Reduction of speckle noise in the reconstructed image of digital holography

Based on the whole process of recording and reconstruction of digital holography, we study the formation causes of speckle noise in the reconstructed image and its characteristics. And we reach the conclusions that the little hologram aperture diffraction aggravates the speckle noise and the speckle noise in the reconstructed image is a kind of multiplicative noise. Serving as the basis of these features, we propose the reduction technique for speckle noise. Firstly, we set an appropriate aperture function matching the recording parameter and an aperture size of hologram and deconvolve the reconstructed image of the hologram with it. Secondly, we converted the multiplicative noise into additive noise by means of homomorphic transform and realize the separation of signal and noise. Thirdly, we applied the Wiener filtering to reduce the additive noise. Finally, we carried out the inverse transformation. The proposed reduction technique is illustrated with experimental results. Therefore, the paper offers a brand-new thought and practical way of reducing the speckle noise in the reconstructed image of digital holography.     

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.     

Speckle reduction by selective spatial-domain mask in digital holography

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.     

Speckle noise reduction by division and digital processing of a hologram

One of the common problems in classical and digital holography is presence of speckle in reconstructed images. Some methods have been predicted for speckle reduction. Speckle has a statistical distribution in size and intensity. Thus complete elimination of them is impossible. But, one can use this statistical characters to reduce its effects.In this paper a method is proposed for reduction of speckle effect by recording a hologram and then dividing it to several sub-holograms. Each sub-hologram is used to produce an image. Final image is constructed by superposition of these images. The final image has a smaller amount of speckle noise. Experimental results verify effectiveness of this technique.     

Digital color holographic recording and reconstruction using synthetic aperture and multiple reference waves

This paper presents a method for recording and reconstructing high-quality digital color holographic images. The synthetic aperture technique is used to improve the resolution of the reconstructed image and also reduce the speckle size in the reconstruction plane. Subsequently, the multiple reference wave technique is used to suppress speckle noise in the reconstructed color holographic image by superposing speckle fields with different distributions. The final high-quality color holographic image is obtained due to the resolution improvement and speckle noise reduction achieved by the two techniques. Our method has potential applications in the digital color holographic displays.     

数字全息技术中散斑噪声滤波算法比较

 在数字全息测量记录过程中,其所记录的全息图易受到散斑噪声的污染造成分辨率下降,同时也严重影响数字全息再现的效果,因此研究适用于数字全息技术中散斑滤波的算法具有重要的实用价值。介绍了中值滤波、Lee滤波、Kuan滤波和SUSAN滤波这四种常用的散斑滤波算法,并将它们运用于数字全息实验所记录图像和数字再现图像的散斑噪声滤波处理中,然后对这四种算法的处理结果进行评价。结果表明,在数字全息技术中使用SUSAN滤波算法进行处理,既明显抑制了散斑噪声,又有效保证了再现图像信息的完整性。     

基于数学形态学的数字全息再现像融合方法

针对数字全息中不同再现距离获得的携带不同聚焦信息的再现像, 提出了一种基于数学形态学的多聚焦再现像融合方法, 以有效扩展成像景深。首先通过小波-Controulet变换获得源图像的高频和低频分量;然后, 针对数字全息中含散斑噪声的特点, 对高频分量采用基于数学形态学区域能量的方法进行融合, 对低频分量采用加权对比度法进行融合;最后, 将融合系数反变换得到融合图像。通过对算法的有效性分析和实验验证, 将本文提出的方法与不加入数学形态学的融合方法进行了对比研究。结果表明, 基于数学形态学的融合方法能充分抑制散斑噪声的影响, 保留更多细节信息, 有效扩展了成像景深范围达11.5 cm。其中, 对于表面粗糙且信息量较少的骰子, 基于数学形态学方法的空间梯度算子提高了11.8%, 熵值提高了2.7%;对于表面光滑且信息量较多的硬币, 其空间梯度算子提高了13.6%, 熵值提高了2.8%。     

Improvement of signal-to-noise ratio in digital holography using wavelet transform

The basic problem in optical and digital holography is the presence of speckle noise in the reconstruction process, which reduces the signal-to-noise ratio (SNR). The presence of speckle noise is serious drawback in optical and digital holography since it substantially reduces the SNR in the reconstructed image. In this paper, we present wavelet filtering to improve SNR in the reconstructed images from digital holograms. Experimental results are presented.     

基于多角度无透镜傅里叶变换数字全息的散斑噪声抑制成像研究

在数字全息成像中,散斑噪声严重影响了再现像的信噪比和成像分辨率,因此为了提高数字全息成像质量,迫切需要抑制再现像的散斑噪声.分析并给出了矩形散射光斑的强度协方差,定量计算了特定实验条件下产生退相关散斑图样的最小角度差.结合透镜的性质设计并搭建了多角度无透镜傅里叶变换数字全息成像系统,利用光纤端面在透镜焦平面的二维机械移动代替传统反射镜的旋转,使照明光束在不改变照明位置和大小的同时,可任意改变光束方向.移动光纤端面使多角度照明满足最小角度差,获取了81幅数字全息图.利用单次快速傅里叶逆变换实现数值再现,对多幅再现像的强度像求平均,实验结果表明散斑对比度降低为单幅再现像的14.6%,使图像信噪比提高了6.9倍.该抑制散斑的多角度数字全息成像方法有效的抑制了散斑噪声,且成像光路结构简单,可操作性强.     

菲涅尔数字全息的脉冲响应

数字全息是用CCD记录全息图并用计算机数值重建全息像的一种全息新方法.在数字全息中,通过对不同记录参数下记录的全息图的数值处理,可以消除零级光和共轭光,从而将数字全息系统看作是一个线性系统.本文依据全息理论和付里叶频谱分析,对菲涅尔数字全息系统的脉冲响应和分辨本领进行了理论分析.结果表明,在矩形等间隔抽样的情形下,菲涅尔数字全息的脉冲响应是由CCD有限大小的孔径衍射斑调制的矩形函数;菲涅尔数字全息的分辨率由CCD的孔径尺寸决定;由于CCD像素具有一定的大小,使得点光源的像发生弥散.     

Increment of lateral resolution in digital holography by speckle noise removal

Experimental features such as wavelength, camera specifications and reconstruction distance determine the theoretical limit for lateral resolution in digital holography. However, the actual experimental resolution limit is about 50% below such theoretical limit due to the high-contrast speckle noise presented in the reconstructed holograms. Recently, a technique has been introduced to reduce the contrast of speckle noise that is based on the superposition of uncorrelated hologram reconstructions of the same static object [5] (Garcia-Sucerquia et al., 2006). By this approach of reducing the contrast of the speckle noise, it is experimentally shown that an improvement of the order of 50% can be reached when 100 reconstructed images are superimposed.     

Automatic focusing in digital holography and its application to stretched holograms

The searching and recovering of the correct reconstruction distance in digital holography (DH) can be a cumbersome and subjective procedure. Here we report on an algorithm for automatically estimating the in-focus image and recovering the correct reconstruction distance for speckle holograms. We have tested the approach in determining the reconstruction distances of stretched digital holograms. Stretching a hologram with a variable elongation parameter makes it possible to change the in-focus distance of the reconstructed image. In this way, the proposed algorithm can be verified at different distances by dispensing the recording of different holograms. Experimental results are shown with the aim of demonstrating the usefulness of the proposed method, and a comparative analysis has been performed with respect to other existing algorithms developed for DH.     

合成孔径数字全息的分析模拟及多参考光合成孔径数字全息

分析了合成孔径数字全息的理论和实现方法,提出了多参考光合成孔径数字全息方法及其应用前景,以双狭缝为例,模拟了单幅及单参考光、多参考光两种合成孔径数字全息术的记录和再现方法,对三种方法再现像的分辨率进行了比较.     

数字无透镜傅里叶变换全息术中非傍轴及离焦像差的校正

对数字无透镜傅里叶变换全息图直接采用逆傅里叶变换进行物场的数值重建时.需要满足两个条件:第一,全息图的记录过程必须满足傍轴近似条件,否则再现过程中会产生非傍轴像差;第二,记录全息图时物平面与参考点光源到全息图记录平面的距离必须相等,否则再现过程中会产生离焦像差.理论分析了非傍轴及离焦记录条件下数字无透镜傅里叶变换全息图的灰度分布特点,并提出了相应的非傍轴及离焦像差的数值校正方法.根据实际的非傍轴或离焦记录情况.分别给所记录的数字全息图灰度分布矩阵乘以适当的非傍轴或离焦校正因子,以消除灰度矩阵中非傍轴或离焦因素的影响.然后再对校正后的伞息图灰度矩阵做逆傅里叶变换处理.即可得到准确的数字再现像.实验结果表明.该数值重建方法能够有效地消除无透镜傅里叶变换全息术中数字再现像的非傍轴像差及离焦像差,提高再现像的质量.     

Studies on aperture synthesis in digital Fresnel holography

Aperture synthesis can improve image resolution in digital holography by increasing the numerical aperture of the system. In this paper, we show that both the lateral resolution and image field of view can be enhanced at the same time using a more general Fresnel holography setup and hologram stitching. The impact of aperture synthesis on the lateral resolution is investigated both theoretically and experimentally. In the experiment, the synthesis is executed by moving the compact digital holographic system in two directions. Nine holograms are recorded and stitched into one hologram. The reconstruction results show that expanding aperture improves the lateral resolution. The lensless Fresnel holography used in this paper is demonstrated to have the ability to provide a larger numerical aperture and can compress the object spectrum in recording process.     

数字全息周期像的产生机理及在抑制零级衍射上的应用

在数字全息成像中, 利用CCD的RGB模式采样全息图时, 全息重构像会出现特定的周期性分布. 本文从理论和实验上详细研究了这种周期像产生的机理、分布特性和应用. 研究结果显示, 由于CCD的光谱滤镜会使全息图的RGB三个单色采样阵列出现部分像素信号的缺失, 因此, 需要通过特定的demosaicing数学算法对缺失的像素信号进行重建以形成完整的单色采样阵列, 这是数字全息再现像周期分布产生的根源. 而基于demosaicing算法的采样阵列重建会在全息图频谱中引入调制函数, 导致物体再现像和零级衍射斑的周期分布差异. 本文揭示了全息图的RGB采样、demosaicing算法与全息重构像周期性之间的内在关联. 最后, 讨论了结合空间移位和图像形态学技术, 利用重构像的周期性抑制零级衍射斑的应用. 所有理论与实验研究结果完全一致. 关键词： 数字全息 图像周期性 零级斑抑制     

相移同轴无透镜傅里叶数字全息的分析与实验

应用菲涅耳衍射和全息理论，详细分析了无透镜傅里叶变换数字全息图的记录、再现方法和再现像的特点，分析了相移数字全息图的记录和再现方法，并进行了相应的实验验证。结果表明：直接对无透镜傅里叶数字全息图进行傅里叶逆变换可同时得到与物体完全相同的再现像及其共轭像；同轴无透镜傅里叶数字全息术能最大程度满足CCD对采样条件的要求，从而可以增大记录物体的尺寸，减小记录距离，明显提高再现像的清晰度和分辨率；相移数字全息术能有效地消除数字再现光场中的零级光场和共轭像，显著提高再现像的信噪比。条件许可时，相移同轴无透镜傅里叶数字全息术是目前解决数字全息术中再现像的分离与满足采样条件之间矛盾的最佳方法。     

基于有限脉冲响应滤波器的数字全息零级像消除

针对数字全息中零级像的存在影响数字再现像的质量,分析数字全息图的记录、再现原理及频谱特性,提出了一种利用有限脉冲响应滤波器消除全息图数字再现中零级像的方法.该方法只需记录一幅数字全息图,不需要相移器材或其他辅助设备,直接利用数字图像处理对数字全息图在空域进行预处理,消除全息再现时的零级像干扰.对比了数字模拟和实验拍摄到的全息图在应用有限脉冲响应滤波器消除零级像前后的再现结果,表明该方法可消除零级衍射像,改善重建像质量,并且算法简单.     

High resolution digital holography

In digital Fresnel holography, present specifications of charge-coupled device cameras require the incident beams to be quasi-parallel. That implies large speckle grain size and low lateral resolution in reconstructed images. Better lateral resolutions are demonstrated in our work, down to 8 μm, allowing the observation of sub-millimetre objects by digital holography. The experimental set-up built samples the incident light distribution with a definition of 500 pixels mm⁻¹. The maximum acceptable angle is then widened and the hologram recording and reconstruction distances are drastically reduced, as well as the speckle size. Interferometric holography was implemented for the measurement of the deformation of a sub-millimetre silicon cantilever and results in phase-contrasts imaging are also reported. The design of dedicated complementary metal-oxide-semiconductor active pixel sensor cameras is also discussed.     

Effect of intensity quantization level in parallel phase-shifting digital holography

Parallel phase-shifting digital holography (PPSDH) enables the instantaneous recording of three-dimensional fields. The three-dimensional field can be reconstructed using a computer by numerical scalar wave propagation. In PPSDH, we record a space-division multiplexed hologram that includes the required phase retardation in the spatial distribution of the reference wave and then interpolate the data at blanked pixels for each phase retardation to obtain the complex amplitude distribution of an object wave. The recorded quality of the multiplexed hologram influences the reconstruction quality. In this study, we investigate the effect of the intensity quantization of a multiplexed hologram on the reconstruction quality. We compare the influence of intensity quantization in PPSDH with that in the conventional phase-shifting method. Random noise is also added to the multiplexed hologram. The required intensity quantization level is helpful for selecting a digital image sensor.