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The results of the research focused on the reference and object beam intensity ratio effect on the transmittance of computer-generated and analog holograms are presented. Particular attention is paid to the hologram synthesis mode in which the intensity of the object beam exceeds the intensity of the reference beam (overmodulation mode). The study is relevant in cases where computer-generated holograms are used in extreme ultraviolet projection photolithography. Mathematical modeling of the physical processes of recording and reconstructing holograms has been performed. The characteristic size of the binary test object was 80 × 80 nm, the radiation wavelength was 13.5 nm, the hologram pixel size was 20 × 20 nm, the distance between the object and hologram planes was 20.4 μm, and the incidence angle of the plane reference wave was 14°42′. Synthesis and reconstruction of holograms were carried out in an overmodulation mode, with different beam paths in the object beam. It is shown that the computer-generated holograms, unless binarized, are always displayed and reconstructed as quantized holograms with a quantization interval depending on the parameters of the synthesis scheme. It has been established that the influence of the overmodulation mode on the quality of the reconstructed image when using computer-generated holograms will be much less than in the case of using analog holograms, but will also be determined by the dynamic range of the object beam intensity in the hologram synthesis plane. It is noted that the influence of overmodulation mode is minimal if an object beam converging at the center of the hologram is used during synthesis. The choice of the adequate quantization interval and the ratio of the intensities of the reference and object beams will ensure high quality of the reconstructed image when using computer-generated Fresnel holograms in extreme ultraviolet projection hotolithography.

Quantization is widely used for compression, storage, transmission and display of computer-generated and digital holograms. It reduces number of hologram gradations to obtain better ratio of the quality of reconstructed 2D and 3D objects to the size of holographic images or videos. This paper proposes 2 methods of non-iterative quantization of holograms: adaptive non-uniform quantization based on histogram processing with peak shift (ANHS method) and additional application of logarithmic companding (ANHSL method). They were compared with 4 methods of iterative and non-iterative quantization on holograms with sizes up to 2048 × 2048 pixels. The proposed methods showed the best reconstruction quality that is higher up to 59 % in comparison with non-iterative uniform quantization. Especially the proposed methods give higher quality at little number of hologram gradations (from 2 to 16 gradations). In comparison with the most qualitative iterative methods (Lloyd-Max and k-means) the quality is increased up to 9 %, and processing time is reduced in 10–1000 times. [ABSTRACT FROM AUTHOR]

Subject of Research.The paper considers the issues of objective quality assessment of computer-generated Fresnel holograms and reconstructed binary object images. Method.The work is performed by the method of projection holographic photolithography based on computer-generated Fresnel holograms. The synthesis of holograms was carried out by mathematical modeling of the physical processes of hologram recording and reconstruction in the original software package using the following parameters: the binary object characteristic size of 80 × 80 nm, laser wavelength of 13.5 nm, hologram pixel size of 20 × 20 nm, the distance between an object and hologram planes equal to 20.4 μm, reference wave incidence angle of 14°42′. The hologram synthesis for each object under research was carried out for the case of two different synthesis setup parameters. The reconstructed images of these objects were compared by three methods: the peak signal-to-noise ratio calculation method, the structural similarity index calculation method, and the method of image quality assessment based on the calculation of the number of levels at the threshold image processing, which is an imitation of photoresist response to actinic radiation exposure. Main Results. Diffraction-based criterion, traditionally applied in optics, is proven to be the most relevant for the quality evaluation of projection photolithographic systems, including systems based on computer-generated holograms. It is established that the methods based on the calculation of the structural similarity index and on estimation of the number of levels at the threshold image processing are the most suitable methods for assessing the reconstructed image quality in the field of projection holographic photolithography. Moreover, the second method does not require any additional calculations. Practical Relevance. Application of an adequate quality assessment method makes it possible to compare quickly and objectively the reconstructed images of the same objects obtained for various parameters of the synthesis setup and select the object representation form and the hologram synthesis setup that is the most suitable for a particular task

Subject of the Research. The paper presents the study on the effect of object representation form on the depth resolution of images reconstructed via computer-generated Fresnel holograms-projectors. Method. The study was performed by mathematical modeling of synthesis and reconstruction of holograms-projectors of various objects with a characteristic size of 20 × 20 in virtual space. The method is based on object wave representation at hologram synthesis as a superposition of object waves emanating from two identical objects located at different distances from the hologram synthesis plane. All numerical experiments were carried out using a specialized software package with the following parameters for the synthesis and reconstruction of holograms: the wavelength of the radiation used is 13.5 nm, the pixel size of the hologram is 20 × 20, the distance between the object and the hologram planes is 20.3 μm, the reference wave angle of incidence is 14.7 °. The quality criterion of the reconstructed image was expressed in terms of the number of gradations at the threshold processing of this image with the intensity distribution in the reconstructed image identical to the intensity distribution in the original object. Main Results. Following the carried out experiments, the dependence was drawn out for the depth resolution of the images reconstructed by computer-generated Fresnel holograms-projectors on object representation form. In particular, it was found that full object representation based on transparencies is the most effective rather than representation of just one its part with the small-sized elements of the object structure. Practical Relevance. The application of this method makes it possible to optimize the object structure parameters, that is, the form and sizes of its elements during the synthesis of Fresnel holograms-projectors with the higher depth resolution for holographic photolithography.

Subject of Research. The paper presents research results on object structure effect, that is the type and size of its elements, on optimal level of computer generated Fresnel holograms binarization in terms of reconstructed image quality. Method. The study was carried out through mathematical modeling of synthesis and reconstruction of holograms of various objects with 80x80 nm pixel size using different binarization levels, and comparing the quality of reconstructed images. All numerical experiments were carried out in a software package for the synthesis and reconstruction of holograms using the following hologram synthesis and reconstruction parameters: wavelength equal to 13.5 nm, hologram pixel size equal to 20x20 nm, distance between object and hologram planes of 20.3 μm, reference beam angle of 18.1. A criterion for the quality estimation of reconstructed image was a number of gradations in the threshold processing of this image, in which the intensity distribution in the reconstructed image would be identical to the intensity distribution in the original object. Main Results. The dependency graphs between binarization level and thenumber of permissible gradations for threshold image processing are obtained. For binary transparency objects used in the research, they are curves with two pronounced peaks, usually located at levels 0.2 and 0.34 along binarization axis.The object structure has an effect on specific value of optimal binarization level and on the maximum number of threshold processing gradations; the general nature of researched dependency remains unchanged even for complex transparency objects. Practical Relevance. The revealed dependency gives the possibility to accelerate the optimal level choice of synthesized holograms binarization.

The paper considers the features of synthesized holograms suitable for practical use. It is established that binary holograms are the first of all suitable ones for successful application in practice. In order to select the most suitable (optimal) level of hologram binarization, we propose a criterion for estimating the quality of an image reconstructed with a binary hologram. An algorithm is developed to find the optimal level. On the basis of the conducted experiments it is established that the introduction of the developed module gives the possibility to reduce the search time of the optimal binarization level of the hologram by eleven times in comparison with manual search.

Multiwavelength digital holographic microscopy (DHM) has been used to improve phase reconstructions of digital holograms by reducing 2π phase ambiguities. However, most samples used as test images have been solid or adhered to a surface, making it easy to determine focal planes and correct for chromatic aberration. In this study we apply 3-wavelength off-axis DHM to swimming protozoa containing distinct spectral features such as chlorophyll and carotenoids. We reconstruct the holograms into amplitude and phase images using the angular spectrum method. Methods for noise subtraction, chromatic aberration correction, and image registration are presented for both amplitude and phase. Approaches to phase unwrapping are evaluated and compared to expected results from simulated holograms. The algorithms used are implemented in plug-ins using the open source Fiji platform and are available for use, significantly expanding the open-source software available for DHM.

Optical scanning holography (OSH) is a distinct digital holography technique, which uses a single two-dimensional (2D) scanning process to record the hologram of a three-dimensional (3D) object. Usually, these 2D scanning processes are in the form of mechanical scanning, and the quality of recorded hologram may be affected due to the limitation of mechanical scanning accuracy and unavoidable vibration of stepper motor’s start-stop. In this paper, we propose a new framework, which replaces the 2D mechanical scanning mirrors with a Digital Micro-mirror Device (DMD) to modulate the scanning light field, and we call it OSH based on Compressive Sensing (CS) using a digital micro-mirror device (CS-OSH). CS-OSH can reconstruct the hologram of an object through the use of compressive sensing theory, and then restore the image of object itself. Numerical simulation results confirm this new type OSH can get a reconstructed image with favorable visual quality even under the condition of a low sample rate. [ABSTRACT FROM AUTHOR]

Low-energy electron holography (LEEH) is one of the few techniques capable of imaging large and complex three-dimensional molecules, such as proteins, on the single-molecule level at subnanometer resolution. During the imaging process, the structural information about the object is recorded both in the amplitude and in the phase of the hologram. In low-energy electron holography imaging of proteins, the object's amplitude distribution, which directly reveals molecular size and shape on the single-molecule level, can be retrieved via a one-step reconstruction process. However, such a one-step reconstruction routine cannot directly recover the phase information encoded in the hologram. In order to extract the full information about the imaged molecules, we thus implemented an iterative phase retrieval algorithm and applied it to experimentally acquired low-energy electron holograms, reconstructing the phase shift induced by the protein along with the amplitude data. We show that phase imaging can map the projected atomic density of the molecule given by the number of atoms in the electron path. This directly implies a correlation between reconstructed phase shift and projected mean inner potential of the molecule, and thus a sensitivity to local changes in potential, an interpretation that is further substantiated by the strong phase signatures induced by localized charges.

Digital holographic microscope allows imaging of opaque and transparent specimens without staining. A digitally recorded hologram must be reconstructed numerically at the actual depth of the object to obtain a focused image. We have developed a high-resolution digital holographic microscope for imaging amplitude and phase objects with autofocusing capability. If the actual depth of an object is not known a priori, it is estimated by comparing the sharpness of several reconstructions at different distances, which is very demanding in means of computational power when the recorded hologram is large. In this paper, we present 11 different sharpness metrics for estimating the actual focus depths of objects. The speed performance of focusing is discussed, and a scaling technique is introduced where the speed of autofocusing increases on the order of square of the scale ratio. We measured the performance of scaling on computer-generated holograms and on recorded holograms of a biological sample. We show that simulations are in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we propose a novel method to detect the best reconstructing distance in phase-shifting digital holography, which can help one to reconstruct high-quality images even though the recording distance is unkonwn. This scheme is based on an algorithm, two-dimensional discrete cosine transform (DCT). Numerical experiments for this method are shown in this paper. It is shown that this method is not only effective but also fast compared to previous schemes for detecting the focal distance in digital holography. Meanwhile, the algorithm can be effective against different types of noise. [Copyright &y& Elsevier]

Digital holographic microscopy (DHM) with time-sequential off-axis illumination directions can synthesize a large aperture, and thus have a higher spatial resolution than the one with on-axis illumination. In this paper, time-sequential off-axis illumination directions are generated by a spatial light modulator (SLM) in DHM, and the residual phases along different illumination directions are suppressed by using the phase compensation technique, as such the image with resolution enhancement is reconstructed. The usage of SLM enables shifting the illumination for different orientations and phase shifts without mechanical motion. The experiments have been conducted to verify the feasibility of this method on the residual phase suppression. [ABSTRACT FROM PUBLISHER]

Abstract: Digitally-recorded holograms are numerically reconstructed to reveal the 3D profile of the object. When the distance of the object to the recording plane is not known a priori, one has to perform reconstruction at discrete steps and decide the best focus distance according to some criteria. There are autofocusing algorithms that utilize a metric to assess the quality of the images, such as the sharpness of the object, to decide the best focus distance. However, the computational cost of this search rapidly increases as the resolution of the system becomes higher since the object image has to be numerically reconstructed from the hologram first. Here, we investigate the behavior of the sharpness curves of the object images obtained from the scaled holograms. We find that it is possible to estimate the autofocus distance accurately with this scaling, and the resultant autofocus speed improvement is on the order of square of the scaling ratio. Experimental results are presented and they are in excellent agreement with the simulations. [Copyright &y& Elsevier]

A modification of the classical calculation algorithm is presented which allows to minimize the effect of the long range oscillations created by the twin object in the reconstruction process of in-line holograms obtained by low energy electron projection microscopy. The reconstructed shapes of a tungsten tip, observed with different magnifications are shown. [Copyright &y& Elsevier]