More than a half century ago Dennis Gabor proposed that conventional lens-based imaging systems, such as the optical microscope, could be replaced by a new type of imaging system that is theoretically free of aberrations and can achieve numerical apertures arbitrarily close to unity. This new idea was, of course, holography, which Gabor referred to as “microscopy by wavefront reconstruction.” In this paper a modern version of Gabor’s original scheme for lensless microscopy is presented that employs a fully coherent (laser) source and no imaging lenses, and generates images on a digital computer using algorithms that mimic (and, actually improve upon) the imaging operation of a diffraction limited lens system. This class of imaging systems, which we will refer to as “digital holographic microscopes” (DHM), generates complex-valued images of amplitude, phase, and even three-dimensional objects by employing digital holography in combination with state-of-the-art computer algorithms to implement the imaging process.
The key experimental ingredient of DHM is phase-shifting holography (PSH), an idea originally conceived by Gabor in the 1950s and later published in 1966. PSH supplies the means of determining the complex amplitude of a coherent wavefield diffracted by an object by digitally recording multiple Gabor (in-line) holograms over an aperture that forms the entrance pupil of the microscope using, for example, a CCD array. Although other schemes exist for performing this task without the need of holography (such as the use of phase retrieval algorithms), PSH is an easily implemented and accurate method that is ideally suited toDHMand is exclusively employed in the microscopes discussed in this paper.
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