Off-axis Holograms Research Articles

Fringe contrast enhancement of digital off-axis hologram via sparse representation

This Letter presents a novel approach to enhance the fringe contrast (visibility) in a digital off-axis hologram digitally, which can save several adjustment procedures. In the approach, we train a pair of coupled dictionaries from a low fringe contrast hologram and a high one of the same specimen, use the dictionaries to sparse code the input hologram, and finally output a higher fringe contrast hologram. The sparse representation shows good adaptability on holograms. The experimental results demonstrate the benefit of low noise in a three-dimensional profile and prove the effectiveness of the approach.

A novel method for identifying the order of interference using phase-shifting digital holography.

In this paper, we introduced a mathematical method for measuring the optical path length differences (OPDs), which is suitable for large OPD values where the fringes connections are difficult to detect. The proposed method is based on varying the width of the fringes, without changing the wavelength of the used coherent source. Also, in this work, we discussed the need for such method in off-axis phase-shifting digital holography. Low-resolution off-axis holograms failed to detect the correct interference order. In general, off-axis phase-shifting digital holography is limited by the resolution of the captured holograms. The results obtained using our proposed technique were compared to the results obtained using off-axis phase-shifting digital holograms and conventional two-beam interferometry. Holograms were given for illustration.

Focal Image Plane Detection Based on Central Coordinate Point Spectral Value in Off-Axis Digital Particle Holography

A method to detect the focal image plane from a single off-axis digital particle hologram is proposed. This method utilizes the central coordinate point spectral value of the reconstructed particle image as focusing criterion to detect the focal image plane. It is found that the central coordinate point spectral values come into maximum when the reconstruction distance is equal to the actual distance that was used in experiment of hologram acquisition. Numerical simulations are given to validate the feasibility and effectiveness of the proposed method. The proposed method is a potential and better option for studying three dimensional particles by using digital holography.

An ultra-compact multiplexed holographic microscope using a multiple-pinhole aperture.

In this paper, we present a low-cost and ultra-compact holographic microscope with multiple imaging areas. Instead of a dual-pinhole aperture as presented in our previous work, a multi-pinhole aperture is employed to filter the light source and to generate a reference wave as well as multiple object waves. The reference wave and the object waves interfere at the digital sensor and form multiplexed off-axis holograms without any lenses, splitters or combiners. The optimal number of object waves is determined which does not only fit our system but also brings some inspiration for traditional multiplexed off-axis holography. Our new system is tested to be able to retrieve quantitative phase images along with the amplitude images of multiple imaging areas at the same time with a lateral resolution of ∼ 2.2µm and an accuracy of the optical path of tens of nanometers.

Single-shot digital holography for fast measuring optical properties of fibers.

We propose a fast method for measuring optical properties, e.g., the refractive index profile and birefringence, of fibers. It is based on recovering the phase distribution of light refracted by a fiber sample at the recording plane from a single-shot digital hologram. During the recovering process, an optimized approach based on the spatial carrier frequency method was utilized. The proposed approach enhances affects that arise from the limited spatial extent of the bandpass filter associated with the implementation of the spatial carrier frequency method. In contrast to the low spatial resolution of off-axis digital holograms, the method ensures the best utilization of the camera support. From the recovered phase information, the optical path difference is measured; thus, the refractive index profile, the mean refractive index, and the birefringence of isotactic polypropylene (IPP) are determined. Experimental results are given for illustration.

Decreasing the noise sensitivity of off-axis hologram imaging

Decreasing the noise sensitivity of off-axis hologram imaging

Reconstruction of three-dimensional sectioning of object by single off-axis Fresnel hologram with compressive sensing

Based on the traditional numerical reconstruction method, the reconstructed image located at different section of the three-dimensional object would be superimposed by the twin image and out-of-focus images. Therefore, a numerical reconstruction method by employing adaptive filtering method and compressive sensing theory is proposed to achieve the reconstruction of three-dimensional sectioning of object from a single off-axis Fresnel hologram. Firstly, the adaptive filtering method is applied to the off-axis Fresnel hologram to eliminate the twin image. And then, according to the compressive sensing theory, images located at different section can be reconstructed with the background out-of-focus images inhibited. The algorithm is presented in detail. A numerical simulation and a digital holography experiment employing test targets are performed to demonstrate the feasibility and validity of the method by the comparison of the traditional numerical reconstruction method.

One-step reconstruction of assembled 3D holographic scenes

We present a new experimental approach for reconstructing in one step 3D scenes otherwise not feasible in a single snapshot from standard off-axis digital hologram architecture, due to a lack of illuminating resources or a limited setup size. Consequently, whenever a scene could not be wholly illuminated or the size of the scene surpasses the available setup disposition, this protocol can be implemented to solve these issues. We need neither to alter the original setup in every step nor to cover the whole scene by the illuminating source, thus saving resources. With this technique we multiplex the processed holograms of actual diffuse objects composing a scene using a two-beam off-axis holographic setup in a Fresnel approach. By registering individually the holograms of several objects and applying a spatial filtering technique, the filtered Fresnel holograms can then be added to produce a compound hologram. The simultaneous reconstruction of all objects is performed in one step using the same recovering procedure employed for single holograms. Using this technique, we were able to reconstruct, for the first time to our knowledge, a scene by multiplexing off-axis holograms of the 3D objects without cross talk. This technique is important for quantitative visualization of optically packaged multiple images and is useful for a wide range of applications. We present experimental results to support the method.

High speed phase retrieval of in-line holograms by the assistance of corresponding off-axis holograms.

Retrieving correct phase information from an in-line hologram is difficult as the object wave field and the diffractions of the zero order and the conjugate object term overlap. The existing iterative numerical phase retrieval methods are slow, especially in the case of high Fresnel number systems. Conversely, the reconstruction of the object wave field from an off-axis hologram is simple, but due to the applied spatial frequency filtering the achievable resolution is confined. Here, a new, high-speed algorithm is introduced that efficiently incorporates the data of an auxiliary off-axis hologram in the phase retrieval of the corresponding in-line hologram. The efficiency of the introduced combined phase retrieval method is demonstrated by simulated and measured holograms.

Generation of Binary Off-axis Digital Fresnel Hologram with Enhanced Quality

Abstract The emergence of high resolution printer and digital micromirror device (DMD) has enabled real, off-axis holograms to be printed, or projected onto a screen. As most printers and DMD can only reproduce binary dots, the pixels in a hologram have to be truncated to 2 levels. However, direct binarizing a hologram will lead to severe degradation on its reconstructed image. In this paper, a method for generating binary off-axis digital Fresnel hologram is reported. A hologram generated with the proposed method is referred to as the Enhanced Sampled Binary Hologram (ESBH). The reconstructed image of the ESBH is superior in visual quality as compare with the one obtained with existing technique, and also resistant to noise contamination.

Singular-value demodulation of phase-shifted holograms.

We report on phase-shifted holographic interferogram demodulation by singular-value decomposition. Numerical processing of optically acquired interferograms over several modulation periods was performed in two steps: (1) rendering of off-axis complex-valued holograms by Fresnel transformation of the interferograms; and (2) eigenvalue spectrum assessment of the lag-covariance matrix of hologram pixels. Experimental results in low-light recording conditions were compared with demodulation by Fourier analysis, in the presence of random phase drifts.

High resolution digital holographic microscopy for the study of aggregated natural cellulose nanowhisker fibers

In this paper, digital holographic (DH) microscopy demonstrates its ability to perform a full characterization of nanofibers. The high resolution and magnification of the presented method to study the nanofibers are tested using standard MIL-STD-150A 1951 USAF resolution test target. In this investigation, aggregated natural cellulose nanowhisker fibers are positioned in the front of the microscopic objective using a 3D translation stage in the object arm of DH setup. The recorded off-axis holograms are refocused using the angular spectrum method. The reconstructed complex field is used to calculate optical phase and intensity distributions of the object at different reconstruction depths. A simple algorithm is used to define the focused image with suitable accuracy. The dimensions and orientation of the fibers can be evaluated from the optical field at different depths. Then, the shape and textures along the aggregated natural cellulose nanowhisker fiber can be presented in a 3D space.

Restoration of digital off-axis Fresnel hologram by exemplar and search based image inpainting with enhanced computing speed

In this paper, we proposed a method for restoration of heavily damaged digital off-axis Fresnel holograms based on image inpainting. Being different from existing image inpainting algorithms, we have employed Artificial Bee Colony search technique to increase the computation efficiency of the inpainting process, resulting in a reduction of over one order of magnitude in the computation time. Briefly, an exemplar and search based image inpainting technique is applied to fill in the damaged regions in the hologram, with the most similar fringe patterns in the intact areas obtained from Artificial Bee Colony search algorithm. Experimental results reveal that with our proposed method, the pictorial information in heavily damaged holograms can be recovered with good fidelity, as compared with those obtained with the original intact holograms. This is the first time that image inpainting technique is applied on hologram restoration.

Fast phase processing in off-axis holography using multiplexing with complex encoding and live-cell fluctuation map calculation in real-time.

We present efficient algorithms for rapid reconstruction of quantitative phase maps from off-axis digital holograms. The new algorithms are aimed at speeding up the conventional Fourier-based algorithm. By implementing the new algorithms on a standard personal computer, while using only a single-core processing unit, we were able to reconstruct the unwrapped phase maps from one megapixel off-axis holograms at frame rates of up to 45 frames per second (fps). When phase unwrapping is not required, the same algorithms allow frame rates of up to 150 fps for one megapixel off-axis holograms. In addition to obtaining real-time quantitative visualization of the sample, the increased frame rate allows integrating additional calculations as a part of the reconstruction process, providing sample-related information that was not available in real time until now. We use these new capabilities to extract, for the first time to our knowledge, the dynamic fluctuation maps of red blood cells at frame rate of 31 fps for one megapixel holograms.

Extracting the parameters of digital reference wave from a single off-axis digital hologram

A novel digital method to extract the precise parameters of digital reference wave (Rd) from a single off-axis digital hologram is proposed. The method makes the Rd closer to the reference wave (R) used in experiment of hologram acquisition. The spectrum expression of off-axis digital hologram multiplied with a digital reference wave or a digital conjugate reference wave is derived for the study of the spectrum characteristics. It is shown that the parameters of the Rd are related to the size of the selected area in the center of the overlapped spectrogram. When Rd = R, the size of selected overlapped area is minimum. Thus, the parameters of the Rd can be determined by finding the minimum of the selected overlapped area. Some simulation results are given to validate the proposed method.

Digital holographic shape measurement using Fizeau microscopy

We present a Fizeau interferometer using a microscopic objective as a tool for surface contouring without the need for a numerical lens for reconstruction. The interferometer is associated with a telescope system to feature the object with collimated light. The experiment is conducted on two objects possessing different step heights. The phase maps from the captured off-axis holograms are calculated numerically, which allows us to deduce the contours of the objects. The great advantages of the presented technique are that it can be done in real time and there is no need for numerical lenses for micro-objects reconstruction.

JPEG 2000-based compression of fringe patterns for digital holographic microscopy

With the advent of modern computing and imaging technologies, digital holography is becoming widespread in various scientific disciplines such as microscopy, interferometry, surface shape measurements, vibration analysis, data encoding, and certification. Therefore, designing an efficient data representation technology is of particular importance. Off-axis holograms have very different signal properties with respect to regular imagery, because they represent a recorded interference pattern with its energy biased toward the high-frequency bands. This causes traditional images’ coders, which assume an underlying 1/f2 power spectral density distribution, to perform suboptimally for this type of imagery. We propose a JPEG 2000-based codec framework that provides a generic architecture suitable for the compression of many types of off-axis holograms. This framework has a JPEG 2000 codec at its core, extended with (1) fully arbitrary wavelet decomposition styles and (2) directional wavelet transforms. Using this codec, we report significant improvements in coding performance for off-axis holography relative to the conventional JPEG 2000 standard, with Bjontegaard delta-peak signal-to-noise ratio improvements ranging from 1.3 to 11.6 dB for lossy compression in the 0.125 to 2.00 bpp range and bit-rate reductions of up to 1.6 bpp for lossless compression.

Phase derivative method for reconstruction of slightly off-axis digital holograms

A phase derivative (PD) method is proposed for reconstruction of off-axis holograms. In this method, a phase distribution of the tested object wave constrained within 0 to pi radian is firstly worked out by a simple analytical formula; then it is corrected to its right range from -pi to pi according to the sign characteristics of its first-order derivative. A theoretical analysis indicates that this PD method is particularly suitable for reconstruction of slightly off-axis holograms because it only requires the spatial frequency of the reference beam larger than spatial frequency of the tested object wave in principle. In addition, because the PD method belongs to a pure local method with no need of any integral operation or phase shifting algorithm in process of the phase retrieval, it could have some advantages in reducing computer load and memory requirements to the image processing system. Some experimental results are given to demonstrate the feasibility of the method.

Digital holographic microscopy for microalgae biovolume assessment

The relative amount of biomass in a body of water is one of the various indicators widely used in water quality evaluation. This implies complex tasks such as identification and characterization of microorganisms and measurement of their biovolume. Particularly, the latter is estimated by assuming simple geometrical shapes for the microorganism and by calculating its dimensions from images taken with a conventional microscope. In order to have a more precise and automatic method for biovolume evaluation, a hybrid methodology based on digital holographic microscopy and image processing is proposed. The whole volume of a specimen under study is obtained combining the phase contrast image of an off-axis hologram with the thickness-profile data of the specimen extracted from the cell silhouette. This technique has been used for determining the biovolume of Ceratium Hirundinella cells in water samples. The methodology proposed also shows that it is possible to estimate accurately an effective refractive index of the microorganism. Experimental results have shown that this technique is not only an efficient and fast alternative, but also suitable for automatizing the entire process.

Optical reconstruction of digital off-axis Fresnel holograms using phase-only LCOS SLM "HoloEye PLUTO VIS"

Optical reconstruction of digital holograms using SLM is used for imaging of 3D scenes, interferometry, microscopy, and etc. In this article reconstruction of digital off-axis Fresnel holograms using phase-only LCOS SLM "HoloEye PLUTO VIS" is described. Experimental and numerically simulated results of reconstruction are presented.