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.
TIRDHM is a technique that allows to analyse the phase change of microscopical sections produced on the prism surface due to material attached on the top. Therefore, due to the evanescence waves properties we can analyse quantitatively the properties and specific morphology located to few nanometers on the top of surface contact. In this work, we study and present an alternative method to off-axis configuration to record and analyse the microscopical phase object information in Total Internal Reflection dispensing with the use of reference arm.
Digital holography (DH) is a well-established interferometric tool in optical metrology allowing the investigation of engineered surface shapes with microscale lateral resolution and nanoscale axial precision. With the advent of charged coupled devices (CCDs) with smaller pixel sizes, high speed computers and greater pixel numbers, DH became a very feasible technology which offers new possibilities for a large variety of applications. DH presents numerous advantages such as the direct access to the phase information, numerical correction of optical aberrations and the ability of a numerical refocusing from a single hologram. Furthermore, as an interferometric method, DH offers both a nodestructive and no-contact approach to very fragile objects combined with flexibility and a high sensitivity to geometric quantities such as thicknesses and displacements. These features recommend it for the solution of many imaging and measurements problems, such as microelectro-optomechanical systems (MEMS/MEOMS) inspection and characterization. In this work, we propose to improve the performance of a DH measurement on MEMS devices, through digital filters. We have developed an automatic procedure, inserted in the hologram reconstruction process, to selectively filter the hologram spectrum. The purpose is to provide very few noisy reconstructed images, thus increasing the accuracy of the conveyed information and measures performed on images. Furthermore, improving the image quality, we aim to make this technique application as simple and as accurate as possible.
Limited depth of field (DOF) is one of the main shortage for many optical imaging systems. This is a limitation that
precludes to get in focus, in a single plane, objects that are located at different distances, but that fall in the same field of
view. Furthermore, the depth of field is reduced as much as greater is the requirement for a high magnification and to
obtain an extended focus image (EFI) of these objects remains one of the major challenges. In this work we propose and
compare two different approaches to build the EFI of holograms recorded on a tilted plane. In the first case, a simplified
three-dimensional (3D) formulation of the angular spectrum method (ASM) is proposed. It allows to generate the entire
stack of propagated images in a single shot. In the second approach, a numerical cubic phase plate (CPP) is included into
the reconstruction process of digital holograms with the aim to enhancing DOF of optical imaging system. Theoretical
formulations of the two approaches are supported by experimental evidences. The obtained results show that the
proposed strategies allow to reconstruct effectively an EFI from holograms recorded on an inclined plane.