Characteristics of Aspheric Lens
Spherical Aberration Correction
One of the most important features of aspheric lenses is their ability to correct for spherical aberration. Spherical aberration is found in all spherical lenses, such as plano-convex or double-convex lens shapes. However, aspheric lenses excel in focusing light to a precise point, resulting in minimal blur and enhanced image quality. Spherical Aberration is the consequence of the uniform curvature of the lens surface and not the result of a manufacturing error. The outer rays converge at a different focal point than the inner rays resulting in blurred or distorted images.
A spherical lens with a significant amount of aberration and an aspherical lens with almost no aberration can be seen(Figure 1). Aspherical Lenses address the issue by deviating from a perfectly spherical shape. An aspheric lens can be designed by modifying the curvature length and adjusting the conic constant and aspheric coefficients of the curved surface of the lens. By carefully shaping the lens, aspheric lenses ensure that all incoming light rays converge to a single focal point. minimizing spherical aberration and improving image quality.
In Figure 1, the difference in focusing performance of spherical lenses and aspheric lenses is further explained by the table below. It compares the performance of a spheric lens and an aspheric lens both with a diameter of 25mm and focal lengths of 25mm (f/1 lenses). The table presents a comparison of spot sizes, or blur sizes, for collimated 587.6nm light rays under different conditions: on-axis (0° object angle) and off-axis (at 0.5° and 1.0° object angles). The spot sizes of the asphere are significantly smaller, differing by several orders of magnitude compared to those of a spherical lens.
Where:
Z: sag of surface parallel to the optical axis
s: radial distance from the optical axis
C: curvature, inverse of radius
k: conic constant
A4, A6, …: 4th, 6th, … order aspheric coefficients
When the aspheric coefficients are equal to zero, the resulting aspheric surface is considered to be a conic. The following table shows how the actual conic surface generated depends on the magnitude and sign of the conic constant, k.Additional Performance Advantages
To achieve the necessary performance of an imaging lens, optical elements designers frequently resort to stopping down, or increasing the f/# of their design. Although the desired resolution goal is obtained, the approach results in a reduction in light throughput. Using aspheric lenses in the design, however, improves aberration correction and enables the creation of high-throughput systems with low f/#s, while also maintaining excellent image quality. The following table compares two designs: an 81.5mm focal length, f/2 triplet lens (depicted in Figure 2) with all spherical surfaces and the same triplet with an aspheric first surface. Both designs utilize identical effective focal length, f/#, field of view, glass types, and total system length. The table provides a comparison of the modulation transfer function (MTF) at 20% contrast for on-axis and off-axis collimated, polychromatic light rays at 486.1nm, 587.6nm, and 656.3nm. The triplet lens with the aspheric surface demonstrates significantly improved imaging performance at all field angles with high tangential and sagittal resolution values, surpassing those of the triplet with only spherical surfaces by factors as high as four.Precision Glass Molding:
Precision Polishing:
- The production of machined aspheric lenses has historically involved the grinding and polishing of each lens individually. Although the fundamental process of creating these lenses one by one has remained largely unchanged over the decades, there have been significant advancements in fabrication technology, particularly in the realm of precision polishing. These advancements have elevated the attainable level of accuracy achievable through this production method.
- In precision polishing, small contact areas, typically on the order of square millimeters, are employed to grind and polish aspheric shapes. These minute contact areas are strategically adjusted in space to mold the aspheric profile during computer-controlled precision polishing, as illustrated in Figure 4. For instances where even higher-quality polishing is demanded, magneto-rheological finishing (MRF) comes into play. MRF involves perfecting the surface using a similar small-area tool that can rapidly adapt removal rates to rectify errors in the profile, as depicted in Figure 3. The technology behind MRF ensures high-performance finishing within a shorter time frame compared to standard polishing techniques, owing to its precise control over removal location and high removal rate. In contrast to many other manufacturing methods that often necessitate a unique mold for each lens, precision polishing makes use of standard tooling. This characteristic makes it the preferred choice for prototyping and low-to-medium volume production, offering practical advantages in terms of versatility and efficiency.
Diamond Turning:
Molded Polymer Aspheres:
Injection Molding
- Injection molding offers advantages in optimizing part cost, tooling complexity, and precision. In this process, molten plastic is injected into an aspheric mold, specially treated to overcome the thermal instability and pressure sensitivity inherent in plastic compared to glass. While plastic lenses may exhibit lower scratch resistance than glass counterparts, their lightweight nature, ease of molding, and compatibility with mounting features make them a favorable choice for creating unified optical elements. Despite a somewhat limited selection of high-quality optical plastics, the cost and weight benefits often sway design choices toward plastic aspheric lenses. Alternatively, plastic aspheres can be shaped using compression molding, wherein a preheated plastic material is positioned in the open lower half of a mold before the top half is pressed down, effectively compressing the plastic to conform to the mold’s shape. Compression molding is particularly employed for lenses where intricate structural details matter, as seen in Fresnel lenses and lenticular arrays. Injection and compression molding techniques can be applied independently or in combination, with the combined method commonly known as coining. Although the options for optical quality plastic are restricted, the advantages in terms of cost and weight are likely to steer certain designs toward the adoption of plastic aspheric lenses.
Selection of Manufacturing Methods for Aspherical Lenses
Fabricating aspherical lenses poses greater challenges due to their complex surface profiles compared to conventional spherical lenses. Various methods are available for producing aspheric lenses, each with its distinct advantages and limitations.
- Precision-Glass-Molded Aspheric Lens:
- Well-suited for mass production.
- Exhibits high thermal stability.
- Ideal for applications demanding high volume, high quality, and thermal stability.
- Precision-Polished Aspheric Lens:
- Offers a short lead time.
- Does not require molds.
- Suitable for sample making and low-volume applications.
- Plastic-Molded Aspheric Lens:
- Characterized by low cost and lightweight.
- Suitable for high-volume applications with moderate quality and low thermal stability.
For optical engineers, a crucial aspect is comprehending manufacturing techniques and selecting the most appropriate method based on lens application, performance requirements, development cost, sample cost, production part cost, and project timeline.
Offerings from Avantier
- Comprehensive Custom Solutions:
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- Avantier specializes in manufacturing a wide variety of custom aspheric lenses, tailored to meet the specific needs of various applications ranging from smartphones to lasers, fiber optics, research, industry, and medicine. This comprehensive approach ensures that clients receive solutions optimized for their unique requirements.
- State-of-the-Art Manufacturing Technology:
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- Avantier employs state-of-the-art grinding and polishing equipment, including computer-controlled precision polishing devices and magneto-rheological finishing (MRF) technology. This advanced technology ensures that the surface quality of the lenses is optimized for their intended applications, meeting high standards of precision and performance.
- Material Flexibility:
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- Avantier offers its aspheric lenses in a range of materials, including glass, crystalline, and plastic substrates. This material flexibility allows customers to choose the most suitable option based on their application requirements, providing versatility and adaptability in optical system design.
- Geometric Variation and Free-Form Optics:
- Avantier’s expertise extends to the manufacturing of aspheric lenses with various geometries, including rotationally symmetric lenses with complex front surfaces. The ability to create lenses with non-constant curvature, and even free-form optics, provides optical engineers with greater design flexibility to address specific challenges in optical system design.
- Quality Surface Profiles:
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- Avantier defines its aspheric lenses by surface profiles, utilizing metrics such as RMS slope departure (Qbfs) and sag departure from a base conic (Qcon). This commitment to defining and maintaining quality surface profiles ensures that the lenses effectively correct optical aberrations, leading to improved image quality in diverse applications.
- Benefits of Aspheric Lenses:
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- Avantier emphasizes the benefits of using aspheric lenses, such as improved image quality, more compact and lightweight designs, and increased design flexibility. These advantages are particularly relevant in applications where size, weight, and optical performance are critical, such as in portable devices like cameras.
- High-Performance Imaging Applications:
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- Avantier highlights the essential role of custom aspheric lenses in high-performance imaging applications, ranging from aerospace and defense imaging to microscope imaging objectives and semiconductor wafer inspection tools. This demonstrates the versatility and importance of Avantier’s products in precision optical systems.
- Cost-Effective High-Performance Optics:
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- While acknowledging that aspheric lenses may be more expensive to manufacture than spherical lenses, Avantier underscores their significance in high-performance optics. The benefits of improved image quality, compact designs, and design flexibility contribute to creating cost-effective optical systems with superior performance.
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Compact and Lightweight Assemblies:
Avantier emphasizes the advantage of using aspheric lenses in compact assemblies, particularly in portable devices like cameras. The controlled curvature of these lenses allows for the creation of thinner and flatter designs, reducing the overall size and weight of optical systems without compromising performance.