粗糙表面对散斑统计特性的影响

为研究高速摄影中激光相干噪声的影响因素,降低激光照明中散斑噪声对高速摄影成像质量的影响,对激光照射表面的粗糙程度与形成的散斑场统计特性之间的关系进行了研究。基于菲涅尔-基尔霍夫衍射原理,分析了高斯光束入射在随机粗糙表面上经反射形成的散斑图样,通过对大量散斑图样的数据统计,得出了激光散斑的信噪比、自相关函数、概率密度函数与入射表面粗糙度之间的变化关系。计算结果表明:粗糙度的增大使散斑场信噪比降低、相干程度减小、概率密度降低,即表面越粗糙散斑噪声越严重。     

Measurement of surface roughness properties by means of laser speckle techniques

The statistical properties of speckle intensity variations produced by coherent light in the far-field diffraction plane of an iluminated area of an object are studied experimentally as a function of the radius of an illuminating beam over the object and are found to have a relation to its surface roughness and correlation lenght. Measurements of the surface roughness and the correlation lenght become possible by investigating the contrast variation of the speckle intensity as a function of the radius of the illuminating beam.     

Speckle analysis in laser scanning display system

The theory of speckle formation in laser scanning display system is established based on the averaging effect of eye response as laser beam scanning through an eye resolution spot. It is analyzed that speckle reduction can be obtained by averaging states of speckle during scanning. The theoretical results show that a smaller correlation length of screen surface and the narrowing of laser beam in scanning direction can reduce speckle contrast for this system.     

Granularity growth of laser speckle and its application to surface inspection

A process of granularity growth in partially developed speckle patterns produced in the near field of wavy object surfaces illuminated using a laser has been experimentally studied by investigating the probability density function and average contrast of their intensity variations. These parameters have been measured as a function of the distance from the object surface to the detector for four typical sample objects having different surface qualities. It has been found that the variation in average speckle contrast with receiving distance is strongly dependent on the surface quality of the objects. The saturated value of the speckle contrast, found from the graph of its dependence on receiving distance, is also related to the maximum height of irregularities in the surface.     

Whole field surface roughness measurement by laser speckle correlation technique

This paper describes a speckle correlation technique for the determination of surface roughness, ranging from 1.6 to 50 μm. Instead of moving the laser beam, the specimen is rotated to achieve angular speckle correlation (ASC) in the far-field plane. The technique is simple and requires minimum optical alignment. The experimental results show a good agreement with the standard specimen of known roughness. An error analysis on the experiment has been carried out. Together with the theoretical curves, the roughness values can be easily related to the change of incidence angle at a particular visibility of the correlation fringes between two speckle patterns.     

Measurement of surface roughness properties using speckle patterns with non-gaussian statistics

The surface roughness properties (i.e. the rms surface roughness and the correlation length) of strong diffuse objects are investigated by using the speckle patterns which obeys the non-gaussian statistics. The intimate linear relation is found to exist between the rms surface roughness of objects and the maximum contrast obtained from the varying curves of the average image speckle contrast as a function of the point spread of an optical imaging system. The correlation length of surface roughness of objects is related to the averaged intensity distribution of speckle patterns produced at the far-field diffraction plane. It now becomes clear that the rms roughness and the correlation length of strong diffuse objects are determined, respectively, from the maximum speckle contrast at the image plane and the averaged speckle intensity distribution at the far-field diffraction plane.     

地面对激光雷达信号散射的统计研究

从理论角度研究了利用高斯光束照射远场目标时,激光散斑的统计特性.导出了散射光场的自相关函数和光强度的二阶矩的解析表达式,计算了激光散斑的面积.研究表明:在接收面上,光强的分布与目标表面高度的相关长度和均方根高度密切相关,激光散斑面积和散斑光强的相关系数只与激光束腰的尺度有关,与目标的均方根高度和相关长度无关. 关键词： 激光散斑 自相关函数 高斯分布 协方差     

Real-time anti-node visualization of vibrating distributed systems in noisy environments using defocused laser speckle contrast analysis

A simple optical technique is described that allows for the direct whole-field visualization of anti-nodal patterns in noisy environments. The technique involves the combination of defocused laser speckle imaging with the laser speckle contrast analysis (LASCA) algorithm to relate local contrast reduction of a laser speckle pattern to the magnitude of the gradient vectors that arise due to the tilt deflection of a vibrating surface. The method requires no reference beam and is tolerant of environmental noise. The sensitivity of the technique is a direct function of the degree of defocus of the speckle pattern and thus can be readily adjusted. By ratioing the instantaneous contrast to the reference image contrast, a simple method for thresholding the noise floor (noise at reference state) is attained, thus increasing the method's tolerance to ambient disturbances. Results from forced vibration via acoustic excitation of a square and rectangular membrane—with and without simultaneous white noise excitation—are shown. Results are compared to theoretical predictions for an ideal membrane, with good agreement.     

Direct Fabrication of Surface Relief Holographic Diffusers in Azobenzene Polymer Films

We propose a direct fabrication of light diffusers using azobenzene polymer films. Holographic recordings of surface relief diffusers are formed on the polymer film by exposure to an Nd:YAG laser (532 nm) through source diffusion materials. No post-treatment is needed, and it can be erased by heating or irradiating uniform laser beam. Diffusion pattern can be controlled by the polarization of the laser. Transmittance of over 88% is obtained at the wavelength of 633 nm.     

Nonuniform intensity distribution of the scattered light by moving diffuser across projection lens pupil and its influence in speckle reduction

The measurement of the nonuniform intensity distribution of scattered light by a moving diffuser using an integrating sphere, which lowers the effective numerical aperture of the projection lens and affects the lowest speckle contrast, is reported. A comparison of the effective numerical apertures realized by several diffusers revealed that JIS #600 or #800 diffusers are most appropriate for efficient speckle reduction     

Surface roughness measurement by speckle contrast under the illumination of light with arbitrary spectral profile

Quantification of surface roughness greater than a micron is desirable for many industrial and biomedical applications. Polychromatic speckle contrast has been shown theoretically to be able to detect such roughness range using an appropriate light source with a Gaussian spectral shape. In this paper, we extend the theory to arbitrary spectral profile by formulating speckle contrast as a function of spectral profile, surface roughness, and the geometry of speckle formation. Under a far-field set-up, the formulation can be simplified and a calibration curve for contrast and roughness can be calculated. We demonstrated the technique using a blue diode laser with a set of 20 metal surface roughness standards in the range 1–73 μm, and found that the method worked well with both Gaussian and non-Gaussian surfaces.     

经成像系统二次调制的动态散斑的空时相关性质

高斯光束经位于成像系统共轭面的两个散射体(其中一个是恒速运动)后,在距像面为菲涅尔区的观测面上形成了串级动态散斑场.理论分析和实验测量结果均表明,这种动态散斑的空-时相关函数与一次散射形成的动态散斑相比,对散射体的运动十分敏感,但其时间相关长度与速度的倒数仍然保持线性关系.     

Detection of a small defect on a rough surface

We report experimental results that show that the far-field correlation function is sensitive to a small local change in rough-surface geometry, where the speckle spatial correlation rather than the sample ensemble average is adopted. The angular cross-correlation function of the far-field speckles scattered by one-dimensional random rough surfaces is measured when a polarized beam of light is incident upon the rough surface from vacuum, where one part of the surface used is a thin dielectric film deposited upon a glass substrate and the other part is identical to the first except for a localized defect. We envisage application of this sensitivity property to inspection of a sample with a defect by means of speckle mapping.     

基于多色散斑延长效应的表面粗糙度测量及影响因素分析

粗糙表面在多波长激光束照射下形成的多色散斑场显示出散斑延长效应,利用此效应可以测量表面粗糙度,并且测量结果在一定条件下不受粗糙表面横向特征的影响。通过模拟计算随机粗糙表面的多色散斑场,以空间平均的多色散斑场局部自相关函数研究了平均散斑延长率〈χ〉对表面轮廓均方根偏差σh的依赖关系,分析了测量系统因素,如入射激光波长组合、成像器件光敏单元尺寸和动态范围对测量结果的影响。结果表明,以空间平均的局部自相关函数代替集平均的散斑自相关函数描述多色散斑延长效应是有效的;为达到一定的粗糙度测量精度,应选择合适的入射激光波长组合和合适的成像器件光敏单元尺寸。     

Variation of the speckle contrast in a graded-index fibre by misalignment of the incident beam

The speckle pattern observed at the output end of a multimode fibre is closely related to the modal noise. In this paper, the average contrast of the speckle pattern is studied as a function of the radial position in the output end plane for three kinds of misalignment of the incident beam, i.e. defocusing, axial misalignment and angular misalignment. It is found that a variation of the speckle contrast is very sensitive to all kinds of misalignment of the incident beam and that its varying form strongly depends on the kind of misalignment of the incident beam. The speckle contrast at the centre of the output end plane is also examined as a function of the magnitude of misalignment. It is found that the contrast increases from a minimum value in the correct alignment to a certain saturated value with an increase of misalignment.     

Laser processing of poly(methyl methacrylate) Lambertian diffusers

Matrix-assisted pulsed laser deposition was used to deposit poly(methyl methacrylate) on silicon wafers and sodium silicate glass slides for the purpose of making optical diffusers. After deposition, the reflectance of the coated substrates was measured as a function of scattering angle. We found that the angular dependence of the reflectance could be described as the sum of two functions. First, a Gaussian describes the specular reflection of the underlying substrate that has been broadened by passage through the film. Second, a cosine function describes the reflectance contribution from the film itself. We found that by increasing the thickness of the deposited film that we could eliminate the specular reflection to obtain Lambertian diffusers. Since we can control the surface roughness by adjusting the ratio of the two matrices in laser processing, this deposition technique offers the possibility of producing a wide range of diffusers of different types.     

Experiments on the space and wavelength dependence of speckle

A comprehensive series of speckle experiments is described for opal glass diffusers ranging in thickness from 10 to 500 μm. Wavelength decorrelation measurements are made using a cw dye laser source; and accurate square-law detection is provided by the use of an optic-fiber, photomultiplier combination rather than film. For this diffuser set intensity distribution functions, polarized and unpolarized, are plotted. Second-order correlation functions are presented for different apertures, relating diffuser motion and probe motion. Remote texture determination using contrast ratio and frequency decorrelation is illustrated.     

斜入射引入动态多重散射机制的激光散斑抑制

散斑抑制一直是激光显示技术的研究热点。对激光光束经SiO₂溶液所形成散斑的特性分析,建立了多重散射与散斑颗粒大小的联系。结合动态光散射理论,提出利用斜入射引入动态多重散射机制的激光散斑抑制方法,并构建由静态散射片和装有粒径为300 nm、摩尔浓度为3.0×10-4 μm-3的SiO₂悬浮液的光通管组成的散斑抑制装置,将其置入激光显示系统的光源部分。针对光束以不同入射角进入SiO₂悬浮液对散斑图像对比度影响进行了系统实验分析。结果表明,光束以约8°进入SiO₂悬浮液能将散斑图像的对比度从0.43降低至0.067。通过该方法实现了散斑颗粒的空间平均和散斑图像的时间平均,提高了散斑抑制的效果。散斑抑制单元无需振动装置且便于集成到激光投影系统中,不仅提高了系统的可靠性也减小了成本。     

半导体激光器内自混合散斑干涉

在散斑和半导体激光器的自混合干涉理论的基础上,提出了基于法布里一珀罗腔的自混合散斑干涉模型,对粗糙表面产生的激光器内自混合散斑干涉效应(SMSI)进行了详细的理论研究和分析,得到了粗糙腔条件下的激光动力学的数值解。模拟产生了高斯相关随机表面及其在夫琅禾费面上产生的散斑场,以及该散斑场与相干光场叠加所形成的光场,分析了这些情况中光强的统计特征,与已知实验情况进行了对比,模拟结果与已知统计结果相同。给出了粗糙表面运动时激光器输出增益的变化及其概率密度分布。对由于照明宽度和外腔长度的变化而给粗糙表面运动时激光器输出增益变化带来的影响进行了分析。实验结果和模拟结果相符。     

Understanding the exposure-time effect on speckle contrast measurements for laser displays

To evaluate the influence of exposure time on speckle noise for laser displays, speckle contrast measurement method was developed observable at a human eye response time using a high-sensitivity camera which has a signal multiplying function. The nonlinearity of camera light sensitivity was calibrated to measure accurate speckle contrasts, and the measuring lower limit noise of speckle contrast was improved by applying spatial-frequency low pass filter to the captured images. Three commercially available laser displays were measured over a wide range of exposure times from tens of milliseconds to several seconds without adjusting the brightness of laser displays. The speckle contrast of raster-scanned mobile projector without any speckle-reduction device was nearly constant over various exposure times. On the contrary to this, in full-frame projection type laser displays equipped with a temporally-averaging speckle-reduction device, some of their speckle contrasts close to the lower limits noise were slightly increased at the shorter exposure time due to the noise. As a result, the exposure-time effect of speckle contrast could not be observed in our measurements, although it is more reasonable to think that the speckle contrasts of laser displays, which are equipped with the temporally-averaging speckle-reduction device, are dependent on the exposure time. This discrepancy may be attributed to the underestimation of temporal averaging factor. We expected that this method is useful for evaluating various laser displays and clarify the relationship between the speckle noise and the exposure time for a further verification of speckle reduction.