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The Complete Guide to Aspheric Lens

May 1, 2024

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.

Structure of an Aspheric Lens

In various industries, ranging from automotive sensors and LED lighting systems to cutting-edge cameras and medical diagnostic devices, the significance of aspheric lenses is steadily growing. These lenses are part of the subset defined by rotationally symmetric optics with a radially varying radius of curvature. Aspheric lenses play an increasingly crucial role in various aspects of the optics, imaging, and photonics industries. This is attributed to the unique advantages they provide compared to traditional spherical optics  and spherical elements.

Unlike spherical lenses, which can be specified solely by the radius of curvature that fluctuates radially from the center of the lens, aspheric lenses exhibit a surface with varying local radii of curvature. The definition of rotationally symmetric aspheres often involves a surface sagitta (the measure of the surface shape in relation to a plane), or sag, expressed through an even aspheric polynomial.
 

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.

Benefit of Aspheric Lenses

Unlike conventional spherical optics, aspheric lenses use less elements to enhance aberration correction. An example would be zoom lenses. Zoom lenses typically use ten or more elements while two aspheric lenses can be replaced for a handful of spherical lenses in order to achieve similar or better optical results. The system size and overall cost of production are also potentially reduced.

Aspheric Surface Tolerances

Surface Accuracy:

  • Surface Accuracy is the measurement of how similar the intended shape is to the desired shape. There are many ways to measure and define surface accuracy and the errors that occur. The errors can be grouped into three categories depending on their frequency across the surface of a part: form errors, waviness, and surface roughness.
  • Form error, or irregularity, is a low frequency variation or a larger-scale error. They are the most important and frequently specified surface criterion for aspheres. These errors directly impact the overall shape and accuracy of aspheres. Form errors often peak one to three times across a part and are crucial considerations in optical specifications. Form errors are commonly defined by the peak-to-valley error measured in waves or fringes, although they can also be expressed as a linear deviation in microns or as an RMS deviation.
  • Waviness is an error that involves mid-frequency variations in the surface profile. It is a measure of the smoother or rougher undulations that occur on a surface. These errors can impact the overall smoothness and precision of a surface. However, these errors rarely occur in whole-aperture polishing performed when making spherical optics. Thus, waviness is only dealt with for aspheres. Waviness is often defined as a slope error over a designated scan length. Its sensitivity varies depending on the application, and many lenses may not be significantly affected by waviness. Therefore, specifying a waviness tolerance is crucial only if it has an impact on your specific application. Due to added testing, costs can increase when adding additional lens requirements. 
  • Lastly, surface roughness pertains to high-frequency variations or irregularities in the texture of the surface. It is the measure of smoothness or quality of the polish on an optic’s surface. These deviations can be microscopic and impact the overall smoothness or roughness of the surface. Surface roughness also has an impact on light scatter and the surface’s ability to withstand high laser power. It is important to note that the measurement of surface roughness involves quantifying the height differences between the peaks and valleys on the microscopic scale. Since analyzing surface roughness can be time-consuming because it requires very special testing, it is best to only specify surface roughness when necessary.

Radius Error:

  • Radius Error, a specific kind of form error, refers to the deviation or discrepancy between the actual radius of a curved surface and the intended or specified radius. It is the most common and in general the easiest error for a system to tolerate since it is often corrected by adjusting the focus position. Radius error is typically measured by assessing the difference between the designed radius and the actual radius of the curved surface. The goal is to help ensure precision and accuracy.
 

Manufacturing Capability

Precision Glass Molding:

  • Precision Glass Molding is an advanced manufacturing process that involves heating optical glass cores to elevated temperatures, rendering their surfaces malleable enough to conform to an aspheric mold (refer to Figure 3). The intricate mold, crafted from exceptionally durable materials to ensure a consistently smooth surface, plays a pivotal role in shaping the glass. The mold’s geometry is meticulously designed, accounting for any potential glass shrinkage to achieve the desired aspheric form.
  • While the initial investment in creating the mold incurs significant startup costs due to the precision required in material selection and geometry planning, the subsequent production benefits are substantial. Once the mold is perfected, the per-unit cost for each lens diminishes significantly compared to conventional manufacturing methods for aspherical lenses. This makes Precision Glass Molding an exceptionally viable option for high-volume production, offering a cost-effective and efficient solution in the optical glass manufacturing landscape.

    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:

    • Akin to grinding and polishing, allows for the production of individual lenses one by one. However, Single-Point Diamond Turning (SPDT) utilizes tools significantly smaller than those in precision polishing, resulting in surfaces characterized by enhanced finishes and form accuracies. SPDT has its limitations, as it is unsuitable for shaping glass, but proves effective with plastics, metals, and crystals. Moreover, SPDT finds application in crafting metal molds for use in glass and polymer molding processes.

    Molded Polymer Aspheres

  • In the realm of molded polymer aspheres, the technique commences with a standard spherical surface, such as an achromatic lens. This surface is then pressed onto a thin layer of photopolymer within an aspheric mold, ultimately yielding an aspheric surface (refer to Figure 7). This method is particularly advantageous for high-volume precision applications where additional performance is imperative, and the quantity justifies the initial tooling costs. The aspheric mold employed in polymer molding is created through SPDT, using a glass spherical lens. The lens surface and the injected polymer undergo compression and UV curing at room temperature, resulting in an aspheric lens.
  • The molding process at room temperature minimizes stress induced in the mold, thereby reducing tooling costs and facilitating easier manufacturing of the mold material. However, the thickness of the polymer layer imposes constraints on the degree of aspheric departure achievable in the resulting asphere. Additionally, the polymer lacks the durability of glass, rendering this method less than ideal for surfaces exposed to harsh environments.

    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.

    1. 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.
    1. Precision-Polished Aspheric Lens:
    • Offers a short lead time.
    • Does not require molds.
    • Suitable for sample making and low-volume applications.
    1. 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:
      • 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:
      • 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:
      • 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:
      • 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:
      • 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:
      • 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:
      • 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.
      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.

    Keys of Manufacturing Capability – Aspheric Lens Element by Avantier

    • Tailored Specifications and Precise Tolerances
      • Specifications and tolerances surpass standard lenses
      • Optimal performance for your specific application
    • Fully Custom Designs
    • MRF Polishing – Experience precision redefined with Q-flex 300 MRF™ Polishing Machine
    • Precision Optics Polishing with Satisloh SPS-200 – ensures precision in producing custom aspheric optics
    • Adaptability
      • Specifications can be easily adjusted according to requests
    • Comprehensive Support
      • Special shapes, product-integrated applications (OEM applications), and small-lot custom orders
      • Commitment to precision and customization for unparalleled optical results

    Custom aspheric lenses play a crucial role in advancing high-performance imaging across various fields. From aerospace applications like night vision imaging optics to defense imaging systems, and from microscope imaging objectives to semiconductor wafer inspection tools, these lenses serve as indispensable components in precision imaging devices. A notable example is the Smite Cassegrain telescope, which utilizes custom aspheric lenses along with reflective elements to mitigate aberrations and achieve superior resolution.

    The advantages custom aspheric lenses bring to high-performance optics are substantial. Particularly, these lenses are an optimal choice when designing systems with a limited footprint, as their inherent characteristics lend themselves well to compact assemblies.

    If you are looking for stock options, visit Stock – Aspheric Lenses.

    For custom requests, visit Aspheric Lens page.

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