Aspherical Solutions for Innovators Using Aspheres to Reduce System Size, Weight, Cost, and Complexity, While Improving Optical Quality Ross Optical Industries | 800.880.5417 | sales@rossoptical.com | www.rossoptical.com Ross Optical Industries | 1410 Gail Borden Place, A3, El Paso, TX 79935 | 800.880.5417 | 915.595.5417 | sales@rossoptical.com | www.rossoptical.com Simplifying Optical Systems Using Aspheres The push for increasingly more compact optical designs, without performance compromises, has fueled increased interest in the design and use of aspherical optics. In applications such as medical devices and instrumentation, scanners, zoom and range-finding optical systems, and hand-held image capture devices, aspheric optics are often the key to reduced size, cost, weight, and component count. For many optical designers, aspheres have proved to be a favorable solution where conventional spherical optics fall short. Because they eliminate spherical aberration, aspheres offer image quality advantages for various types of imaging systems. Aspheres can also improve image brightness with increased effective aperture dimensions, which is particularly important in miniaturized and light-starved systems. This guide is intended to provide a better understanding of aspheric optics and what they can accomplish. Addressing Shortcomings of Spherical Optics Designers generally turn to aspheric optics in response to the limitations of more conventional spherical optics solutions. Some of the more compelling reasons for considering aspheric elements include: Reducing Parts Count and Complexity Doing More with Less One asphere can do the job of two, three, or more spherical lenses. Ross Optical has helped customers to reduce the complexity of existing designs by incorporating aspheres into the optical system. Correcting for Quality Aspheric lenses are capable of higher aberration order correction than their spherical counterparts, which means they can be used to improve image quality for a variety of optical systems. In working with one customer, Ross was able to eliminate the need for other correction optics by employing an asphere solution. Ross Optical has also developed a new design using aspheric optics for a type of popular optical device currently on the market. The new design would result in a compact product that reduces the length of the device by 1/3 and maintains similar optical performance. With proper design, aspheric optics can reduce the number of lenses needed for properly conditioning light in an optical path. Using aspheric designs, a single lens element can often provide the functions of two or more conventional lenses, using less space and requiring fewer alignment procedures. For complex lens assemblies, such as zoom lenses, one or two aspheric lenses can provide the function otherwise requiring more spherical lenses. Component count reduction helps to decrease size and weight, as well as simplifying optical assembly. Reducing or Eliminating Optical Aberrations An aspheric surface profile can be designed and fabricated to reduce or eliminate spherical aberration that can cause a variety of problems for imaging or other optical systems. In effect, spherical aberration occurs when incident light focuses at different points, some distance from the theoretical focal point. For imaging systems, for example, this causes blurring. With an aspheric lens, light can be controlled to focus to a point, so that there is little or no perceptible blurring and improved image quality. The use of aspheric lens solutions can thus help to eliminate the need for additional corrective optics that compensate for spherical aberration and related problems. The surface of an aspheric lens is designed to reduce or even eliminate spherical aberration, providing diffraction-limited spot sizes. Challenging Applications Aspheres can be the answer for difficult applications where space and available illumination are at a minimum. A single aspheric lens can often replace elements in a much more complex multi-lens system and provide complex functions in systems including wideangle optics and fast normal lenses. In some cases, aspheric components can be used in combination with spherical lenses. Hybrid aspheric lenses, for example, can help to provide color correction within an imaging system. Aspheric surfaces can increase the numerical aperture of a lens with reduced system aberrations. With its expertise in systems design and mounting solutions, Ross can help customers optimize designs using aspheres to reduce the size and cost of system optics. Ross Optical Industries | 1410 Gail Borden Place, A3, El Paso, TX 79935 | 800.880.5417 | 915.595.5417 | sales@rossoptical.com | www.rossoptical.com Describing the Aspherical Surface Profile The surface profile of at least one surface of an aspheric lens or asphere is non-spherical and non-cylindrical, and its potential shapes are virtually limitless. The aspheric lens is rotationally symmetric, but its radius of curvature varies radially with respect to its center. The radius of curvature of the aspheric surface changes with distance from the optical axis, unlike a spherical surface, which has a constant radius (Figure 1). The distinctive shape of a particular aspheric lens enables it to deliver improved optical performance compared to standard spherical surfaces. Figure 1A. Spherical lens profile Figure 1B. Convex aspherical lens profile Figure 2. Aspherical gull-wing shaped Schmidt corrector lens in cross section Figure 1B shows a simple asphere design in cross section along the optical axis. As the surface distance to the axis changes, the effective radius changes with respect to the spherical surface (dashed line). Figure 2 shows a typical surface profile of a more complex asphere. In this “gull wing” shape, there are both convex and concave portions. How It’s Made Ross Optical provides aspheres that are formed from glass materials using a variety of techniques, from machining to molding. Mathematical expressions for aspherical surfaces are considerably more complex than those for spherical surfaces. Recent developments in the optical community are rapidly changing the classical model for defining the asphere shape and for providing mathematical expressions that are inherently more accurate and well-behaved in terms of making minor changes and modifications, so that surface shape can be more accurately predicted and fabricated. In practice, these expressions, along with the associated variable parameters for the lens, are generated by the design software that computes the aspheric surface for the optical designer. Practical Considerations for Specifying and Using Aspheres In order to better understand some of the possibilities and constraints that aspheres offer, it is useful to consider some practical considerations that relate to challenges the asphere fabricator faces. Material Selection Considerations when choosing the best-fit material for your application include: Cost Weight Geometric shape Application environments such as temperature Durability required Precision required Choice of raw materials or refractive indices Ross Optical Industries | 1410 Gail Borden Place, A3, El Paso, TX 79935 | 800.880.5417 | 915.595.5417 | sales@rossoptical.com | www.rossoptical.com Manufacturing Challenges of Providing the Aspheric Shape An initial consideration for the asphere fabricator is the overall shape of the surface, convex (as in Fig. 1), concave, or some combination with both convex and concave portions (as in Fig. 2). For tooling, the tool radius is constrained by the minimum value of the local radius. Factors of tool clearance, rates of radius change, and measurability also play a part in determining manufacturability for the aspheric surface. Lens aspect ratio is another criterion for machinability and polishing, since there is some part movement that can vary with differences in aspect ratio. Polishing techniques for aspheric surfaces typically employ tools that have a contact area that is much smaller than the clear aperture. The path of polishing tools must also be considered, sometimes requiring additional aperture to accomodate polishing procedures and tool overrun. Molding Techniques for Asphere Fabrication Molded Aspheric Lenses from Ross Optical Ross Optical provides a range of molded asphere optical components, including lenses from 1.5 to 100 mm diameter. For more details on Ross Optical’s capability, visit http://www.rossoptical.com/resources. Small glass aspheric lenses can be made using precision molding, allowing mass production once a prototype design is tested and approved. Offering low cost and good performance, molded aspheres are commonly used in inexpensive consumer cameras, camera phones, and personal devices such as CD players. Molded aspheric lenses are widely used as collimators for laser diodes and for coupling light to and from optical fibers. Machining Techniques for Asphere Fabrication Larger aspheric lenses, such as those used in projection devices, guidance systems, telescopes, and aerial instrumentation are typically formed by various machining techniques. Methods such as point-contact contouring are used to provide the basic contour that can then be polished, in successive stages, to finished shape. In other designs, such as in the Schmidt optical system, the aspheric corrector plate is formed by vacuum, distorting a parallel optical plate into a curve that can be polished to be flat on one side. Aspheric surfaces can also be made by polishing with a small tool with a compliant surface that conforms to the optical component, although precise control of the surface form and quality can be challenging, and results may change as the tool wears. Single-point diamond turning is also used for shaping some types of larger aspheric surfaces. For this function, a computer-controlled lathe uses a diamond tip to directly cut the desired profile into a piece of glass or another optical material. Diamond turning has proved to be particularly useful for infrared optics. Diamond turning can be a slow process and has some limitations with respect to materials and surface quality. Non-spherical curvature can also be formed by grinding the curvatures off-axis. Dual rotating axis grinding can be used for high index glass that isn’t easily spin molded. Polishing and Coating Finishing and Coating Services for Aspheres Finishing methods used to improve the surface quality of the polished surface include ion-beam finishing, abrasive water jets, and magneto-rheological finishing, which directs a a magnetically guided fluid jet to remove material from the surface. Because of its precise control of the removal location and high removal rate, magneto-rheological finishing provides a high performance finished surface in less time than is typically achieved when using standard polishing techniques. While more specialized manufacturing techniques require specialized molds, polishing typically uses standard tooling, particularly advantageous for designing and testing prototypes as well as for some low volume production. Ross Optical provides various coatings for aspheric surfaces, including anti-reflective coatings. Applications Where Aspheres Make a Difference For more information, see http://www.rossoptical. com/capabilities/optical-coatings. Medical Imaging and Instrumentation Aspheric lenses improve optical systems in a range of fields, including: Aspheres offer particular advantages in applications that are light-starved and where the aperture size needs to be maximized for larger field of view. Consumer Imaging Aspheres for consumer cameras on a range of devices offer high quality imaging from molded optical materials. Ross Optical Industries | 1410 Gail Borden Place, A3, El Paso, TX 79935 | 800.880.5417 | 915.595.5417 | sales@rossoptical.com | www.rossoptical.com Security Imaging and Sensing Aspheric designs simplify lens structure and improve image quality, eliminating components in some cases. Illumination Aspheric lenses and lens arrays help to direct light efficiently from the illumination source. Solar Energy Aspheric prism surfaces and aspheric lens arrays help to concentrate light for a number of solar energy devices. Communications Aspheres can be used to collimate light emitted from a laser diode or from an optical fiber. Figure 3. Aspheres for laser collimation Ross Optical: Focusing on Asphere Solutions Aspheres from Ross Optical serve a number of light collection, projection, illumination, detection, and condensing applications. Aspheric solutions help to improve image quality, reduce the number of required system elements, and lower costs in optical designs. From digital cameras and CD players to high-end microscope objectives and fluorescence microscopes, aspheric lenses are growing into every facet of the optics, imaging, and photonics industries due to the distinct advantages that they offer compared to traditional spherical optics. Look to Ross Optical for: Design Services for Optics Components and Assemblies Ross Optical offers design services that help our customers identify the right solutions for their systems. Glass Aspheres We supply aspheres in a range of optical materials. Standard Catalog and Specialty Aspheric Products In addition to standard catalog aspheres, we provide custom solutions such as assembly and a range of coating options. To discuss how Ross Optical’s aspheric solutions can benefit your application, contact sales@rossoptical.com or (915) 595-5417. Ross Optical Industries | 1410 Gail Borden Place, A3, El Paso, TX 79935 | 800.880.5417 | 915.595.5417 | sales@rossoptical.com | www.rossoptical.com
