Measuring aspheric lenses aspheres

Erin McDonnell looks at next-generation solutions for the measurement of

An aspheric lens — sometimes known as an asphere — is a lens that’s not part of a sphere or cylinder and it can depart somewhat from a sphere. The more complex surface profile of an asphere not only reduces or eliminates spherical aberration but can also reduce other optical aberrations such as astigmatism. The result is a lens that creates sharper and clearer images. 

Aspheric surfaces are used in a variety of optical systems and are a powerful way of reducing the number of elements required. This simultaneously enhances a system’s performance, and ultimately reduces its size and weight. Manufacturers working in the optics sector are highly concerned with producing aspheres that are both cost-effective and that conform precisely to the design intent. However, testing the accuracy of an asphere’s shape and deviation from its design can be a considerable challenge, as it requires the measurement of the tiniest nanometer deviations in shape.

The industry landscape for aspheres is competitive because of the large number of players in the market. This competition has been a driving force for innovation and development, leading to large-scale growth in the aspheric market.  As a result, ultra-high precision measurement systems are critical as it is impossible to determine if an asphere was made to design without measuring it. The production process for aspheres can be iterative, with multiple part generations and measurement steps to achieve the required level of precision.  The goal is to minimise the number of iterations and reduce the time it takes to produce a surface.  The onus is on cutting-edge and ultra-precise metrology tools to deliver accurate measurement data quickly and efficiently.

The metrology of aspheres

Smaller aspheric lenses are typically manufactured using moulding technologies, which produce low-cost and relatively high-performance lenses. These types of high-volume production aspheres are typically used for applications such as cameras, hand-held mobile devices, and contact lenses. 

Larger, more precise, and expensive aspheres are made through grinding and polishing a glass substrate. These aspheres are often used in safety-critical and demanding applications such as telescopes, lithography equipment, and defence applications where the highest standard of precision and quality is required. 

There are several parameters to consider when selecting a metrology tool for aspheric optics. Test surface size, aspheric departure, number of surfaces, and measurement time are a handful of considerations that help guide tool selection or measurement technique. It is imperative for manufacturers to understand these critical measurement decisions, removing the complexity from complex surfaces. 

As manufacturers continue to produce a growing number of different optics of varying size and shape, they need a flexible metrology system that can measure multiple parts without major re-tooling. More than ever, companies are responding to changing research and design changes throughout the manufacturing process—they are looking to next generation metrology solutions that allow them to easily switch between designs and generate quick response. 

Next generation asphere metrology

Optical metrology manufacturer Zygo Corporation — a business unit of Ametek Inc — has been at the forefront of the development of interferometry-based metrology solutions for the measurement of aspheres for many years. The Verifire Asphere+ (VFA+) is the latest member of the Verifire series of laser Fizeau interferometers.

The Verifire series represents a complete line of high-performance metrology instruments for the measurement of plano, spherical, and aspherical surfaces and material characteristics, and provides a variety of proprietary acquisition techniques to ensure optimum metrology in a wide range of environments. 

The VFA+ supports the trend in industry for the prolific use of aspheric optics across numerous industry sectors, and leverages the benefits of Fizeau interferometry through a unique combination of precise, high-resolution, fast, and full aperture metrology for axisymmetric aspheres. This allows faster convergence on deterministic polishing feedback for more efficient surface generation, measurement of the vertex radius to describe the shape of an aspheric surface, and coverage of aspheric form error and mid-spatial frequency surface characteristics—all with one instrument.  

The system is also flexible, with the ability to measure a range of aspheres with only the change of the reference optic. The VFA+ is equipped with an optional secondary stage which supports a computer generated hologram (CGH) extending the asphere shape capability to nonsymmetric freeforms and off-axis aspheric optics. This future compatible investment opens the doors for flexible optical fabrication technology. With the ability to measure a part two different ways, quality assurance becomes more cost effective, is easier to execute, and delivers a high level of confidence in measurement results. 

The VFA+ is now supported by ZYGO’s Mx metrology software. The new VFA+ application was developed with a critical focus on creating an intuitive user experience for better ease-of-use and significantly improved measurement reliability for aspheres. This means both faster setup of new designs in the R&D and prototyping phase, and better efficiency in production.  The VFA+ is also has an option for multi-part automated measurement of trays of optics. This is a capability and facilitates high throughput for significantly improved efficiency in the volume production of aspheres. 

Summary

With the introduction of the VFA+, the challenges faced by the optics industry — providing a measurement system that gives optics manufacturers the flexibility and precision needed to measure a wide range of aspheric designs — has been addressed. The VFA+ pushes the boundaries of what is possible in the design and production of ever more precise freeform and aspheric optics, and as such is an enabling technology, stimulating innovation in the optics sector. 

Erin McDonnell is the Product Manager for Laser Interferometers at Zygo Corporation.