Sizing calibration in digital lensless holographic microscopy via iterative Talbot self-imaging

In this work, an iterative method to calibrate the length measurements in digital lensless holographic microscopy (DLHM) is presented. In DLHM, the correct sizing is controlled by the accurate knowledge of the illumination wavelength, the digital camera specifications, and the source-to-screen distance. While the two formers are provided by the manufacturers, the latter has to be somehow measured, usually by mechanical means. As an alternative for performing that key measurement in DLHM, the Talbot self-imaging effect, and hence the Talbot distance, is used as a measurement tool in this work. Amplitude reconstructions of a monolayer of self-organized spheres of polystyrene produce the self-imaging phenomenon, with the Talbot distance controlled by the illuminating wavelength, the reconstruction distance, and the period of the monolayer. As the latter can be varied in DLHM by changing the source-to-screen distance while keeping fixed the wavelength and the camera specifications, the source-to-screen distance is iteratively varied until the correct Talbot distance is found. The proposed method has the advantages of i) increased sensitivity because of the axial quadratic dependency on transversal measurements, ii) not requiring in-focus images that demand the use of focusing criteria, and iii) can use non-approximated propagation methods to produce axial intensity profiles to determine the correct source-to-screen distance. The feasibility of the calibration method is tested by reconstructing a USAF 1951 test of resolution.