The aim of this work is to develop a holographic method that provides the shape reconstruction with high, interferometric accuracy and an extended measurement range. The method requires recording of a set of <i>n</i> holograms obtained for selected combination of illumination angles θ𝑛. The difference between the optical phases corresponding to on-axis φ0 and offaxis φ𝑛 fields allows calculating the object height. To maintain high-accuracy evaluation of height the dedicated shape reconstruction algorithm is proposed. The algorithm consists of n steps, each with several substeps. Each substep is divided into: (1) calculation of the height from φ0 and φ𝑛 ; (2) propagation of the optical fields. In this paper, the proposed algorithm is numerically validated using three types of objects.
This paper proposes an improvement of the measurement method of focusing high gradient microobjects of small and large radius of curvature. The measurement process is carried out in a Fourier digital holographic microscope with spherical illumination, which ensures maximum usage of numerical aperture of the optical system. The interference pattern is the result of interference between the deformed, quasi-spherical object and spherical reference waves. The noise influence on the phase measurement is analyzed and minimized with block-matching 3-D (BM3D) filtering method. The accurate shape reconstruction of the object and localization of the imaging reference plane are provided by (1) the aberration compensation procedure, and (2) the calibration procedure. To obtain the final shape, the Local Ray Approximation approach is used. In this work an effective BM3D method was applied in the process of the reconstruction of the object beam, improving accuracy of the measurement method. The validation of our approach is presented for spherical objects of large radius of curvature and high gradient of shape.
In this contribution, we propose a method of digital holographic microscopy (DHM) that enables measurement of high
numerical aperture spherical and aspherical microstructures of both concave and convex shapes. The proposed method
utilizes reflection of the spherical illumination beam from the object surface and the interference with a spherical
reference beam of the similar curvature. In this case, the NA of DHM is fully utilized for illumination and imaging of the
reflected object beam. Thus, the system allows capturing the phase coming from larger areas of the quasi-spherical
object and, therefore, offers possibility of high accuracy characterization of its surface even in the areas of high
inclination. The proposed measurement procedure allows determining all parameters required for the accurate shape
recovery: the location of the object focus point and the positions of the illumination and reference point sources. The
utility of the method is demonstrated with characterization of surface of high NA focusing objects. The accuracy is
firstly verified by characterization of a known reference sphere with low error of sphericity. Then, the method is applied
for shape measurement of spherical and aspheric microlenses. The results provide a full-field reconstruction of high NA
topography with resolution in the nanometer range. The surface sphericity is evaluated by the deviation from the best
fitted sphere or asphere, and the important parameters of the measured microlens: e.g.: radius of curvature and conic
This paper presents the study on the accuracy of topography measurement of high numerical aperture focusing
microobjects in digital holographic microscope setup. The system works in reflective configuration with spherical wave
illumination. For numerical reconstruction of topography of high NA focusing microobjects we are using two algorithms:
Thin Element Approximation (TEA) and Spherical Local Ray Approximation (SLRA). In this paper we show comparison
of the accuracy of topography reconstruction results using these algorithms. We show superiority of SLRA method.
However, to obtain accurate results two experimental conditions have to be determined: the position of point source (PS)
and imaging reference plane (IRP).Therefore we simulate the effect of point source (PS) and imaging reference plane (IRP)
position on the accuracy of shape calculation. Moreover we evaluate accuracy of determination of location of PS and IRP
and finally present measurement result of microlens object.