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We measured sample aspheres using multiple techniques: profilometry, null interferometry, and subaperture stitching. We also obtained repeatability and reproducibility (R&R) measurement data by retesting the same aspheres under various conditions. We highlight some of the details associated with the different measurement techniques, especially efforts to reduce bias in the null tests via calibration. We compare and contrast the measurement results, and obtain an empirical view of the measurement uncertainty of the different techniques. We found fair agreement in overall surface form among the methods, but meaningful differences in reproducibility and mid-spatial frequency performance.
Therefore after each fabrication step we want to know how the surface features at different lateral sizes evolved, so that we can optimize the choice of the next fabrication step. A spec like PSD often doesn’t inform the fabricator whether it’s failing because of a localized error (such as edge roll), continuous texture, or an artifact of metrology or computation. So while it may indicate the need for additional fabrication steps, it is not ideal for guiding specifically which fabrication step ought to be undertaken next. We have developed analyses to help determine what surface characteristics are failing spec, and thus optimize the next fabrication step. Finally, we demonstrate an example of how we have applied these techniques to fabricate parts with demanding slope and MSF specifications.
To investigate how zoom affects the MSF measurement capability, we measured a spherical surface with significant MSF content over a range of lateral magnifications. Two methods were used to obtain equivalent lateral magnifications: zoom and changing the transmission sphere. Differences in the relative MSF content were observed between the two methods. For further comparison, the same surface was measured with the same transmission spheres on a different interferometer with a fixed magnification coherent viewing system. We report on differences observed in measured MSF content between the two interferometers.
This course provides an overview of how aspheric surfaces are designed, manufactured, and measured. The primary goal of this course is to teach how to determine whether a particular aspheric surface design will be difficult to make and/or test. This will facilitate cost/performance trade off discussions between designers, fabricators, and metrologists.
We will begin with a discussion of what an asphere is and how they benefit optical designs. Next we will explain various asphere geometry characteristics, especially how to evaluate local curvature plots. We will also review flaws of the standard polynomial representation, and how the Forbes polynomials can simplify asphere analysis. Then we will discuss how various specifications (such as figure error and local slope) can influence the difficulty of manufacturing an asphere. Optical assembly tolerances, however, are beyond the scope of this course - we will focus on individual elements (lenses / mirrors).
The latter half of the course will focus on the more common technologies used to generate, polish, and/or measure aspheric surfaces (e.g. diamond turning, glass molding, pad polishing, interferometry). We'll give an overview of a few generic manufacturing processes (e.g. generate-polish-measure). Then we'll review the main strengths and weaknesses of each technology in the context of cost-effective asphere manufacturing.
This course provides attendees with a broad overview of optical surface metrology, with a focus on how to choose tools and techniques to support modern optical manufacturing processes. First we will review metrology principles and definitions of measurement capability (e.g. accuracy, lateral resolution, etc.). After establishing this basic language, we will discuss the metrology challenges that modern optical applications present (e.g. greater aperture sizes, improved accuracy specifications, and more complex shapes such as aspheres and free-forms).
We will next compare the capabilities and limitations of various tools for the measurement of figure, mid-spatial frequencies, and finish (e.g. Fizeau interferometers, stylus profilometry, interference microscopes, various null tests for aspheres). Examples of "real" data from some measurement tools will be provided. Finally we will review how to identify measurement performance limitations, and techniques for extending capability such as error calibration, averaging, and subaperture stitching.
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