Binary Computer-generated Holograms Research Articles

Cryptographic computer generated hologram watermarking technology based on DWT-SVD

To improve the security and invisibility of image encryption and concealment, an encrypted computer generated hologram (CGH) watermarking technology based on discrete wavelet transform (DWT) and singular value decomposition (SVD) is proposed. Firstly, the image to be encrypted is encoded and converted into a binary real-valued CGH. The encrypted CGH is then embedded into the host image as a watermark, implementing image encryption and concealment. During decryption, the encrypted CGH watermark is first extracted using the inverse DWT-SVD and the correct keys. Subsequently, the decrypted image is obtained by applying the correct optical key for computational holographic reproduction. The method introduces computational holographic coding and optical keys into the digital watermarking technique and combines the characteristics of DWT and SVD, solving the problem of the high false positive rate of the SVD algorithm. Simulation results show that the security of image encryption and concealment is greatly improved with the introduction of CGH into watermarking technology, especially in terms of robustness to watermark attacks and noise. This method can be applied in the fields of copyright protection and anti-counterfeiting for digital and printed images.

A region based random multi-pixel search algorithm to improve the binary hologram reconstruction quality

Direct binary search (DBS) is an effective method for the generation of binary computer-generated holograms (CGHs), which has been widely used in wavefront synthesis, optical tweezers and 3D display. However, the correlation of pixels in holograms is neglected in DBS, resulting the algorithm easily converging to local minimum and leading to an unsatisfactory reconstruction quality. To ameliorate this disadvantage, we proposed a new algorithm by randomly selecting multiple related pixels in the same region to generate hologram. Compared with DBS, the mean square error (MSE) of reconstructed image in simulation has declined from 844.85 to 445.74, the peak signal to noise ratio (PSNR) has increased from 18.86 to 21.64 and structural similarity (SSIM) has increased from 0.336 to 0.459. The reconstructed image in experiment shows fewer noise points and clearer outline. Both the numerical and experimental results demonstrate that the reconstruction quality can be improved by this algorithm, which will exhibit great applications in the fields of data storage, microscopy, and true sense dynamic 3D display.

Binary Computer-Generated Holograms by Simulated-Annealing Binary Search

The binary computer-generated hologram (BCGH) has attracted much attention recently because it can address the high-speed binary spatial light modulator (SLM), such as a digital micromirror device (DMD) SLM. In this paper, our concern is the development of an algorithm to produce high-quality BCGHs. In particular, simulated annealing (SA) is an efficient algorithm used to produce a phase-only computer-generated hologram. In the study of SA for the production of a BCGH, we found some inherent shortcomings of SA, and the quality of the produced BCGHs is limited. Accordingly, we have modified SA and propose the simulated-annealing binary search (SABS) algorithm. We have also proposed a method to quickly determine the parameters for SABS. In the comparison with SA, the mean square error of the SABS BCGHs decreases by 32% on average. Therefore, the SABS is a promising technique for a high-quality holographic display by DMD.

Efficient wavefront sensorless adaptive optics based on large dynamic crosstalk-free holographic modal wavefront sensing.

The correction of wavefront sensorless adaptive optics (WFSless AO) can be significantly accelerated by using a holographic modal wavefront sensor (HMWFS). The HMWFS is realized by a computer-generated hologram (CGH) into which all aberration modes to be detected are encoded and only a single-shot image is required for simultaneous measurement of multiple modes. The conventional HMWFS suffers from a quite limited dynamic range and severe inter-modal crosstalk which deteriorates the sensing accuracy. We proposed a novel HMWFS with a large dynamic range and no crosstalk and validated its performance by simulation and experiment. In the improved HMWFS scheme, the aberration is represented by Lukosz modes whose gradients are orthogonal and the modal coefficients can be estimated independently. Instead of using a binary CGH in conventional HMWFS, a kinoform CGH with high diffraction efficiency is adopted in the improved HMWFS. The kinoform CGH is produced by a phase-only liquid-crystal spatial light modulator (LCSLM) which also serves as a wavefront corrector in our WFSless AO system.

Integrated binary hologram to monitor cargo release from a drug-eluting film

Nowadays, holography translates from a pure technical tool for recording the phase and amplitude of the light wave to a widely applicable research-based method. Holographic devices are used for security enhancement, entertainment, 3D display technologies and augmented reality. Binary computer-generated holograms (amplitude or phase-based) are of specific interest. They are easy to compute and their manufacturing methods are fast and robust. In this work, a method of manufacturing amplitude-only binary holograms out of biopolymers films is proposed. Opaque cargo bits made out of different bioactive substances (antibiotics, dyes, etc.) absorb or scatter specific parts of the incoming light wave. Cargo release was conducted by submerging the produced holograms into the aqueous solution at specific pH and temperature levels. Drug elution from the film was traced out by the numerical analysis of the far-field diffraction pattern, which otherwise could only be measured post-factum using sophisticated spectroscopic or chromatography devices. This approach broadens the application of general holography over to the field of biomedical research relevant to quantitative monitoring of the drug elution.

Performance Estimation of Intensity Accumulation Display by Computer-Generated Holograms

The image generated by binary computer-generated holograms (CGHs) always suffers from serious speckle noise. Thanks to the fast frame rate of the binary spatial light modulator, the speckle can be significantly suppressed by intensity accumulation, i.e., the sequential display of multiple CGHs of the same scene. If enough randomness is added to the CGHs, the speckle noise can be mostly averaged out. Intuitively, the quality of the reconstructed image should be proportional to the number of intensity accumulation. However, there is no simple method to predict the dependence of the average noise and accumulation number, and we can only know the results after finishing the full computation. In this paper, we propose an empirical formula of the average noise based on the speckle phenomenon in a laser projector. Using this model, we have confirmed that the randomness induced by random phase is equivalent to that induced by random down-sampling for the generation of binary CGHs. In addition, if the computational efficiency is a concern, the CGH calculated with iterations is not recommended for intensity accumulation display. Finally, there is an upper-quality limit of the reconstructed image by intensity accumulation. Thus, a strategy for efficient intensity accumulation is suggested.

Generation of Mathieu beams based on the detour phase encoding method

We introduce a new method for precisely generating Mathieu beams by designing binary computer-generated holograms (CGHs). The CGHs are obtained by encoding the complex filed information through the use of the Lohmann-type detour phase coding method. Accordingly, a family of high-quality Mathieu beams can be engineered precisely. The good consistency between experimental results and theoretical predictions show that the proposed method is a valuable approach for modulating and generating complex structured light fields. In other words, one can use a single CGH mask to simultaneously regulate the phase and amplitude of one beam hence its propagation is controlled.

Real-time gradation-expressible amplitude-modulation-type electroholography based on binary-weighted computer-generated hologram

In amplitude-modulation-type electroholography, the binary-weighted computer-generated hologram (BW-CGH) facilitates the gradation-expressible reconstruction of three-dimensional (3D) objects. To realize real-time gradation-expressible electroholography, we propose an efficient and high-speed method for calculating bit planes consisting of BW-CGHs. The proposed method is implemented on a multiple graphics processing unit (GPU) cluster system comprising 13 GPUs. The proposed BW-CGH method realizes eight-gradation-expressible electroholography at approximately the same calculation speed as that of conventional electroholography based on binary computer-generated holograms. Consequently, we were able to successfully reconstruct a real-time electroholographic 3D video comprising approximately 180,000 points expressed in eight gradations at 30 frames per second.

3D display by binary computer-generated holograms with localized random down-sampling and adaptive intensity accumulation.

In this paper, we proposed a new technique to realize a high-quality three-dimensional (3D) display by using binary holograms. First, we applied a localized random down-sampling (LRDS) mask to down-sample the object function and generated a binary CGH by direct sign-thresholding. Subsequently, we devised the display by adaptive intensity accumulation (AIA). In AIA, multiple CGHs of the same object are generated. However, selective sampling points of the same scene are removed according to the reconstructed image of previous binary CGHs as the second and more binary CGHs are generated. Finally, these holograms are sequentially displayed on a fast spatial light modulator, a digital micromirror device (DMD). Thus, a high-quality 3D image is reconstructed without artifacts and speckle noise.

Second-harmonic computer-generated holographic imaging through monolithic lithium niobate crystal by femtosecond laser micromachining.

The computer-generated holography technique is a powerful tool for three-dimensional display, beam shaping, optical tweezers, ultrashort pulse laser parallel processing, and optical encryption. We have realized nonlinear holography in ferroelectric crystals by utilizing spatial light modulators in our previous works. Here, we demonstrate an improved method to realize second-harmonic (SH) holographic imaging through a monolithic lithium niobate crystal based on binary computer-generated holograms (CGHs). The CGH patterns were encoded with the detour phase method and fabricated by femtosecond laser micromachining. By the use of the birefringence phase-matching process in the longitudinal direction, bright nonlinear holograms can be obtained in the far-field. The realization of SH holography through monolithic crystal opens wide possibilities in the field of high power laser nonlinear holographic imaging.

High conversion efficiency second-harmonic beam shaping via amplitude-type nonlinear photonic crystals

An efficient two-dimensional arbitrary harmonic wavefront shaping has been demonstrated in amplitude-type nonlinear photonic crystals, where the phase-matching condition is fulfilled through the birefringence and nonlinear Raman–Nath effects in longitudinal and transverse phase matching, respectively. The binary modulated nonlinear photonic crystal was fabricated by femtosecond laser micromachining based on binary computer-generated holograms. Three second-harmonic Hermite–Gaussian beams, H G 10 , H G 11 , and H G 12 , were achieved by pumping a nanosecond pulsed fundamental Gaussian beam, with the measured normalized conversion efficiency of 8.4 % W − 1 c m − 2 in the first diffraction order of the H G 11 structure. The amplitude-type nonlinear photonic crystal opens wide possibilities in the field of efficient harmonic beam shaping and mode conversion.

Real-time interactive holographic 3D display with a 360° horizontal viewing zone.

To realize a real-time interactive holographic three-dimensional (3D) display system, we synthesize a set of 24 full high-definition (HD) binary computer-generated holograms (CGHs) based on a 3D fast-Fourier-transform-based approach. These 24 CGHs are streamed into a digital micromirror device (DMD) as a single 24-bit image at 60 Hz: 1440 CGHs are synthesized in less than a second. Continual updates of the CGHs displayed on the DMD and synchronization with a rotating mirror enlarges the horizontal viewing zone to 360° using a time-division approach. We successfully demonstrate interactive manipulation, such as object rotation, rendering mode switching, and threshold value alteration, for a medical dataset of a human head obtained by X-ray computed tomography.

Enhanced direct binary search algorithm for binary computer-generated Fresnel holograms

The direct binary search (DBS) algorithm was originally invented for the synthesis of a binary Fourier hologram, and was applied for the generation of a binary Fresnel hologram recently. DBS performs quality evaluation on every pixel. Therefore, both the quality and diffraction efficiency of the generated binary hologram are better among various algorithms of the binary hologram. However, DBS is a time-consuming algorithm and thus is impractical for the generation of high-definition computer-generated holograms. In this paper, we proposed an enhanced DBS (E-DBS) method to speed up the hologram computation. E-DBS is based on the same pixelwise evaluation strategy of DBS, but the diffraction field of a single pixel is precomputed as a lookup table. In evaluating any pixel value, only a small area in the region of interest affected by the diffraction field of single pixel is calculated. In addition, it is also found that qualified results can be obtained by using only 4% of the area of the diffraction field. As a result, the computing complexity of E-DBS can be reduced by at least 2 orders of magnitude in contrast to conventional DBS.

An Adaptive and Secure Holographic Image Watermarking Scheme.

A novel adaptive secure holographic image watermarking method in the sharp frequency localized contourlet transform (SFLCT) domain is presented. Based upon the sine logistic modulation map and the logistic map, we develop an encrypted binary computer-generated hologram technique to fabricate a hologram of a watermark first. Owing to the enormous key space of the encrypted hologram, the security of the image watermarking system is increased. Then the hologram watermark is embedded into the SFLCT coefficients with Schur decomposition. To obtain better imperceptibility and robustness, the entropy and the edge entropy are utilized to select the suitable watermark embedding positions adaptively. Compared with other watermarking schemes, the suggested method provides a better performance with respect to both imperceptibility and robustness. Experiments show that our watermarking scheme for images is not only is secure and invisible, but also has a stronger robustness against different kinds of attack.

Fast and improved bioimaging via temporal focusing multiphoton excitation microscopy with binary digital-micromirror-device holography.

Conventional temporal focusing-based multiphoton excitation microscopy (TFMPEM) can offer widefield optical sectioning with an axial excitation confinement of a few microns. To improve the axial confinement of TFMPEM, a binary computer-generated Fourier hologram (CGFH) via a digital-micromirror-device (DMD) was implemented to intrinsically improve the axial confinement by filling the back-focal aperture of the objective lens. Experimental results show that the excitation focal volume can be condensed and the axial confinement improved about 24% according to the DMD holography. In addition, pseudouniform MPE can be achieved using two complementary CGFHs with rapid pulse-width modulation switching via the DMD. Furthermore, bioimaging of CV-1 in origin with SV40 genes-7 cells demonstrates that the TFMPEM with binary DMD holography can improve image quality by enhancing axial excitation confinement and rejecting out-of-focus excitation.

Programmable CGH on photochromic plates coded with DMD generated masks.

Computer Generated Holograms (CGHs) are used for wavefront shaping and complex optics testing. Present technology allows for recording binary CGHs. We propose a Digital Micro-mirror Device (DMD) as a reconfigurable mask, to record rewritable binary and grayscale CGHs on a photochromic plate. Opaque at rest, this plate becomes transparent when it is illuminated with visible light of suitable wavelength. We have successfully recorded the very first amplitude grayscale CGH, with a contrast greater than 50, which was reconstructed with a high fidelity in shape, intensity, size and location. These results reveal the high potential of this method for generating programmable/rewritable grayscale CGHs, which combine DMDs and photochromic substrates.

Gradation representation method using binary-weighted computer-generated hologram

We propose a simple gradation representation method for a reconstructed three-dimensional (3-D) image without controlling the brightness of the reference light. In the proposed method, we use multiple bit planes comprised of binary-weighted computer-generated holograms (CGHs) with various light transmittances. Binary-weighted CGH is generated by changing the white in the conventional binary CGH to gray. The light transmittance of a binary-weighted CGH is less than that of a conventional binary CGH. The object points of a 3-D object are assigned to multiple bit planes according to the gray level of the object points. The multiple bit planes are displayed sequentially in a time-division multiplex manner. Consequently, the proposed method realizes a gradation representation of a reconstructed 3-D object.

Design quadrilateral apertures in binary computer-generated holograms of large space bandwidth product.

A new approach for designing the binary computer-generated hologram (CGH) of a very large number of pixels is proposed. Diffraction of the CGH apertures is computed by the analytical Abbe transform and by considering the aperture edges as the basic diffracting elements. The computation cost is independent of the CGH size. The arbitrary-shaped polygonal apertures in the CGH consist of quadrilateral apertures, which are designed by assigning the binary phases using the parallel genetic algorithm with a local search, followed by optimizing the locations of the co-vertices with a direct search. The design results in high performance with low image reconstruction error.

A QDCT- and SVD-based color image watermarking scheme using an optimized encrypted binary computer-generated hologram

Based on quaternion discrete cosine transform (QDCT) and singular value decomposition (SVD), a novel color image watermarking scheme using an optimized encrypted binary computer-generated hologram (CGH) as the watermark is presented. A Fibonacci transform-based binary CGH technique that uses particle swarm optimization (PSO) algorithm to improve the reconstructed image quality has been proposed to generate a hologram of a watermark. By use of QDCT, the host color image is treated holistically as a vector field. The component of the host color image after QDCT, which is suitable for embedding watermark, is analyzed. With SVD, the mark CGH is inserted into the host color image by modifying the U matrix. To embed the binary CGH, multiple embedding strength factors are used in this study. To withstand the geometric attacks, based on Zernike moment algorithm and invariant centroid, an improved geometric distortion detection algorithm is proposed. Considering the significant color information, this algorithm is performed on the color invariance model. To obtain better imperceptibility and robustness, PSO is used for finding the proper multiple embedding strength factors. Compared with traditional geometric estimation method, the improved algorithm can estimate the transform parameters of the geometric distorted color image more accurately. Experimental results show that the proposed color image watermarking is not only secure and invisible, but also robust against common image processing operations and geometrical distortions.

Computer-Generated Very Large Space-Bandwidth Product Binary Hologram for Laser Projector

A new approach for designing the binary phase-only hologram with arbitrary-shaped polygonal apertures is proposed. The diffraction of the aperture was computed with the analytical Abbe transform. To explore the very high number of degrees of freedom available in the computer-generated hologram (CGH) of very large pixel count, a two-step optimization including the genetic algorithm with local search for encoding binary phases of apertures followed by the direct search for floating covertices of the elementary triangular apertures was developed for high performance and low image-reconstruction error of the binary CGH.