When it comes to asphere production, is there one part of the process that turns out to be more important than the others? Not really. Ignoring materials, intended usage, manufacturability, or measurability often leads to a lens too expensive to produce and too difficult to measure. The pieces need to fit together to ensure maximized performance that is cost effective.
Materials Affect Design
In a very loose translation of the expression “form follows function,” the optical designer will want to know not only the intended use of the asphere, but the environment it will be used. Only certain materials can withstand heat, cold, pressure or any number of other conditions. Usage and environment are two parameters that must be considered in conjunction with the type of material in order to ensure the success of the design. Some common materials used for aspheres include the following, and they are all suited to a function in their own way.
Pick the wrong one, that does not mesh with the method of manufacture, and performance can be affected. Over specify the glass, and costs are increased unnecessarily. Can the material be polished or ground or molded, melted, or some combination thereof? These are questions to be considered.
What Manufacturing Method will be Used?
Aspheres are complex to manufacture. Used most frequently, these methods include Plastic Injection Molding, Glass Molding, Diamond Turning, CNC & MRF Polishing and even ion beam figuring (IBF). The molding methods do not remove material while the other methods are subtractive — some amount of glass, plastic, or crystalline material is removed.
So, the dovetail here is determining the manufacturing method that will work with the specified material — producing an asphere design that performs as intended and isn’t budget busting.In addition, certain methods are more suited to “one-at-a-time,” low volume runs.Others handle high volume much more effectively.
There are trade-offs. Benefits for one method could be high volume and low cost per asphere, but the setup and tooling costs are expensive. CNC, MRF, and IBF work well on optical glass, but not plastics. Diamond Turning can handle a variety of shapes, including free form, but is not suited to high volume production.
The interaction between design and material selection is a balancing act in which the scale can’t be tipped too heavily in favor of one or the other.
Now, Let’s Measure the Asphere
Not only are they challenging to manufacture; they are also challenging to measure. Unlike a traditional spherical lens, the surface of an axisymmetric asphere can be thought of as a collection of spheres of continuously varying radii.
As with spheres, there is a need to measure how accurately the manufactured asphere matches its designed shape. Depending upon the design and specified tolerances, can the available measurement equipment provide data for form error (irregularity), waviness, and roughness?
The two most common methods for measuring aspheres include interferometry and profilometry. Interferometric measurements provide surface figure data by comparing two wavefronts — one representing a high-quality reference (transmission sphere) and the other the test part (asphere). The resulting interference pattern is a map of the part’s shape, and with spheres this is straightforward. However, this is not as easy as it sounds when the reference must be able to compare to a surface with varying radii.
Until recently, standard interferometric techniques included the use of null lenses and computer generated holograms (CGH). Null lenses can be difficult to setup, and are not cost effective for low-volume production or R&D. A CGH makes it possible to create a desired wavefront of nearly any shape, and the resulting interference pattern directly indicates the deviation between the actual aspheric surface and the nominal surface — in essence a good quality reference.
With the Recent Introduction…
ZYGO recently introduced a new product for measuring aspheres — one that does not require a null lens or specially made CGH. The VFA+ System combines the industry standard VerifireTM HD with Distance Measuring Interferometry (DMI). This system also includes Mx software with a VFA application designed to make measuring a variety of aspheres possible and easy to do.
The VFA+ measurement employs a unique combination of two interferometric techniques: noncontact, three-dimensional, optical phase-shifting interferometry and displacement measuring interferometry (DMI). Setup is simplified, alignment is simplified, and a built-in design check tool speeds up the measurement process while providing confidence that the measured asphere meets design specifications.
Contact ZYGO for more information on the VFA+ System.