CCD成像辅助激光散斑实验

激光照射到粗糙物体的表面上在空间随机相干叠加形成散斑.激光散斑随物体在自身平面内作同步运动,这一现象可用来测量物体的微小位移.利用氦氖激光器作为光源,结合CCD成像技术辅助采集散斑图像,采用计算机程序处理图像,验证了散斑位移理论,并指出CCD成像辅助的激光散斑教学中应注意的问题.     

激光斑点漂移的研究

用激光照射粗糙表面时,可以观测到颗粒状的细小斑点,当人眼或相机移动时,观测到斑点也发生移动.分析了激光散斑产生和漂移的原因,将散斑漂移简化为几何光学和波动光学问题,得到一致的散斑漂移速度计算公式.利用Mathematica计算光强关联函数,证实圆斑越大散斑越小.实验测量了散斑漂移速度与相机移动速度的关系,结果符合理论公式.     

微型激光光学引擎中光学噪音的抑制

分析了微型激光投影引擎的两种主要光学噪音|干涉网格和激光散斑产生的基理|提出采用振动的随机位相板同时实现干涉网格和激光散斑抑制的方法并进行了理论分析|自行设计了微型激光投影引擎,利用微型电控马达带动随机位相板振动,用CCD相机在不同的F#及曝光时间下对投影图像进行了采集,并对图像的散斑对比度行了测试.结果表明,通过静止的随机位相板可以很好地抑制干涉网格噪音|通过使随机位相板以大于视觉暂留效应形成的融合频率进行振动,可以同时抑制干涉网格和激光散斑,使得散斑对比度小于5%,达到投影观察的要求.     

合成孔径激光成像雷达(Ⅵ):时空散斑效应和外差探测信噪比

研究了合成孔径激光成像雷达成像过程中激光散斑效应对于光学外差探测的影响,考虑了光学天线孔径对于散斑的审间平滑效应,特别是提出了时间变化的散斑统计特性并进行了数学分析,因为线性调频激光在一次激光曝光时间过程中激光散斑的花样是变化的.最终给出了考虑时空散斑效应的光子受限外差探测信噪比.     

激光散斑漂移现象的理论模型与成像实验

基于傅里叶光学原理,建立了数值计算客观及主观激光散斑图样的方法.利用几何光学成像原理,推导了主观散斑现象中散斑及光斑漂移速率的表达式.搭建了成像实验装置,观察主客观散斑及其漂移现象.结合相关函数、快速傅里叶变换算法以及卷积定理,建立了基于图像处理定量计算散斑及光斑漂移量的方法.实验结果、数值计算结果、几何光学理论均互相符合,表明了数值方法与几何光学理论的正确性,为激光散斑漂移现象提供了系统性的研究方法和直观的理解.     

激光散斑的漂移现象以及数值模拟方法

当一束激光照射到粗糙表面上时会产生散斑.即使接收端保持静止,收集到的散斑图案依然会发生变化,看起来像是在“漂移”一样.我们的研究发现,此时散斑漂移的主要原因来自粗糙表面吸热膨胀导致反射光的相位变化,从而使得干涉图样发生变化.我们采集大量实验数据并且使用相关系数及其变化率对散斑的动态变化进行描述和分析.另外,我们还通过数值计算模拟了散斑漂移现象.     

微型激光光学引擎中光学噪音的抑制

分析了微型激光投影引擎的两种主要光学噪音:干涉网格和激光散斑产生的基理;提出采用振动的随机位相板同时实现干涉网格和激光散斑抑制的方法并进行了理论分析;自行设计了微型激光投影引擎,利用微型电控马达带动随机位相板振动,用CCD相机在不同的F#及曝光时间下对投影图像进行了采集,并对图像的散斑对比度行了测试.结果表明,通过静止...     

激光散斑效应对激光雷达探测性能的影响

分析了激光散斑效应对激光雷达探测性能的影响. 给出了远场情况下接收物镜所采集激光散斑数的表达式. 在物镜所采集的散斑和光电子数均较大情况下，提出了光电子所遵守的分布. 分别导出了机载和星载激光雷达的探测概率. 分析表明激光雷达探测性能与所采集的散斑数密切相关，接收的散斑数仅与激光束腰和物镜的口径有关，随着接收散斑数的增加，散斑噪声的影响将减弱. 关键词： 激光雷达 激光散斑 探测概率     

采用大口径投影光学系统监测远场散斑特征参数

大口径投影光学系统采用低成本、大口径菲涅耳透镜制作,可将远场散斑强度分布投影到CCD成像探测器上。通过CCD图像处理,能够对给定孔径上的接收功率、闪烁指数进行量化评估;在接收孔径足够大、保障散斑不会因为光束漂移效应而脱离菲涅耳透镜的条件下,该系统还可以对光束漂移和特征半径进行量化评估。同时讨论了CCD像元响应非均匀性误差及其影响、CCD辐照响应函数和图像几何投影系数的定标方法。实验表明,系统能够对激光大气传输过程中的远场散斑特征参数进行监测。特别对自由空间激光通信系统而言,可以为大气衰减和多种大气湍流效应综合作用下的中值电平慢衰落研究和检测阈值优化设计提供实验数据支撑。     

像面散斑平均尺寸对激光散斑成像的影响

以激光散斑衬比分析为基础的激光散斑成像技术,是一种无需扫描的全场光学成像方法,在监测生理及病理状态下组织血流动态变化中的应用日益广泛.在实际应用中,像面散斑平均尺寸等多种因素影响散斑衬比值,使得该技术在反映血流变化的准确性方面受到影响.采用一种成像散斑计算机模拟方法研究了像面散斑平均尺寸对成像散斑统计特性的影响,分析了成像参数与像面散斑尺寸的定量关系,并通过物理模型实验对模拟结果进行了验证.研究结果确认了合理的像面散斑平均尺寸计算公式,证实了散斑衬比值随像面散斑平均尺寸增大而减小的现象,并为确定合理的成像参数提供了依据.     

基于散斑抑制的合成孔径激光成像雷达的结构和工作模式

目标漫反射产生的激光回波散斑效应严重影响合成孔径激光成像雷达(SAIL)的成像质量。在体系结构上提出了抑制散斑效应的系统性解决方案,建立了SAIL结构和工作模式设计的理论基础。研究了SAIL中与目标分辨单元尺寸、啁啾波长变化、目标相关性质和接收面光强随机分布有关的散斑统计特性。定义了SAIL光学接收天线的散斑孔径积分场复相干函数,它是天线孔径相关函数和目标分辨单元相关因子的卷积,其宽度就是可实现的孔径合成长度,给出了实现较大的孔径合成长度的发射口径、接收口径和实际孔径合成长度的设计原则,发现和分析了由啁啾散斑移动产生的拍频信号波动。最后建议采用滑动聚束模式来有效使用散斑效应造成较短的孔径合成尺度,因为其光束扫描宽度对SAIL移动距离有放大作用。同时也提出了具有多发射机/多接收机的多通道结构以提高回波散斑光场的探测率。     

偏振激光散斑的积分光强的统计特性

本文讨论了偏振激光散斑的统计特性,得出了部分偏振激光散斑场的积分光强的概率密度函数的精确表达式,通过实验研究了当检测孔径面积和平均光强变化时偏振激光散斑的积分光强的概率密度函数。 关键词：     

激光光斑形状和尺寸对扫描显示中散斑对比度的影响

计算了圆形、矩形激光光斑对于激光扫描显示系统中散斑对比度的影响.对于圆形光斑,当光斑尺寸大于散射表面相关长度时,得到的散斑对比度随着光斑尺寸变小而下降;当光斑尺寸接近表面相关长度大小时,由于光斑中包含的散射颗粒变少,得到的是非高斯散斑,散斑的对比度反而会变大;随着激光斑进一步变小,由于镜面反射效果散斑的对比度会很快下降.对圆形光斑部分结果给出了实验验证.为了保证光斑中有足够的散射颗粒,只在一个方向压窄光斑,分别计算了平行于扫描方向和垂直于扫描方向压窄光斑得到的散斑对比度.平行于扫描方向压窄光斑,散斑的时间相关性会下降,平均效果变明显,散斑的对比度变小;垂直于扫描方向压窄,散斑对比度变化不大.     

Application of laser speckle analysis for the assessment of cementitious surfaces subjected to laser cleaning

This paper focuses on the application of laser speckle technique for the assessment of the effectiveness of laser cleaning of cementitious surfaces. Laser speckle-based methods are non-contact, highly resolving techniques for the measurement of displacement, rotation, and strain of an illuminated area on a rough surface. Since the intensity of reflected light depends on the geometrical microstructure and colour of the samples, any alterations of the surface result in different speckle images. Analysis of speckle images presented here is based on the analysis of the distribution of intensity of reflected light obtained in a selected plane, and analysis of statistical parameters describing such distribution (skewness and kurtosis). A wide range of laser-cleaned mortar samples with different geometrical microstructure and moisture content has been subjected to the assessment by He-Ne laser. Laser speckle method has been successfully used to identify the effectiveness of the laser cleaning process with respect to different surface conditions. It appears that the changes in kurtosis and skewness should be mainly associated with the alterations of geometrical microstructure. Whereas, mean light intensity seemed to depend predominantly on the mortar's absorption characteristics (colour).     

Detection of fatigue damage in steel using laser speckle

We investigated a method to detect fatigue damage of steels without contact using laser speckle. In the earlier stage of fatigue in steels, slipbands appear on the surface and microscopic shear strain is stored in the slipbands. The slipbands appear more densely with progress of fatigue damage. When a laser illuminates the surface of the fatigued steel, light intensity distribution of the laser speckle pattern formed by the reflected light changes with the change of surface properties caused by slipbands. It has been clarified that the width of the speckle pattern broadens corresponding to spatial frequency distribution of the surface profile and thus it is presumed that speckle pattern broadens with increase of slipband density. This shows that we can detect fatigue damage by observing the laser speckle pattern on material surface. The method presented in this paper is based on this phenomenon. We observed change of the speckle pattern during fatigue loading under constant stress amplitude using a steel specimen and the relation between speckle pattern, number of loading cycles and also magnitude of loading was considered. We investigated the possibility of detection of fatigue damage using this method and also proposed a method to estimate fatigue life by observing change tendency of the speckle pattern depending on the number of loading cycles in the earlier stage of fatigue before crack initiation.     

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.     

A new speckle analysing interferometer for the measurement of double exposure laser speckle records

A new speckle analysing interferometer has been devised for the measurement of double exposure laser speckle records. The advantage is that it displays simultaneously, side-by-side on a screen, the fringe pattern and an incoherent record of the speckle negative image, upon which the actual spot under interrogation is illuminated.The device is simple to operate and inexpensive to manufacture. Reflected and back-scattered components of a laser beam shining obliquely through the negative are collated and passed to a screen. The surface relief effect on the negative is sufficient to provide all the necessary information for a noise-free bright record of the fringes. The actual photographic image on the speckle negative is focussed onto the screen via a projection system, where it can be photographically printed, processed and returned to the screen to form an index to mark the spot at which speckle analysis takes place.     

Speckle-mirror laser

We report on a new laser feedback mechanism that uses speckle-pattern phenomena to create highly dispersive feedback. This speckle mirror is capable of creating laser emission as narrow as that from laser cavities using a number of dispersive elements such as gratings and prisms. Contributions from the peaks of intensity of the speckle patterns and a limited pumping-beam diameter are the basic conditions for the observation of this effect.     

Monitoring foam coarsening using a computer optical mouse as a dynamic laser speckle measurement sensor

In this paper, we present an experimental approach to track coarsening process of foam using a computer optical mouse as a dynamic laser speckle measurement sensor. The dynamics of foam coarsening and rearrangement events cause changes in the intensity of laser speckle backscattered from the foam. A strong negative correlation between the average speed of the cursor and the evolution of bubble diameter was found. We used microscopic images to demonstrate that decrease in speed is related to increase in bubble size. The proposed set-up is not very expensive, is highly portable and can be used in laboratory measurements of dynamics in other kinds of opaque materials.     

Basic physical principles of defocused speckle photography: a tilt topology inspection technique

This paper describes the basic physical principles of defocused speckle photography, and how tilt (differential) topology variations on a scatter surface may be separated from linear displacements. Several rather special features about the scattering process have been revealed, such as the laser source distance from the scatter surface must be considered as well as the light incidence angle. Also, the surface must be treated as mirror facet discontinuities for linear displacements using ray and intensity analogies, and surface topology must be treated in terms of phase and pathlength changes, and a continuous smooth surface. The complementary relationship between focused speckle photography and defocused (topological) speckle photography is shown using specially developed separation techniques. This demanded a detailed look at the ‘speckle process’. The basic physical principles, together with an appropriate analysis technique for the developed speckle photographs, have enabled very small faults and indentations on engineering structures to be detected and measured.