Digital lensless holographic microscopy (DLHM) allows the design of cost-effective systems using off-the-shelf materials, making this type of lensless microscope accessible to many users worldwide. However, these materials may have a limited optomechanical performance that is aggravated due to the sought compactness and the intended cost-effective manufacturing process. This problem particularly affects the illumination source, which is of critical importance for DLHM, as it defines the optical performance of the microscope. While recent reports show that the required point source can be built from a low-cost laser diode coupled to an also low-cost aspheric lens, the resulting illumination has a distorted wavefront that limits the performance of the microscope. A simple homemade setup to correct the distortion of such illumination source and its integration into a compact, cost-effective, DIY, and open-source-certifiable digital lensless holographic microscope, is presented. The distortion-corrected DLHM is validated by imaging calibrated test targets and biological samples, achieving a 12-fold extension on the distortion-free magnification range of previous designs and a doubling of the effective spatial resolution without significant increments in its overall cost.
In this work, the content for an undergraduate-level holography workshop is presented. The session is divided into two parts: an instructional section and a hands-on application activity supported by research-grade open-source software. The first section starts with a brief theoretical review of conventional imaging, interference, and diffraction as the underlying physical phenomena. These concepts are then used for the description of the recording, processing, and reconstruction stages of analog holography, emphasizing in each case the phenomenology rather than the mathematical framework. Finally, the translation of these stages to digital holography is presented, introducing the principles of digital recording and numerical reconstruction. The contents of the instructional section are then applied in a lecturer-guided activity, in which the participants generate a computational off-axis hologram and calculate its reconstruction. All the operations are performed using the “Numerical Propagation” plugin of the open-source software ImageJ. This research-grade software allows the modeling and manipulation of complex-valued wavefields from a user-friendly graphical interface. It thus allows the participants to recreate step-by-step the recording and reconstruction stages of the holographic process, while directly identifying when each physical phenomenon is at play. The proposed content can be implemented either as a stand-alone workshop or as an applied component of an undergraduate optics course.
Since the discovery of the wave behavior of light, diffraction has been a cornerstone in optics. The teaching of the diffraction theory has been usually done theoretically based on a mathematical approach that could hinder the understanding of the physical phenomena. In this work, the simplicity in the architecture of an accessible, cost-effective, and 3D-printed digital lensless holographic microscope is used as an educational tool to study the diffraction theory by providing experimental validations of the phenomena for undergraduate students. The recording and reconstruction steps of the lensless holographic technique take the students to the bidirectionally of the diffraction phenomena in a completely hands-on approach. The integration of the theory with an accessible experimental setup generates an innovative way of teaching the diffraction phenomena in a classroom.
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