The need for rapid, high resolution, accurate metrology of mass produced aspheric and freeform surfaces continues to grow as the application space evolves. Expanding the use cases of standard interferometers, computer generated holograms enable snapshot, megapixel resolution measurements of freeform surfaces that may have large and non-symmetric departure from spherical. CGH metrology at production scales can be realized without specialized engineers or in-house knowledge of setup. In this paper, we highlight the impact of the CGH vendor to provide not only a hologram but a suite of hardware to streamline high volume aspheric and freeform metrology. Alignment becomes straightforward, data acquisition remains standard to the interferometer and data is processed automatically. Early integration of this metrology solution in product planning helps users scale from prototype to manufacture.
Arizona Optical Metrology supplies Computer Generated Holograms (CGHs) that are used around the world for projects in industry, research, and defense. CGHs enable high-accuracy snapshot measurements of complex optical surfaces, such as cylinders, rotationally symmetric aspheres, conic sections and freeforms. The growing markets that use such high performance optics, along with technical advances in CGH capabilities, have created a demand for technical training in CGH metrology. AOM has produced a CGH education kit for the expressed purpose of donation to colleges and university optics programs for training students and faculty in the capabilities and usability that CGH metrology offers.
It is well known that computer generated holograms enable standard interferometers to measure nearly any shape with accuracy of a few nanometers. Most high-precision non-spherical optical surfaces are manufactured based on CGH measurements. Historically, the application of CGH interferometry required a skilled optical engineer to define the test configuration, set up and align the hardware, and reduce the data. In this talk we discuss a different scenario, where the CGH vendor provides not only the hologram, but a kit of hardware and software that eliminates the need for a specialized engineer. The alignment is straightforward, the data acquisition follows the standard procedures for measuring spheres, and the data processing is automatic. Several examples illustrate this paradigm shift, including an autonomous system that loads the optics with a robot and performs snapshot measurements without any adjustment at all. This takes CGH interferometry from the laboratory to the manufacturing floor.
The Thirty Meter Telescope primary mirror consists of 492 1.4 m diameter hexagonal segments. A CGH-assisted interferometric testbed has been developed to quickly and accurately measure surface figure error of the 82 different segment prescriptions. In this paper, technical aspects of the testbed will be described, including interferometer design, techniques to reduce sensitivity to vibration and turbulence, use of CGH phase fiducials for 6 degree-of-freedom alignment of the interferometer to the Test Plate, proximity sensors for segment-to-Test Plate alignment, synthetic extended source technique to mitigate coherent artifacts, characterization of instrument transfer function, and system calibration. A surface figure error map of a "Type 0" full-scale segment will also be presented.
We propose an in situ method for establishing the amplification coefficient (height scale) for an interference microscope as an alternative to the traditional step height standard technique for routine calibration. The method begins by determining the properties of the microscope illuminator equipped with a narrow-band spectral filter, using a spectrometer to provide traceability to the 546.074nm 198Hg line. A data acquisition with the interference microscope links this wavelength standard to a calibration of the properties of the optical path length scanning mechanism of the interferometer. A capacitance sensor in the scanner maintains this calibration for subsequent measurements. A targeted k=1 uncertainty of 0.1% is favorable when compared to calibration using physical artifacts, and the calibration procedure is easier to perform and less sensitive to operator error.
Optical 3D profilers based on Coherence Scanning Interferometry (CSI) provide high-resolution non-contact metrology
for a broad range of applications. Capture of true color information together with 3D topography enables the detection of
defects, blemishes or discolorations that are not as easily identified in topography data alone. Uses for true color 3D
imaging include image segmentation, detection of dissimilar materials and edge enhancement. This paper discusses the
pros and cons of color capture using standard color detectors and presents an alternative solution that does not rely on
color filters at the camera, thus preserving the high lateral and vertical resolution of CSI instruments.
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