基于正弦和三角波条纹投影的三维测量方法

在现有的针对复杂物体表面形貌的三维测量方法中, 为了完成绝对相位的测量, 通常需要处理至少6幅条纹图像, 限制了测量速度。提出了采用2幅正弦条纹和2幅三角波条纹图来获得物体三维形貌的方法。利用两步相移正弦条纹和两步相移三角条纹得到截断相位, 再利用两步相移三角波条纹得到条纹级次, 减少了投影条纹幅数, 提高测量速度。在得到条纹级次时, 计算三角波条纹强度调制和强度对比度, 与计算相位相比, 可以减少数据处理的时间, 进一步提高测量速度, 同时能减小物体表面反射率的影响, 提高了测量精度。测量最大高度为39 mm的阶梯状标准块, 得到的最大绝对误差和最大的RMS误差分别为0.045 mm和0.041 mm。验证了该方法的有效性和实用性, 在高速实时的复杂形貌三维测量中有广泛的应用前景。     

基于二进制条纹加相位编码条纹离焦投影的三维测量方法

复杂表面的精密三维测量在工业无损检测中非常重要。二进制条纹离焦投影方法在快速三维测量中有重要的应用前景,但该方法难以实现复杂表面高精度三维测量。为此,提出了基于二进制条纹加相位编码条纹离焦投影的三维测量方法。由于离焦投影滤除了高次谐波和高频噪声,可以克服投影仪的非线性伽马效应,与传统投影正弦条纹方法相比,提高了其测量精度。针对离焦投影时,随着相位编码条纹频率增大,条纹级次判决困难,出现周期错位,导致相位解包裹出错,提出了相移编码方法来解决以上问题。采用相移编码方法校正周期错位,使条纹级次判决准确,进一步提高其测量精度。实验结果表明,其测量精度可以达到0.044 mm,验证了本方法的有效性和实用性。     

基于二进制编码条纹的三维测量方法

提出一种二进制编码条纹代替正弦条纹的方法,以降低三维测量系统的非线性误差。首先,通过对传统正弦条纹一个周期内呈正弦变化的光强值进行二进制编码,将所有二进制编码的每一位分别组合生成二值条纹,投影到被测物体表面。然后,将采集到的多幅二值条纹进行叠加,生成正弦条纹。最后,结合四步相移技术,并使用互补格雷码辅助相位展开进行实验验证。实验结果表明,所提方法相比于传统四步相移方法可以有效降低非线性伽马效应导致的相位误差,提高测量精度。对直径为50.8140 mm的高精度标准球进行测量,所得的局部点云与拟合标准球的平均距离为0.0697 mm。     

基于系统非线性校正与滤波的相位测量

相位测量系统中投影仪输出与CCD输入变形条纹光强之间的非线性导致频谱混叠,影响测量精度,为了消除他们之间的非线性关系,提高相位测量精度,分析了系统非线性产生的基本原因,提出了一种基于系统非线性校正与滤波的相位测量方法.首先校正由投影仪中伽马畸变产生的系统非线性,然后采用低通滤波器对校正后输出的频谱进行滤波,只保留频谱中的基频成份,最后采用四步相移法对变形条纹进行相位测量.MATLAB仿真与实际实验结果表明,所提方法的系统非线性校正与滤波效果较好,有助于提高相位测量精度.     

基于点阵投影的彩色复合光栅傅里叶变换轮廓术

传统傅里叶变换轮廓术(FTP)采用单一频率条纹测量时存在着由于待测物体高度变化剧烈导致的相位展开困难。结合数字点阵和正弦条纹投影的复合编码三维数字成像方法能有效解决相位模糊和误差传播的问题,但在实际测量中需要分别采集点阵图像和正弦条纹图像,影响了测量的实时性。提出一种新型彩色复合光栅测量方法,实现了从一帧图像中分别得到点阵图像和正弦条纹图像,并以点阵图像为依据对正弦条纹图的截断相位进行展开。理论分析和实验结果表明,该方法适合于不连续物体的三维测量,同时抑制误差传播能力强,具有较高的测量效率。     

基于圆形条纹投影的相位测量轮廓术研究

编码的圆形正弦条纹在圆心具有零相位,该点可以作为后续相位计算和展开的参考.但是圆形条纹提供的载频相位具有非线性特点,当其用于三维面形测量时,如果圆心在物面上,获取物面高度信息的方法与投影单频直条纹的情况不同.研究基于圆形条纹投影的相移轮廓术,讨论了物面高度引起的图像点坐标的位移量和绝对相位的计算方法,并完成相应的误差分...     

利用散斑嵌入条纹和查找表进行三维面形测量

为了快速、准确地重建复杂场景的三维面形,提出了一种利用散斑嵌入相移条纹图案和截断相位-高度查找表的三维面形测量方法。将散斑作为辅助信号嵌入到四步相移条纹图案的相位分布中,得到待投影的复合条纹图案。利用相移算法解调出被测物体的截断相位和额外的散斑信息。利用查找表得到截断相位对应的多个候选高度,再利用散斑对条纹周期进行标记以解决高度歧义问题,唯一确定截断相位所对应的正确高度,以实现被测物体三维面形的快速重建。该方法直接从截断相位信息中重建三维面形,无需投影任何附加图案,也无需进行相位展开。理论分析和实验结果表明,该方法可以实现动、静态复杂场景三维形貌的高精度测量且鲁棒性好。     

高速旋转物体频闪在线相位测量轮廓术

为了实现高速旋转工件的在线三维面形检测,提出一种高速旋转物体频闪在线相位测量轮廓术(PMP)三维测量方法。利用圆形正弦光栅替代线形正弦光栅,采用高密度二元编码光栅替代灰度编码光栅,设计专用频闪同步控制单元同时触发投影系统频闪投射相移光栅和图像采集单元同步获取图像,获得物体在同一"冻结"位置下的相移变形条纹后,采用静态PMP相移算法重建物体的三维面形信息。实验结果表明,该方法具备可行性和实用性,可有效避免工件离线检测时多次装夹不一致引入的加工误差,能够提高工件的检测效率和加工效率,可用于其他高速旋转物体的三维面形重建。     

基于格雷码和相移算法的动态三维测量技术

格雷码辅助实现的相位测量轮廓术具有较好的鲁棒性和抗噪能力,是三维测量领域的一个研究热点。在已有测量方法的基础上,提出了一种把格雷码嵌入相移条纹进行复合图案投影的三维面形测量方法,通过将传统的3幅格雷码图案整体向右移动半个条纹周期,共得到6幅格雷码图,将其交替嵌入三组相同的四步相移条纹中并结合二值离焦技术依次进行投影。通过拍摄得到一系列变形复合条纹图案后,沿时序综合前后三组变形复合图案信息并进行解码,从而获得两组错位的条纹级次,这可从源头上避免格雷码级次边沿的错误;同时采用传统相移算法计算得到对应的截断相位分布,时序上的综合解码结果可用于指导截断相位产生并重建被测物体的三维面形信息。静态和动态场景的三维测量实验结果验证了该方法的可靠性、高效性和鲁棒性。该方法每多记录5幅图案就可以对应新重建出一个三维面形,在高速动态三维测量中具有较好的实用性。     

基于双2+1相移法的高动态范围三维测量

针对基于2+1相移法的高反光表面三维测量,分析了强度饱和条纹图案的傅里叶频谱,引入了强度饱和条纹的三阶傅里叶级数表达形式,建立了强度饱和所致的包裹相位误差模型,提出了双2+1相移法。从精度和效率两方面进行了对比实验：对比传统的2+1相移法和自适应条纹投影的2+1相移法,双2+1相移法的相位误差分别降低了69.9%和65.2%;对比多曝光2+1相移法和自适应条纹投影的2+1相移法,双2+1相移法的测量效率分别提高了91.9%和63.9%。     

傅里叶变换轮廓术中相位失真的预矫正方法

介绍了一种基于傅里叶变换轮廓术的三维面形测量系统中相位失真的预矫正方法。由于投影系统和成像系统的空间三角位置关系、投影仪的发散照明和两套系统蕴含的光学畸变,投影一幅相位与空间坐标成理想线性关系的标准正弦光场,拍摄到的条纹相位和空间坐标不再呈线性分布,引起相位失真,甚至会影响系统测量精度。该方法借鉴反向条纹投影的思想,计算拍摄光场的非线性相位分布与理想的线性相位分布之间的关系,预先矫正,反算出一个新的待投影光场。实验结果表明这种方法能有效地减小该类相位失真所导致的测量误差,获得了更好的测量结果．     

Image fusion by color texture extraction

In this paper, we present a method to extract shape and color information simultaneously of a colorful object by projecting sequentially sinusoidal fringe patterns onto object's surface. Distorted fringe patterns are captured by digital CCD color camera. It is applied the phase shifting method to evaluate the phase of the projected fringes. We obtain both topography details and color texture information.     

A novel fringe adaptation method for digital projector

Aimed to obtain high-quality sinusoidal fringe projection, a new method to correct the output fringe of digital projector is presented. The method is based on the proposed fringe transform model, which describes the relationship of the input and output fringe pattern. Firstly, a series of fringe patterns are projected and from the fringe images, the transform function is calculated by the pattern shifting method. At last, by modifying the input fringe pattern, a standard sinusoidal output fringe can be achieved. Different from the previous methods, the waveform nonlinearity is estimated by varying the intensity of the projected fringe pixel by pixel; thus the waveform nonlinearity can be estimated precisely and the time cost is considerably reduced. Experimental results show that by modification of the input projection patterns the projector can project fringe with high-quality sinusoidal waveform leading to high performance of the projection system.     

基于傅里叶频谱分析的相位测量轮廓术系统Gamma非线性校正方法

数字投影和成像系统的Gamma非线性效应是导致相位测量轮廓术（PMP）测量误差的重要原因。目前大多采用多帧条纹图进行Gamma校正,使测量的实时性受到限制。提出了一种Gamma校正方法,利用正交光栅像的傅里叶频谱分布计算Gamma值,再根据此Gamma值对投影相移条纹进行Gamma逆变换,实现投影条纹输入值的提前校正,以获取具有良好正弦性的结构条纹,从而降低PMP相位测量误差。校正过程中只需一帧条纹图,而且考虑了测量系统的离焦效应。实验证实了该方法的有效性和正确性。     

光学三维扫描仪光强传递函数的测量和校正

由于数字光栅投影仪的光强传递函数对于正弦投影条纹的质量以及相位测量精度起着至关重要的作用,本文提出了一种校正光学三维扫描仪光强传递函数的新方法。首先,分析了由于投影仪非线性响应引起的光栅谐波的相位测量误差;然后,通过投影一组不同灰度级的图像,并利用光功率计测出数字投影仪投出图像的亮度。接着,通过分析得到数字投影仪的非线性响应特性曲线,再经过数据处理,即可获得投影仪的光强传递函数;最后,对光强传递函数进行反函数逆变换,得到一个校正后的非正弦光栅,利用投影仪对该光栅的投影即可在被测物体表面上获得一个正弦光栅。数字投影仪对标准平板的测量结果表明,校正前平均误差为0.71 mm,校正后为0.55 mm;对于标准量块的测量,校正前的平均误差为0.62 mm,校正后为0.15 mm。上述结果表明,本文提出的方法可以减小由于系统非线性响应引起的测量误差并提高测量精度。     

An improved stair phase encoding method for absolute phase retrieval

An improved phase unwrapping method is proposed to reduce the projection fringes in three-dimensional (3D) surface measurement. Color fringe patterns are generated by encoding with sinusoidal fringe and stair phase fringe patterns in red and blue channels. These color fringe patterns are projected onto the tested objects and then captured by a color CCD camera. The recorded fringe patterns are separated into their RGB components. Two groups of four-step phase-shifting fringe patterns are obtained. One group of the stripes are four sinusoidal patterns, which are used to determine the wrapped phase. The other group of stripes are four sinusoidal patterns with the codeword embedded into stair phase, whose stair changes are perfectly aligned with the 2π discontinuities of sinusoidal fringe phase, which are used to determine the fringe order for the phase unwrapping. The experimental results are analyzed and compared with those of the method in Zheng and Da (2012. Opt Express 20(22):24139–24150). The results show that the proposed method needs only four fringe patterns while having less error. It can effectively reduce the number of projection fringes and improve the measuring speed.     

A composite-structured-light 3D measurement method based on fringe parameter calibration

This paper proposes a novel method for reducing measurement error caused by spectrum overlapping in composite-structured-light 3D measurement systems. For a composite-structured-light 3D measurement system, spectrum overlapping causes parameters of each deformed phase-shifting fringe to change, and therefore leads to phase measurement errors. The proposed fringe parameter calibration method is based on the fact that variations in each deformed fringe's parameters are independent of height and reflectivity of the measured object. Three frames of composite grating are projected on the reference plane, and each carrier channel includes the information of three phase-shifting sinusoidal gratings used in Phase Measuring Profilometry (PMP). With the parameter calculation formulas of PMP, the parameters of fringes demodulated from the same carrier channel can be calculated, and therefore parameter relation coefficients between fringes demodulated from different carrier channels may be obtained. When an object is measured, these relation coefficients can be used to calibrate the parameters of the deformed phase-shifting fringes. A new 3D measurement mathematical model is established to reconstruct the shape of the object. Experimental data proved that the proposed method can effectively restrain the effect of spectrum overlapping and improve measurement accuracy by more than three times.     

Optimum projection pattern generation for grey-level coded structured light illumination systems

Structured light illumination (SLI) systems are well-established optical inspection techniques for noncontact 3D surface measurements. A common technique is multi-frequency sinusoidal SLI that obtains the phase map at various fringe periods in order to estimate the absolute phase, and hence, the 3D surface information. Nevertheless, multi-frequency SLI systems employ multiple measurement planes (e.g. four phase shifted frames) to obtain the phase at a given fringe period. It is therefore an age old challenge to obtain the absolute surface information using fewer measurement frames. Grey level (GL) coding techniques have been developed as an attempt to reduce the number of planes needed, because a spatio-temporal GL sequence employing p discrete grey-levels and m frames has the potential to unwrap up to p^m fringes. Nevertheless, one major disadvantage of GL based SLI techniques is that there are often errors near the border of each stripe, because an ideal stepwise intensity change cannot be measured. If the step-change in intensity is a single discrete grey-level unit, this problem can usually be overcome by applying an appropriate threshold. However, severe errors occur if the intensity change at the border of the stripe exceeds several discrete grey-level units. In this work, an optimum GL based technique is presented that generates a series of projection patterns with a minimal gradient in the intensity. It is shown that when using this technique, the errors near the border of the stripes can be significantly reduced. This improvement is achieved with the choice generated patterns, and does not involve additional hardware or special post-processing techniques. The performance of that method is validated using both simulations and experiments. The reported technique is generic, works with an arbitrary number of frames, and can employ an arbitrary number of grey-levels.