Extended focus imaging in digital holographic microscopy: a review

The microscope is one of the most useful tools for exploring and measuring the microscopic world. However, it has some restrictions in its applications because the microscope's depth of field (DOF) is not sufficient for obtaining a single image with the necessary magnification in which the whole longitudinal object volume is in focus. Currently, the answer to this issue is the extended focused image. Techniques proposed over the years to overcome the limited DOF constraint of the holographic systems and to obtain a completely in-focus image are discussed. We divide them in two macro categories: the first one involves methods used to reconstruct three-dimensional generic objects (including techniques inherited from traditional microscopy, such as the sectioning and merging approach, or multiplane imaging), while the second area involves methods for objects recorded on a tilted plane with respect to hologram one (including not only the use of reconstruction techniques and rotation matrices, but also the introduction of a numerical cubic phase plate or hologram deformations). The aim is to compare these methods and to show how they work under the same conditions, proposing different applications for each. (C) The Authors.