Speckle Noise In Digital Holography Research Articles

Reduction of speckle noise in digital holography using a neighborhood filter based on multiple sub-reconstructed images.

The application of digital holography in several fields is limited since speckle destroys the original information of the reconstructed image. This paper proposes a neighborhood filter based on multiple sub-reconstructed images according to the random distribution of speckle noise. In this method, the denoised value is equal to the weighted sum of neighboring pixel values, and the weight is calculated by the degree of correlation between different positions of multiple sub-holograms. The experimental results show that the method can greatly reduce the speckle noise, and its noise reduction performance is superior to traditional digital image processing algorithms.

Speckle reduction in digital holography with low-dimensional reconstruction.

Speckle reduction is a crucial technique since the presence of speckle disturbs the quality of the reconstruction in digital holography. In this paper, we present an easy, fast, and efficient single-shot method to reduce speckle noise in digital holography. The method reconstructs subholograms from a single hologram. Then, sub-reconstruction images are randomly shuffled and divided into several groups and low-dimensional noise-reduced images can be achieved by averaging sub-reconstruction image groups by groups. Next, these low-dimensional noise-reduced images are combined to obtain a noise-reduced image. Finally, the noise-reduced image is processed by a mean filter to obtain a final image, which has substantially less speckle noise while preserving the dimensions of the original image. The experimental results demonstrate the effectiveness of the proposed method and indicate its potential in real-time digital holography.

Suppression of Speckle Noise in Digital Holography With Spatial and Temporal Domain Depolarization

Speckle noise in digital holography has a great influence on image quality, and one of the reasons for the formation of speckle noise is that the degree of coherence for the reference light and the object light is too high and unnecessary noise is introduced to the recorded hologram. In this paper, the spatial depolarization and the temporal domain depolarization are analyzed to reduce the speckle noise. The spatial depolarization utilizes the depolarization performance of the depolarizer. By adding a quartz depolarizer to the object light, the linearly polarized light in the object light becomes randomly polarized light, thereby reducing the degree of coherence with the reference light, realizing the suppression of speckle noise, and by utilizing the characteristics of pupil parameters towards the depolarization degree, it is to realize the control for the degree of coherence. The temporal domain depolarization uses two quarter-wave plates and adjustable liquid crystal to realize the depolarization superposition. By controlling the phase delay of the liquid crystal phase variable retarder, the multi-depolarization accumulation in a short time is realized, and the experimental results show that this method preserves the detail portion of the image while better removing speckle noise. Finally, the Non-Local Means filtering algorithm is used to make the reconstructed image smoother. The method proposed in this paper has great applicable value for improving the image quality of digital holography system.

Quasi-Noise-Free and Detail-Preserved Digital Holographic Reconstruction

The shortcoming of the coherence of digital holographic light sources leads to the generation of holographic reconstruction speckle noise, which seriously affects the quality of holographic reconstruction. Although many advanced algorithms for reducing speckle noise in digital holography (DH) have achieved good results, in the process of processing image speckle noise, the details of the image are blurred, and the information relating to the image texture is lost. Therefore, we propose a new algorithm, which combines automatic GrabCut and guided filtering to solve the problem of blurred image details in filtering reconstruction. The new algorithm is applied to the problem of holographic speckle removal for the first time. In this paper, the new algorithm not only achieves quasi-noise-free (approximate the noise-free state) DH reconstruction but also realizes the retention of details in the image. The quality of the resulting holographic reconstruction is comparable to that achieved by incoherent technology, which exceeds the current level of DH and produces a good visual effect.

Speckle reduction by selective spatial-domain mask in digital holography**Project supported by Guangdong Provincial Science and Technology Plan Project of China (Grant Nos. 2015B010114007 and 2014B050505020).

An improved method of using a selective spatial-domain mask to reduce speckle noise in digital holography is proposed. The sub-holograms are obtained from the original hologram filtered by the binary masks including a shifting aperture for being reconstructed. Normally, the speckle patterns of these sub-reconstructed images are different. The speckle intensity of the final reconstructed image is suppressed by averaging the favorable sub-reconstructed images which are selected based on the most optimal pixel intensity sub-range in the sub-holograms. Compared with the conventional spatial-domain mask method, the proposed method not only reduces the speckle noise more effectively with fewer sub-reconstructed images, but also reduces the redundant information used in the reconstruction process.

Strategies for reducing speckle noise in digital holography

Digital holography (DH) has emerged as one of the most effective coherent imaging technologies. The technological developments of digital sensors and optical elements have made DH the primary approach in several research fields, from quantitative phase imaging to optical metrology and 3D display technologies, to name a few. Like many other digital imaging techniques, DH must cope with the issue of speckle artifacts, due to the coherent nature of the required light sources. Despite the complexity of the recently proposed de-speckling methods, many have not yet attained the required level of effectiveness. That is, a universal denoising strategy for completely suppressing holographic noise has not yet been established. Thus the removal of speckle noise from holographic images represents a bottleneck for the entire optics and photonics scientific community. This review article provides a broad discussion about the noise issue in DH, with the aim of covering the best-performing noise reduction approaches that have been proposed so far. Quantitative comparisons among these approaches will be presented.

Reduction of speckle noise in digital holography by combination of averaging several reconstructed images and modified nonlocal means filtering

Although the speckle noise in hologram reconstructed image can be effectively suppressed by averaging multiple reconstructions with different speckle patterns, the approach usually needs a lot of the reconstructions and encounters an improvement limit. In this paper, we propose a speckle noise suppression method that combines averaging several reconstructed images and a modified non-local means (MNLM) filtering method. First, the probability density function (PDF) of the averaged speckle field after averaging multiple hologram reconstructions is deduced, then the number of hologram reconstructions for the averaging process is inferred for making the speckle noise distribution approaches Gaussian distribution. Second, considering the statistical model of the multiplicative Gaussian noise, we modify the NLM filtering for effectively removing the residual noise in the averaged image. The experimental results show that the proposed method can reduce the speckle noise more than 90% and achieve nearly speckle-free. Compared with another similar method, the presented method is more effective and feasible to suppress speckle noise and preserve image contrast based on a small number of reconstructions.

Speckle noise suppression using part of pixels in a single-exposure digital hologram

A method is proposed to suppress speckle noise using only part of the pixels in a single-exposure digital hologram. Different holographic patterns are first generated from a single-exposure digital hologram using specially designed binary masks; then, these holographic patterns are reconstructed according to the Fresnel transform. The reconstructed images are superposed and averaged on the intensity to achieve the suppression of speckle noise. The entire denoising process does not need any additional digital holograms or specific requirements for recording a hologram. Theoretical simulation and experiment verification were carried out and confirm that the proposed method is a very convenient and effective way to suppress speckle noise in digital holography. The proposed method has wide applications in holographic imaging, holographic storage, and art display.

Two-stage method to suppress speckle noise in digital holography

The two-stage method is proposed to suppress speckle noise in the digital hologram. Three kinds of optical denoising ways are analyzed and compared at first. The optimal one is used to reduce speckle preliminarily. At the same time, the statistical property of the speckle is changed by the optical way. Then the optimized NLM algorithm is adopted to further suppress speckle noise. The experimental system is set up, and the performance indices are calculated. The results are compared with other algorithms. It is demonstrated that the presented method can effectively suppress speckle noise in the digital hologram and the processed image is very vivid.

Study on the reduction of speckle noise in the reconstructed image of digital hologram

As one of the most important 3-D display technique, reduction of speckle noise in the reconstructed image of digital holography should be grounded on the digital hologram itself. Based on the whole process of the recording and reconstruction of digital holography, the optical distributions of recorded object and reconstructed image of digital holography have been studied. It has been proposed that the root formation cause of speckle noise in its reconstructed image is the speckle noise formed on the recorded object surface when illuminated by coherent light because of its optical roughness. A novel approach has been presented to reduce speckle noise in digital holography by changing the interference structure of hologram itself. First, by reducing the speckle noise in the reconstructed image, the distribution of ideal reconstruction light with reduced speckle noise is acquired. Then in turn, taking the ideal reconstruction light with reduced speckle noise as ideal object light, a new hologram can be rebuilt, which can reconstruct the ideal object light. The experimental results are given to confirm the proposed method. Therefore, it offers a brand-new thought and practical way to reduce the speckle noise in the reconstructed image of digital holography.

Research on speckle denoising by lensless Fourier transform holographic imaging with angular diversity

The signal-to-noise ratio and resolution of the reconstructed image is seriously influenced by speckle noise in digital holography, so it is essential to reduce the speckle noise and improve the image quality. Intensity correlation between two speckle patterns with the rectangle speckle spot is analyzed and deduced, and the minimal angular difference of illumination beams is given quantitatively under a special situation. A lensless Fourier transform holographic imaging system with angular diversity is designed based on the lens property, in which the direction of the wave can be changed by shifting the fiber instead of conventionally rotating the mirror, and the formor can change the direction of illumination with a fixed illumination size and location. Eighty one holograms with uncorrelated speckle patterns are recorded at different illumination angles by shifting the fiber. Then a digital reconstruction is achieved only by a fast Fourier transform, and the multiple reconstructed images are averaged. Experimental results show that the speckle contrast of the averaged reconstructed image can be reduced to 14.6% that from a single reconstructed image, and the signal-to-noise ratio is improved 6.9 times. This proposed digital holographic imaging method can suppress the speckle noise greatly, has a simple setup, and is easy to operate.

Improvement of speckle noise suppression in digital holography by rotating linear polarization state

An improved polarization recording approach to reduce speckle noise in digital holography is proposed. Multiple off-axis holograms are obtained by rotating the linear polarization state of both illumination and reference wave simultaneously. By averaging the intensity fields, the speckle noise in the reconstructed images is well suppressed. Statistical evaluation of the experimental results shows the effectiveness and improvement of the proposed method.

SPECKLE SUPPRESSION BY INTEGRATED SUM OF FULLY DEVELOPED NEGATIVELY CORRELATED PATTERNS IN COHERENT IMAGING

A coherent imaging system images a frame or an object onto a changing difiuser and projects the resulting pattern which generally contains speckles. Using a spatial light modulator (SLM) as the changing difiuser, the speckles in the pattern are suppressed without the need for any other mechanisms. With M random phasor arrays being displayed in the SLM during the integration time of a detector, a suppression factor (Cf) of speckles, 1= p M, is achievable in the projected pattern, which is the sum of the intensity of M uncorrelated patterns. This paper shows both theoretically and in simulations that the Cf of the sum pattern was considerably reduced when two elementary patterns with fully developed speckles were negatively correlated. With the correlation coe-cients of the elementary patterns found at (i0:3, i0:25), the Cf of the sum of 10negatively-correlated speckle patterns was 48% lower than the Cf of the sum of 10uncorrelated speckle patterns. The negatively correlated patterns can be implemented using spatial light modulators or difiractive optical elements, and are used to suppress speckle noise in digital holography, laser projection display, and holographic display projections with relatively high e-ciency.

消除数字散斑噪声的一种有效方法

An effective method for reducing the speckle noise in digital holography is proposed in this paper. Different from the methods based on classical filtering technique, it utilizes the multiple holograms which are generated by rotating the illuminating light continuously. The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns. Hence by properly averaging the reconstructed intensity fields, the speckle noises can be reduced greatly. Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.

Speckle noise reduction in digital holography by multiple holograms

An effective method for reducing speckle noise in digital holography is proposed in this paper. The proposed method does not require classical filtering technique; instead it utilizes multiple holograms of an object generated by rotating an illuminating light continuously. The reconstructed hologram intensity fields possess different speckle patterns, and by properly averaging the intensity fields, speckle noise in the reconstructed images is reduced significantly. A merlion sculpture is evaluated and experimental results show that the proposed method is simple and effective in reducing speckle noise in digital holography.

Improvement of accuracy in digital holography by use of multiple holograms

Speckle pattern decorrelation reduces the accuracy of interferometric shape and deformation measurements. We introduce a technique for the reduction of speckle noise in digital holography. The method is not based on classical filtering techniques such as median filters. Instead it utilizes the shift theorem of the Fourier transform. For this method several holograms of the same object under test are recorded. The reconstruction leads to a set of object wave fields with different speckle patterns. A proper averaging procedure, taking into account the properties of the wrapped phases, leads to an improvement of the accuracy in the resulting phase difference. The theory of the applied method is described and our first results for technical components with an improvement of accuracy up to 1/57 of the wavelength are presented.

Reduction of speckle noise in digital holography by using digital image processing

A fundamental problem in optical and digital holography is the presence of speckle noise in the reconstruction process. Many approaches have been carried out in order to overcome such a problem ranging from modifying the spatial coherence of the illumination (optical techniques) to image processing techniques (digital techniques). This work shows the merged use of digital image processing techniques in order to reduce the speckle noise in digital reconstruction of optically recorded Fresnel's holograms. The proposed filtering techniques are illustrated with experimental results.