light Scattering and Diffraction

• Scattering: Light spreads in all directions, usually occurring on the surface of solids and within transparent materials. Ideally, eyeglass lens surfaces should be free from scattering due to polishing during the grinding process. However, dirt or surface fogging can cause scattering. Internal scattering is minimal but can occasionally cause lenses to appear yellow or milky.
• Diffraction: When light waves encounter small obstacles, changing direction. Diffraction can cause abnormal interference on the lens surface, especially if the lens is damaged. This phenomenon should be given attention.

Refractive Index The refractive index of a transparent medium is the ratio of the speed of light in a vacuum (C) to the speed of light in the medium (V), expressed as N=C/V. This ratio is unitless and greater than 1. A higher refractive index results in a greater deviation of light entering the medium from the air. The deviation or refraction from air to a transparent medium with a refractive index can be calculated using Snell’s law: sin I=N sin R. The value of the refractive index varies with wavelength, typically referenced at a specific wavelength (e.g., 546.07 nm in Europe and Japan).

Dispersion Dispersion occurs when the refractive index changes with the wavelength of light, causing white light to spread into different colors. As described by the Abbe number, dispersion varies by material. Higher refractive index materials have greater dispersion, leading to noticeable color fringes around the edges of off-axis objects.

Transmittance Light absorption by the material follows Lambert’s law, with transmittance decreasing exponentially with lens thickness. The transmittance of light through a lens is the total amount of light not reflected or absorbed. Factors affecting visual perception include the intensity and spectrum of incident light, lens absorption and spectral selectivity, and eye sensitivity to different wavelengths. Choose materials with minimal surface reflection and good transmittance (e.g., CR39 material, with 92% visible light transmittance and 4% surface reflection).

Mechanical Properties These reflect the characteristics of solid materials, determining the quality, volume, and size, as well as resistance to deformation and impact. Choose lightweight materials for comfort and high hardness for safety. Terms like elastic modulus (E) and impact resistance are relevant. For example, the FDA’s impact resistance test in the US ensures safety.

Thermal Properties These describe material changes and characteristics under temperature influence. Important parameters include thermal conductivity, specific heat, linear expansion coefficient, melting point, boiling point, and stress temperature.

Electrical Properties These describe the characteristics of materials under electromagnetic and electrical effects, sometimes linking optical and electrical properties. Parameters include dielectric strength and dielectric loss coefficient at a predetermined frequency.

Chemical Properties These describe the material’s reaction to chemicals during manufacturing and daily use or under extreme conditions. Two main tests are accelerated aging (testing material reliability using cold water, hot water, acids, and organic solvents) and fire resistance (international standards assessing fire resistance of lens materials).

II: Lens Materials

Lens materials are mainly divided into inorganic (glass) and organic materials. Inorganic materials (glass) include ordinary glass, high refractive index glass, colored glass, and photochromic glass. Organic materials include thermoset materials, ordinary resin, high refractive index resin, colored resin, photochromic resin, and thermoplastic materials.

III: Lens Classification

Lenses are classified from different perspectives, resulting in various names:

1. By material: natural lenses, glass lenses, optical resin lenses, special lenses. Common natural lenses are crystal lenses. Glass lenses include white lenses, red lenses (Crox lenses), blue lenses (Crox lenses), 1.6 glass lenses, 1.7 ultra-thin glass lenses, 1.8 ultra-thin glass lenses, and 1.9 ultra-thin glass lenses. Optical resin lenses include CR-39, acrylic PMMA, and polycarbonate PC.
2. By refractive principle: myopia lenses, hyperopia lenses, astigmatism lenses (myopic and hyperopic), and mixed astigmatism lenses.
3. By curvature design: spherical lenses, aspherical lenses (single and double aspherical).
4. By refractive index:
   • Glass: 1.523, 1.6, 1.7, 1.8, 1.9
   • Resin: 1.499, 1.5, 1.56, 1.6, 1.67, 1.74
5. By focal point: single-focus lenses, bifocal lenses, trifocal lenses, multifocal lenses.
6. By protective principle and decoration: sunglasses (tinted lenses), photochromic lenses, anti-radiation lenses, and polarized lenses.

IV: Features of Each Type of Lens

Discussion on the characteristics of each lens material:

1. Natural Material—Crystal Lenses
   • The main component is silicon dioxide (SiO2), with high purity in pure crystal.
   • Mined from quartz.
   • Colorless: high SiO2 purity.
   • Tea-colored (tea crystal): contains other elements, reducing visible light transmittance, making it more comfortable for photophobic individuals, and beneficial for eye health.
   • Hard material, resistant to wear, low thermal expansion coefficient, moisture-resistant.
   • Cannot fully absorb short-wave UV light. High UV transmittance.
   • Does not reduce infrared transmittance, causing fatigue when worn.
   • Uneven density, impurities cause stripes and bubbles, leading to double refraction, and affecting vision. Not suitable for optical lenses.
   • Expensive.
2. Glass:
   • Glass is a unique amorphous material, solid at room temperature, hard but brittle, and viscous at high temperatures.
   • No fixed chemical structure, and no exact melting point. Softens and becomes viscous with rising temperature, transitioning from solid to liquid.
   • Optical glass has strict quality requirements like refractive index, dispersion, and transmittance. Mainly composed of SiO2 (60-70%), calcium oxide, sodium, boron trioxide (B2O3), and phosphorus pentoxide (P2O5). Antimony sulfide enhances transmittance.
   • Good hardness, comparable to crystal.
   • Can be colored by adding various oxides, reducing strong light stimulation, and absorbing harmful rays like UV and infrared. Has a history of over 300 years.
   • Heavy; higher refractive index increases weight, causing discomfort.
   • Poor impact resistance easily causes eye injuries from mechanical damage.
3. White Lens: Also known as white lenses, optical white lenses, are mainly composed of sodium-calcium silicate, colorless and transparent, high clarity, absorb UV below 330A. Adding CeO2 and TiO2 makes it UV white, absorbing UV below 346A. Visible light transmittance is 91-92%, refractive index 1.523.
4. Red Lens—Crox Lens: Also known as Crox lenses, adding CeO2 and MnO2 to white lenses enhances UV absorption. Appears pale red under sunlight and incandescent light, hence called red lenses. Absorbs UV below 350A, transmittance above 88%.
5. Blue Lens—Crox Lens: Also known as Crox lenses, the transmittance is 87%. These lenses have a dual-color effect, appearing pale blue under sunlight, hence called blue lenses. Absorbs UV below 340A, some infrared, and 580A yellow visible light.
6. 1.60 Glass Lens—Yabo Lens: Refractive index is 1.60, thinner than ordinary glass lenses (1.532), and lighter than ultra-thin lenses (1.70), suitable for moderate prescriptions, with a smooth curvature design and larger diameter.
7. High Refractive Index Glass Lenses (1.7, 1.8, 1.9): Manufacturers have found ways to increase refractive index while maintaining low dispersion by adding new elements to glass. In 1975, lenses containing titanium had a refractive index of 1.7 and an Abbe number of 41. Ultra-thin lenses (1.7) add TiO2 and PbO, increasing refractive index to 1.70, with high surface reflection, about 1/3 thinner than ordinary lenses, suitable for high myopia, and aesthetically pleasing. However, low Abbe number and high chromatic aberration can reduce peripheral vision and cause color fringes. The curvature design is flatter, and the center thickness is within the safety range. In 1990, lenses containing lanthanum had a refractive index of 1.8, Abbe number 34; in 1995, lenses with a refractive index of 1.9 added niobium, Abbe number 30. These lenses are lighter, and thinner, with aspherical treatment reducing edge distortion. However, increased refractive index also increases material density, offsetting weight reduction.
8. Colored Glass Lenses: Mixing certain metal salts into glass materials creates colored effects. Adding nickel and cobalt (purple), cobalt and copper (blue), chromium (green), iron, cadmium (yellow), gold, copper, and selenium (red). Used mainly for non-prescription sunglasses or protective lenses. Demand for corrective lenses is low due to uneven color distribution in lenses with different thicknesses.
9. Photochromic Glass: Photochromic effect changes light absorption properties, responding to sunlight intensity. It involves reversible changes in material, darkening under UV radiation, and lightening under high temperatures. Introduced in 1962, photochromic glass mainly contains silver halide. Silver atoms and chlorine atoms exchange electrons under UV radiation, causing darkening. When UV decreases, electrons return to chlorine atoms, restoring clarity. Common colors are gray and tea (gray and tea photochromic). All lenses can use photochromic materials, not limited to 1.5 refractive index.
10. Organic Lenses: Two types of materials, thermoset (e.g., CR-39) and thermoplastic (e.g., PMMA, PC).

• CR-39: Good optical performance, lightweight, high impact resistance, UV protection, easy coloring, versatile, but low scratch resistance and refractive index.
• PMMA (polymethyl methacrylate): Lightweight, has good impact resistance, and is safe. Thermoplastic, softens at 75°C, changing refractive index. Not suitable for eyeglass lenses, used for reading glasses, sunglasses. Lower hardness and optical performance than glass.
• PC (polycarbonate): Lightweight, suitable for rimless frames, high impact resistance, 60 times that of glass, 10 times that of resin, making it more durable. Thick lenses (2.5 cm) can be bulletproof. 100% UV protection, low Abbe number, surface easily scratched, poor thermal stability, softens at 100°C.

11. Bifocal Lenses: Feature two focal points on one lens, with a small lens on a regular lens. Mainly for presbyopia, allowing users to see near and far without changing glasses. However, it has a small field of view and the image jumps during the transition (prism effect).
12. Progressive Multifocal Lenses: Have multiple focal points on one lens, with gradually changing diopters. Advantages over bifocals include uninterrupted vision from near to far, clearer intermediate vision, aesthetically pleasing without visible lines, no image jump, suitable for walking on stairs and streets, thinner than single vision lenses, reduced eye fatigue, and improved visual health. Suitable for early presbyopia, those dissatisfied with wearing two pairs of glasses, or bifocals.

V: Lens Manufacturing Processes and Techniques

Overview of manufacturing processes for various lenses:

Glass Lens Processing: Manufacturing involves processing the front and rear surfaces of provided glass blanks. First, components are melted in a furnace to form blanks, then precise curvature surfaces are created to produce finished blanks. Steps include: a) Ingredient Preparation: Key to stable refractive index and uniform glass. b) Melting: Includes melting, clarifying, homogenizing, and distributing. A stable glass liquid flow is provided for molding, with precise temperature control. Glass flows out of the feeder pipe, cut into predetermined weights by special scissors. c) Automatic Molding: Drops from the feeder pipe fall into molds, and automatic molding machines press them into lens blanks. d) Annealing: Eliminates stress. e) Inspection

Resin Lens Manufacturing: Molds are assembled using gaskets, then liquid monomer is injected into the cavity and cured at 40-85°C for 24 hours. After curing, the molds are opened, and lenses are removed, followed by edging, cleaning, and bevelling.

Thermoplastic Material Production: Careful processing is required when grinding and edging thermoplastic materials, differing from CR-39. For example, polycarbonate manufacturing involves heating transparent granules and injecting them into lens molds. The screw acts as a piston, pushing the material into the mold cavity, ensuring plasticity. After injection and cooling, molds are opened, and lenses are removed.

Hard Coating: Both inorganic and organic lenses can be scratched by dust or grit (SiO2). Organic materials are more prone to scratching. Hard coating, also known as scratch-resistant coating, involves applying a high polymer coating to the surface of CR39 resin lenses to increase hardness and scratch resistance, making lenses more durable and high-quality. Hard coatings can be dyeable or non-dyeable. Production involves immersing lenses in hardening liquid at a specific temperature, then drying them.

Anti-Reflective Coating: Light passing through the lens surface can reflect, creating white light and affecting appearance. Optical theory suggests lenses should form a clear image at the far point, with light bending and focusing on the retina. Different curvatures of the lens surfaces cause reflections, affecting clarity and comfort. The anti-reflective coating uses interference principles, applying thin layers to reduce reflection. The lens surface is cleaned with high-energy argon ions before coating, ensuring tight bonding and quality.

Anti-Fouling Coating: Lenses with multiple anti-reflective layers are prone to smudges, affecting performance. Anti-fouling coating applies a thin top layer to resist oil and water. Common materials include fluorides. Methods include immersion and vacuum coating. After anti-reflective coating, fluorides are applied using vapor processes, covering the porous anti-reflective layer and reducing water and oil adhesion.

VI: Aspherical Lens Design

Aspherical lenses break optical limitations on lens shape design, allowing smoother curvature without losing non-central optical performance. Benefits include thinner, lighter lenses with reduced curvature and improved peripheral vision.

VII: Standards for Lens Appearance

To understand manufacturing processes and coatings, we need to know how to assess lens appearance quality.

Common lens surface defects include foreign objects (US), scratches (SC), cotton-like fibers (CO), and pits (PIT). Defect severity standards: B2, B3, C1, C2. Defect positions: center 30mm (zone 1), from outer edge minus 10mm to center 30mm (zone 2), and outer 10mm (zone 3).

Each lens can have up to 5 defects, excluding B2 and B3 in zone 3. Only 1 B2 defect is allowed in zone 1. Zone 2 allows 4 defects, with a maximum of 1 C1, no C2. Zone 3 allows 5 defects, excluding B2 and B3.

Inspection Methods: Visual inspection: Use an 8W daylight lamp, hold the concave side of the lens perpendicular to the lamp (lens marking facing down) at 200mm, tilt slightly, and observe the lens surface for defects (bright spots, crystal points, scratches, tear marks, uneven coating).

Coating Testing: Place multi-coated lenses on dark or deep blue velvet, and observe the color tone for natural, non-dazzling shades. Under daylight lamps, check for consistent coating color. Test adhesion by pressing adhesive tape on the lens surface, removing it after 15 minutes or half an hour, and observing the coating.

Top Coat Testing: Pour water on the lens’s concave surface, and observe sliding. Create a fog layer on the lens, and check for fog dissipation.

Transmittance Check: Place the lens on white paper, and check for the base color. Test for UV protection coating.

Refractive Index Check: Determine the refractive index by material (monomer). Dyeing and boiling tests indicate a refractive index, with higher refractive index materials being harder to dye. The density affects the sound when dropped, helping identify the approximate refractive index. Higher refractive index materials are more brittle and prone to edge cracks.

Different Materials’ Smell During Edging: Materials have distinct smells during edging, with harder lenses emitting stronger odors, indicating a higher refractive index.

This comprehensive introduction covers lens material properties, manufacturing processes, and inspection methods. We hope this article provides valuable information to help you better understand the world of lenses. For more professional knowledge or custom services, visit the official website of XIAMEN ISUNNY PACKING CO., LTD. The next article will cover major lens production bases and well-known optical lens manufacturers in China.

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