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1. Height reconstructions from geometric wavefronts using vision ray metrology

   https://doi.org/10.1364/AO.539226  

   Ramirez-Andrade, Ana Hiza; Falaggis, Konstantinos (November 2024, Applied Optics)

   A recently reported vision ray metrology technique [Opt. Express29,43480(2021)OPEXFF1094-408710.1364/OE.443550] measures geometric wavefronts with high precision. This paper introduces a method to convert these wavefront data into height information, focusing on the impact of back surface flatness and telecentricity errors on measurement accuracy. Systematic errors from these factors significantly affect height measurements. Using ray trace simulations, we estimate reconstruction errors with various plano-concave and plano-convex elements. We also developed a calibration technique to mitigate telecentricity errors, achieving submicron accuracy in surface reconstruction. This study provides practical insights into vision ray metrology systems, highlighting validity limits, emphasizing the importance of calibration for larger samples, and establishing system alignment tolerances. The reported technique for the conversion of geometric wavefronts to surface topography employs a direct non-iterative ray-tracing-free method. It is ideally suited for reference-free metrology with application to freeform optics manufacturing. 

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2. Telecentric broadband objective lenses for optical coherence tomography (OCT) in the context of low uncertainty metrology of freeform optical components: from design to testing for wavefront and telecentricity

   https://doi.org/10.1364/OE.27.006184  

   Xu, D; Chaudhuri, R; Rolland, J (March 2019, Optics express)

   Freeform optical components enable significant advances for optical systems. A major challenge for freeform optics is the current lack of metrology methods with measurement uncertainty on the order of tens of nanometers or less. Towards addressing this challenge, optical coherence tomography (OCT) is a viable technique. In the context of low uncertainty metrology, the design requirements pertaining to the sample arm of an OCT metrology system are explicitly addressed in this paper. Two telecentric, broadband, diffraction limited, custom objective lens designs are presented with their design strategies. One objective lens was fabricated and experimentally tested for wavefront performance and telecentricity. This lens demonstrates near diffraction limited performance and a maximum deviation from telecentricity of 8.7 arcseconds across the full field of view, correlating to measurement uncertainty of less than 12 nm in simulation. The telecentricity test method developed completes the loop with respect to the design requirements and strategies presented and provides further intuition for telecentric lens designs in general. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement 

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3. High-Dynamic Range, High-Resolution Freeform Metrology with Optical Differentiation Wavefront Sensing

   https://doi.org/10.1364/AOMS.2020.OF4B.6  

   Swain, B. R.; Dorrer, C.; DeFisher, S.; Qiao, J. (January 2020, Imaging and Applied Optics Congress, OSA Technical Digest (Optical Society of America, 2020))

   Wavefront generated by freeform phase plate is measured with an optical differentiation wavefront sensor. Comparison of the measurement with a commercial scanning interferometer shows M0 (A=633 nm) agreement, demonstrating its potential for metrology applications. 

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4. Implementation of a null test for freeform optics using a high-definition spatial light modulator

   https://doi.org/10.1364/OE.473853  

   Chaudhuri, Romita; Wansha, Alexander; Porras-Aguilar, Rosario; Rolland, Jannick P. (January 2022, Optics Express)

   We report the implementation of an interferometric null test using a high-definition spatial light modulator (SLM) as a reconfigurable alternative to a computer-generated hologram. We detail the alignment process chain, including novel techniques using the SLM to project alignment fiducials on the test part. To validate the alignment protocol, we measure a mild off-axis conic with the SLM-based system and cross-validate with conventional interferometry within 30 nm root-mean-square (RMS) surface figure. Finally, we report the null test of a 65 mm clear aperture concave freeform with 91 μm peak-valley sag departure from the base sphere. The measured surface figure of the freeform is within 40 nm RMS compared to the measurement with a commercial metrology instrument. 

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5. Estimating the irradiance of a perturbed surface under an extended source

   https://doi.org/10.1364/OE.521368  

   Kim, Wooyoun; Cassarly, William J; Pomerantz, Michael; Rolland, Jannick P (May 2024, Optics Express)

   This paper presents a method for evaluating the irradiance of a single freeform surface deviation under extended source illumination. The method takes advantage of a well-known concept, the pinhole image. First, the irradiance of the perturbed freeform surface under point source illumination is computed. Second, a pinhole image of the extended source is obtained by placing a small aperture (pinhole) on the freeform surface. Then, the extended source irradiance pattern change can be quickly calculated by convolving the pinhole image with the perturbed point source irradiance change. The method was experimentally verified, demonstrating the efficacy of the underlying concept. The proposed method alleviates the computational demands during extended source tolerancing, expediting the process. 

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