The influence of hologram aperture on speckle noise in the reconstructed image of digital holography and its reduction

Based on the whole process of the recording and reconstruction of digital holography, we study the formation cause of speckle noise in its reconstructed image and acquire the conclusion that the small size of hologram aperture diffraction aggravates the speckle noise of reconstructed image and the speckle noise has been one of primary noise sources in the reconstruction process. In order to reduce the speckle noise resulting from little hologram aperture diffraction, we set an appropriate aperture function matching the recording parameter and aperture size of hologram and deconvolve the reconstructed image with it. The validity has been proved in theory and experiment. Therefore, it offers a brand-new thought and practical way to reduce 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 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.     

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.     

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。     

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.     

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%。     

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.     

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.     

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.     

长工作距离显微成像数字全息合成孔径方法(英文)

提出了一种用于长工作距离下显微成像的数字全息合成孔径方法。首先,在不同斜入射照明条件下记录多幅包含不同物光频谱范围的数字全息图。然后,利用每一幅全息图通过数字方法重构并放大物体强度像。最后,通过非相干叠加这些强度像得到分辨力提高且散斑噪声减小的合成物体强度像。理论分析和实验结果表明,此方法可应用于长工作距离下的原位显微观测。     

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.     

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.     

预放大数字全息显微系统的特点

为了提高预放大数字全息显微系统的成像质量,采用理论分析与实验验证相结合的方法,分别对采用平面及球面参考光记录的预放大数字全息显微系统进行了研究和比较。结果表明:在通常的实验条件下,系统的横向分辨力主要取决于显微物镜的成像分辨力;记录距离较小时,两种系统的横向分辨力均随着记录距离的增大略有降低;但当记录距离较大时,球面参考光预放大数字全息系统的横向分辨力降低得更为明显,即平面参考光预放大数字全息显微系统较为优越;在记录距离为0的情况下,即像面数字全息成像情况下,两种系统的再现像均具有最高的分辨力,在利用普通工业用传感器条件下,横向分辨力远超过了2.19 m,且像质较好。因此,尽可能减小全息图的记录距离,或者采用像面数字全息系统,可以有效提高数字全息系统的成像分辨力。     

基于支持向量回归的patch近场声全息研究

为了解决patch近场声全息中全息数据外推问题,提出一种基于支持向量回归的一步式patch近场声全息技术。该方法首先把初始全息面上的数据当成训练样本进行学习,构造出回归函数,然后利用回归函数实现全息数据外推,最后基于统计最优近场声全息进行重建。数值仿真和实验研究的结果表明:在各个分析频率下,该方法都可以实现小孔径全息面的近场外推。从近场声全息重建结果看,即使初始全息数据受到噪声干扰,该方法也是一种有效的patch近场声全息技术。      

基于两步正交相移干涉的振幅图像光学加密技术

提出一种基于两步正交相移干涉的光学图像加密技术.这种相移干涉数字全息只要记录两幅干涉图,不需要记录物光波和参考光波的强度信息,就可以再现没有零级像和共轭像的再现像.物光波对应的光路经过两次菲涅尔变换,并结合双随机相位编码.参考光分别引入0和π/2相位,用数字化记录介质记录两幅数字全息图作为加密图像.解密时只要获得正确的密钥,经过简单的计算就可以重建清晰的原始图像.模拟实验验证了它的可行性和有效性,分析了抗裁剪和噪音的鲁棒性.     

Speckle noise reduction by the composition of diffused Fraunhofer holograms

The image reconstructed from a composite hologram is compared with that obtained by multiple-exposure of images independently reconstructed from a single element and also with that from a synthetic-aperture hologram. The experimental results show the difference in the principle of speckle noise reduction with these three methods and prove that the composite hologram arranged only in a row is sufficient for reducing the speckle noise.     

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.