21 October 2004 Focal length assessment by self-imaging
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Proceedings Volume 5622, 5th Iberoamerican Meeting on Optics and 8th Latin American Meeting on Optics, Lasers, and Their Applications; (2004) https://doi.org/10.1117/12.591641
Event: 5th Iberoamerican Meeting on Optics and 8th Latin American Meeting on Optics, Lasers, and Their Applications, 2004, Porlamar, Venezuela
Abstract
Several classical and modern approaches were developed in the literature for measuring the focal length of a lens. We focus our attention on those that employ self-imaging methods. In our proposal, a collimated laser beam illuminates a test lens. After the lens, the wavefront becomes convergent or divergent according to the lens power. Attached to the lens, we place a Ronchi grating. This procedure gives rise to several classical Talbot images, although magnified due to the non-parallel illumination. Key to our presentation is the use of the property that the pitch of each self-image is directly related to the focal length of the collimating lens. To obtain the pitches, a lensless CCD camera is positioned in any two consecutive self-images planes. The respective images are captured and processed. We designed software that allows first to precisely focus each self-image plane by a best-contrast algorithm, and then takes an intensity histogram along a direction perpendicular to the grating lines. By measuring the distance between consecutive self-image planes and their pitches, the focal length is determined. This method provides a reference testing with higher setting sensitivity and an increase in the accuracy compared to previous methods. Note that the proposed method represents a practical improvement over other existing methods.
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Myrian C. Tebaldi, Nestor A. Bolognini, Alberto A. Tagliaferri, Roberto Daniel Torroba, "Focal length assessment by self-imaging", Proc. SPIE 5622, 5th Iberoamerican Meeting on Optics and 8th Latin American Meeting on Optics, Lasers, and Their Applications, (21 October 2004); doi: 10.1117/12.591641; https://doi.org/10.1117/12.591641
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