CCJC OPTICS

Manufacturing Process of Aspherical Lenses

Juncheng Optics' aspheric surface processing technology

2025-12-17 10:58

116

Aspherical lenses, also known as aspheres, are lenses whose surface profiles are not portions of a sphere or cylinder.

The asphere's more complex surface profile can reduce or eliminate spherical aberration and also reduce other optical aberrations such as astigmatism, compared to a simple lens. A single aspheric lens can often replace a much more complex multi-lens system, making the resulting device smaller and lighter. They are used in the design of multi-element wide-angle and fast normal lenses to reduce aberrations. In addition, aspheric lenses are used in combination with reflective elements in catadioptric systems, such as the aspherical Schmidt corrector plate used in Schmidt cameras and Schmidt-Cassegrain telescopes. Small molded aspheres are often used for collimating diode lasers.

The manufacturing of aspherical lenses is a sophisticated and precision-driven process that demands strict control over every stage, as their non-spherical surface profiles (unlike traditional spherical lenses) require advanced techniques to achieve the desired optical performance. Below is a detailed breakdown of the core manufacturing steps, along with key considerations for each phase:

1. Material Selection & Preparation

The first critical step is choosing the appropriate optical material, which depends on the lens’s intended application (e.g., visible light imaging, infrared detection, laser systems). Common materials include optical glass (e.g., BK7, fused silica for high-transmittance and low dispersion), plastic (e.g., PMMA, PC for cost-effective, lightweight applications), and special crystals (e.g., germanium for infrared use).

For glass aspheres: Raw glass is typically sourced as bulk ingots or preforms. These preforms undergo initial processing to remove surface impurities, such as grinding or lapping to achieve a rough but uniform shape. For plastic aspheres: Thermoplastic resins are dried to eliminate moisture (which can cause defects like bubbles during molding) and then melted for subsequent processing.

2. Primary Shaping (Forming the Aspheric Contour)

This stage aims to create a rough blank with a surface profile close to the target aspheric shape. The method varies significantly between glass and plastic lenses:

Glass Lenses: Traditional methods include grinding and lapping using diamond or abrasive tools. For high-precision glass aspheres, single-point diamond turning (SPDT) is widely used—this process uses a sharp diamond tool to cut the glass surface directly, creating the aspheric contour with sub-micron accuracy. However, SPDT is limited to certain glass types (e.g., infrared-transparent glasses) that are ductile enough for cutting; brittle glasses may require additional processes like ion beam figuring later.
Plastic Lenses: The most common method is injection molding, which is cost-effective for mass production. A precision mold (with the inverse aspheric profile) is created first—this mold is typically manufactured using SPDT to ensure its surface accuracy. Molten plastic is injected into the mold under high pressure, cooled, and then demolded to form the aspheric lens blank. For ultra-precision plastic aspheres, compression molding (lower pressure, slower cooling) may be used to reduce internal stresses and improve surface quality.

3. Precision Finishing (Optimizing Surface Quality & Accuracy)

After primary shaping, the lens blank requires finishing to achieve the exact aspheric profile, ultra-smooth surface, and minimal optical aberrations. Key processes include:

Lapping & Polishing:For glass lenses, lapping uses fine abrasives to refine the surface roughness, while polishing (e.g., with cerium oxide slurries) creates a smooth, transparent surface. For aspheric profiles, computer-controlled polishing (CCP) is essential—machines use feedback from precision metrology tools to adjust the polishing pad’s pressure and movement, ensuring the surface matches the designed aspheric curve.
Ion Beam Figuring (IBF): A advanced finishing technique for high-precision aspheres (e.g., those used in telescopes or laser systems). IBF uses a focused ion beam to etch the lens surface at the atomic level, removing tiny amounts of material with extreme accuracy (nanometer-level precision). This process is ideal for correcting minor deviations in the aspheric profile after initial polishing.
Plastic Lens Finishing: Injection-molded plastic aspheres often require minimal additional finishing, but may undergo coating preparation (e.g., plasma cleaning to remove mold release agents) or light polishing to fix minor surface defects.

4. Metrology & Quality Inspection

Given the strict optical requirements of aspherical lenses, comprehensive metrology is integrated throughout the manufacturing process to detect and correct deviations. Critical inspection techniques include:

Profilometry: Tools like contact profilometers (for surface roughness measurement) and non-contact profilometers (e.g., white-light interferometers, laser profilometers) scan the lens surface to verify the aspheric profile accuracy. Non-contact methods are preferred for delicate surfaces to avoid damage.
Interferometry: Used to measure optical wavefront errors, which directly affect the lens’s performance. Phase-shifting interferometers or digital holographic interferometers compare the wavefront passing through the aspheric lens to a reference wavefront, identifying aberrations like spherical aberration or astigmatism.
Dimensional Inspection: Checking key parameters such as lens thickness, diameter, and center deviation (to ensure the optical axis aligns with the mechanical axis).

Lenses that fail inspection are either reworked (e.g., re-polished, re-figured) or discarded, as even minor deviations can compromise their optical performance.

5. Coating & Final Assembly (Optional)

Many aspherical lenses require optical coatings to enhance performance:anti-reflective (AR) coatings reduce light reflection at the lens surface (improving transmittance), while protective coatings (e.g., scratch-resistant coatings for plastic lenses) increase durability. Coatings are applied using techniques like vacuum deposition or sputtering.

For lenses used in complex optical systems (e.g., cameras, telescopes), final assembly may involve mounting the aspheric lens into housings with precision alignment to ensure optimal performance with other optical components.

Key Manufacturing Challenges & Considerations

Profile Accuracy: Aspheric surfaces have non-uniform curvature, making it harder to control their shape compared to spherical lenses. Advanced metrology and computer-controlled processing are essential to meet tight tolerance requirements.
Surface Quality: Even tiny scratches, pits, or impurities on the lens surface can scatter light and reduce optical performance. Cleanroom environments (class 100 or higher) are required during manufacturing to minimize contamination.
Material Compatibility: Different materials require tailored processing—for example, brittle glass is prone to cracking during cutting, while plastic lenses may deform under high temperatures during coating.
Cost: High-precision glass aspheres (e.g., those used in aerospace) are expensive to manufacture due to complex processing and metrology, while plastic aspheres offer a more cost-effective option for consumer electronics (e.g., smartphone cameras).

In summary, the manufacturing of aspherical lenses combines advanced material science, precision machining, and rigorous quality control. The choice of process depends on the lens’s material, intended application, and performance requirements, with continuous advancements in technology (e.g., AI-driven process control, next-generation metrology tools) further improving the accuracy and cost-effectiveness of aspheric lens production.