Proper autofocus for better particle measurements

M. Zs. Kiss

doi:10.1117/12.2508896

1 March 2019 Proper autofocus for better particle measurements

M. Zs. Kiss

Proceedings Volume 10944, Practical Holography XXXIII: Displays, Materials, and Applications; 109440I (2019) https://doi.org/10.1117/12.2508896
Event: SPIE OPTO, 2019, San Francisco, California, United States

Abstract

Digital holographic imaging is a good tool for automatic particle size and morphology measurements. One of its benefits is that flowing samples can be observed, but the focus –the reconstruction distance- should be well defined automatically to reach the best quality image reconstruction during real time measurements. Now, we present a new way to increase the accuracy of the autofocus. It is not another autofocus algorithm, but it can be used with different autofocus algorithms. Our idea was to change the input images of the autofocus algorithm. The theoretical foundation of this modification is the known connection between the numerical aperture and the depth of field. A focused target beam contains central rays with smaller and extremal rays with higher converging angles. Imaging without the central rays, the rays that increase the depth of focus will be missing but the rays which carry the details will be kept . Using only the extremal rays at the imaging we can see more clearly whether the image is in focus or not, even though it has missing information. Therefore, we reconstructed the input images of the autofocus algorithm from the hologram without the central rays. The reconstruction was made with the angular spectrum method. The central rays were eliminated with a proper frequency-filtering of Fourier transform of the hologram. This new method was tested with different autofocus algorithms, and its efficiency can be clearly observed. We used it in an in-line holographic setup, which was built to observe algae from algae cultures.

Conference Presentation

Citation Download Citation

M. Zs. Kiss "Proper autofocus for better particle measurements", Proc. SPIE 10944, Practical Holography XXXIII: Displays, Materials, and Applications, 109440I (1 March 2019); https://doi.org/10.1117/12.2508896

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