Full Hologram Research Articles

Binary hologram compression using context based Bayesian tree models with adaptive spatial segmentation

With holographic displays requiring giga- or terapixel resolutions, data compression is of utmost importance in making holography a viable technique in the near future. In addition, since the first-generation of holographic displays is expected to require binary holograms, associated compression algorithms are expected to be able to handle this binary format. In this work, the suitability of a context based Bayesian tree model is proposed as an extension to adaptive binary arithmetic coding to facilitate the efficient lossless compression of binary holograms. In addition, we propose a quadtree-based adaptive spatial segmentation strategy, as the scale dependent, quasi-stationary behavior of a hologram limits the applicability of the advocated modelling approach straightforwardly on the full hologram. On average, the proposed compression strategy produces files that are around 12% smaller than JBIG2, the reference binary image codec.

Computationally efficient processing of in situ underwater digital holograms

AbstractUnderwater digital in‐line holography can provide high‐resolution, in situ imagery of marine particles and offers many advantages over alternative measurement approaches. However, processing of holograms requires computationally expensive reconstruction and processing, and computational cost increases with the size of the imaging volume. In this work, a processing pipeline is developed to extract targets from holograms where target distribution is relatively sparse without reconstruction of the full hologram. This is motivated by the desire to efficiently extract quantitative estimates of plankton abundance from a data set (>300,000 holograms) collected in the Northwest Atlantic using a large‐volume holographic camera. First, holograms with detectable targets are selected using a transfer learning approach. This was critical as a subset of the holograms were impacted by optical turbulence, which obscured target detection. Then, target diffraction patterns are detected in the hologram. Finally, targets are reconstructed and focused using only a small region of the hologram around the detected diffraction pattern. A search algorithm is employed to select distances for reconstruction, reducing the number of reconstructions required for 1 mm focus precision from 1000 to 31. When compared with full reconstruction techniques, this method detects 99% of particles larger than 0.1 mm2, a size class which includes most copepods and larger particles of marine snow, and 85% of those targets are sufficiently focused for classification. This approach requires 1% of the processing time required to compute full reconstructions, making processing of long time‐series, large imaging volume holographic data sets feasible.

Region specific magnification on digital holograms at 100 frames per second

We report a fast method for magnifying a selected region of the pictorial content that has been recorded in a digital hologram, without the need of regenerating the latter from the original object scene. In essence, the hologram is first back-projected to a complex, two dimensional virtual diffraction plane (VDP) which is located at close proximity to the original object points. Next, the fringe patterns in the selected region of the VDP, which corresponds to the complex light wave emerged from the part of the object within the coverage of the region, is transformed to achieve the magnification effect. Subsequently, the processed field distribution on the VDP is expanded into a full hologram. We have applied our proposed method to realize a magnifying glass effect, so that the reconstructed image of the hologram will zoom into a specific region of the object scene. For a medium size digital hologram comprising of 2048 × 2048 pixels, the magnifying process can be realized in less than 10ms (equivalent to a rate of around 100 frames per second). To the best of our knowledge, this is the first time region specific magnification at such a high frame rate is considered and achieved in the context of digital holography.

Enhancing the pictorial content of digital holograms at 100 frames per second

We report a low complexity, non-iterative method for enhancing the sharpness, brightness, and contrast of the pictorial content that is recorded in a digital hologram, without the need of re-generating the latter from the original object scene. In our proposed method, the hologram is first back-projected to a 2-D virtual diffraction plane (VDP) which is located at close proximity to the original object points. Next the field distribution on the VDP, which shares similar optical properties as the object scene, is enhanced. Subsequently, the processed VDP is expanded into a full hologram. We demonstrate two types of enhancement: a modified histogram equalization to improve the brightness and contrast, and localized high-boost-filtering (LHBF) to increase the sharpness. Experiment results have demonstrated that our proposed method is capable of enhancing a 2048x2048 hologram at a rate of around 100 frames per second. To the best of our knowledge, this is the first time real-time image enhancement is considered in the context of digital holography.

Real-time relighting of digital holograms based on wavefront recording plane method

Relighting is an important technique in photography which enables the optical properties of a picture to be modified without retaking it again. However, different from an optical image, a digital hologram cannot be relit by simply varying the value of individual pixel, as each of them is representing holistic information of the entire object scene. In this paper, we propose a fast method for the relighting of a digital hologram. First, the latter is projected to a virtual wavefront recording plane (WRP) that is located close to the object scene. Next, the WRP is relit, and subsequently expanded into a full hologram. Experiment results have demonstrated that our proposed method is capable of relighting a 2048x2048 hologram at a rate of over 50 frames per second. To the best of our knowledge, this is the first time relighting is considered in the context of holography.

Improvement of thermal stability in photochromic holograms

Significant improvement of the thermal stability of the hologram recorded on photochromic materials had been achieved via covalent bonding of the photochromic dye to the polymer matrix as compared to the host-guest systems. For the photochromic materials with covalent bonding of the dye to the polymer matrix the level of thermal stability is demonstrated such that at temperatures (T) ca. 100 degrees C there was no detectable diffusion-type degradation of the hologram's diffraction efficiency after 6 h of exposure to elevated T. The same heating of the hologram in photochromic host-guest polymer led to a full hologram erasure within 40 min.

Enhancement of photochromic hologram thermal stability

Significant enhancement of the thermal stability of the hologram recorded on photochromic materials had been achieved via covalent bonding of the photochromic dye to the polymer matrix as compared to the host-guest systems. One time partial reduction of the hologram’s initial diffraction efficiency due to thermal exposure was observed for both—polymers with attached dye as well as hostguest materials. Such one time reduction is interpreted to be due to the thermal relaxation of the polymer network induced with photochromic transition in the dye molecule. A gradual hologram erasure at elevated temperatures was observed for the host-guest system, which is assumed to be due to dye’s diffusion between highly lit and dark areas. For the photochromic materials with covalent boding of the dye to the polymer matrix the level of thermal stability is demonstrated such that at temperatures ca. 100C there was no detectable diffusion type degradation of the hologram after 6 hours of exposure to elevated temperature. Same heating of the hologram in photochromic host-guest polymer led to a full hologram erasure within 40 minutes. In both cases the one time initial DE reduction due to polymer matrix relaxation at elevated temperatures had comparable value of about 0.5 of hologram’s initial DE.

Use of Area Array Detector for X-Ray Fluorescence Holography Allows Simultaneous Recording of the Full Hologram without Sacrificing Angular Accuracy

Use of Area Array Detector for X-Ray Fluorescence Holography Allows Simultaneous Recording of the Full Hologram without Sacrificing Angular Accuracy

Recording of x-ray holograms on a position-sensitive detector

An unconventional x-ray fluorescence holography experiment was carried out by using an area detector in combination with an absorption filter. The high angular resolution and the very precise detection of intensities allowed the reconstruction of images of distant, as well as light, atoms. The simultaneous recording of the full hologram opens the possibility of one-shot imaging at atomic resolution. The hologram of a CoO single crystal was recorded on the imaging plate and the images of atoms located up to more than 7 Å far from the emitter were obtained.

Instrumental development of X-ray atomic holography at ESRF

We have developed an X-ray Fluorescence Holography (XFH) setup at the European Synchrotron Radiation Facility (ESRF). The main difficulty inherent to XFH is that the holographic signal is in the low 10 −3 range compared to the background isotropic fluorescent radiation. This requires a very pure fluorescent signal and the highest possible count rate. Therefore, we designed a focusing analyser system with large solid angle acceptance. The photons of the focused beam were counted by an avalanche photodiode in single-photon counting mode up to a few MHz. At even higher intensity, we switched to a Si diode working in the photocurrent mode. Due to the relatively weak requirements on the bandwidth of the exciting incident radiation, we could directly use the undulator beam, discriminating lower harmonics by an absorber and higher ones by reflecting the beam on a Si mirror. By developing a fast, continuous “spiral” scanning technique, a full hologram could be recorded in 30 sec.

Optical Wave Reconstruction from Acoustically Acquired Data

The difference between the recording and reconstructing wavelengths used in acoustical holography leads to a longitudinal distortion in the image. Furthermore, one of the important advantages of the three-dimensional nature of the hologram image, namely, the parallax effect, cannot be utilized unless resolution in the image is sacrificed by not using the full hologram aperture. This paper describes the experimental apparatus and a technique whereby the principles of multiplexing by holography are extended to 3-D representation of acoustically acquired data. It will be shown that full aperture sharing among the recorded wavefront of B-scans is compatible with off-hologram-plane sampling, so that the convenience of off-hologram-plane sampling can be combined with the image properties characteristic of the full aperture. Experimental results demonstrating the usefulness of the technique in ultrasonic diagnostics and nondestructive testing are presented. The possibility of using it in earth holography also will be discussed.