Aspheric Optics: Design, Fabrication & Test

Like going back to college, about 30 interested attendees sat in on a four-hour educational course on aspheric optics presented by Robert Fischer of Optics 1 Inc. on Wednesday at Optifab 2009.

One of the many introductory courses offered at the conference, the program covered aspheric applications, capabilities, designs, fabrications, testing and even blunders in the industry. The course was intended for those in the optical systems design industry to make them aware of what’s actually going on with aspheric surfaces today.

Aspheric and non-spherical surfaces of lenses are very beneficial for optical performance. Aspheres are used in many products such as conventional lenses, telescope optics, digital cameras, cell phone cameras, inserts for plastic molding and glass molding, CD’s and DVD’s and injection molding plastics. Chief manufacturing methods for aspheric surfaces include diffractive optics, compression-molded glass, deterministic microgrinding, conventional fabrication, single-point diamond turning and magnetorheological finishing; a method for high quality optical polishing.

Traditional methods of manufacturing equipment can be expensive and time consuming, while also requiring unique tooling with continuous change and reconditioning, also iterate trial and error processes are involved and complex surfaces of aspheres can be especially hard to make. It was stated that many optical shops are currently using out-of-date shaping, grinding and polishing equipment making manufacturing even more arduous. Nevertheless, over the past 10 years, at the Center for Optics Manufacturing, there have been several developments in optical machines, hopeful to turn production around.

A product that is ideal for aspheric surface applications is the digital camera. Digital camera sales have increased dramatically since 2000, but a problem with the growing market is the demand for new features, better zooming capabilities and smaller imaging systems. According to Fischer, because of such high demand, optics in the cameras have been a progressively challenging development. An increase in chip size (pixels) would require an increase of focal length so as to be proportionate for the same field of view. However, by developing a better design, more aberrations also develop leading to difficult design schemes and more aspheric surfaces.

Aspheric surfaces have also been useful for conformal optics to advance aerodynamics of missile domes used by DARPA in 1997. Airbags are also equipped with imaging systems integrated with aspheres as the system can redirect the deployment of an airbag away from babies that may be in bassinets or car seats; protecting them from any harm that may be caused by the immense force, Fischer said.

Concisely, aspheric surfaces are an extremely important tool in optical design applications. Though it is important for those involved in the industry to know that aspheric surfacing should be used in moderation; especially if stronger asphericities are necessary, lower orders should also be applied first unless for longer radii, which typically utilizes a 4th order coefficient. Additionally, designers are recommended to make sure they can test surfaces and lenses, select an optimal location for the asphere, consider effects of scattering and identify surface roughness and maximum slope. When using optics, equations to create a product have to be absolutely precise. Any miscalculation, even a slight divergence in measuring, could completely change the characteristics of a lens model such as the example made by Fischer of “The Hubble Space Telescope Optical Systems Failure Report.”

Optics 1 Inc. of Westlake Village, Ca. is an independent optics and optical systems design and manufacturing company, which focuses on precision optics, opto-mechanical design, electro-optical, engineering and system integration of precision optical products.

Amanda D. Francoeur
amanda.francoeur@laurin.com