Research on the influence of light intensity on digital image correlation measurement accuracy in the testing of concrete
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摘要: 在复杂光照条件下采用数字图像相关技术(DIC)测量混凝土结构表面应变场时，光强变化会引起图像的退相关效应。针对此问题，基于传统2D-DIC测量系统，探究了光强变化对DIC测量精度与计算效率的影响规律。试验结果表明：平均灰度190为最优光强；以最优光强下的图片作为参考图像，光强增大时应变误差呈指数增长，光强减小时应变误差呈线性增长；计算时间与计算点匹配率对光强的小范围波动不敏感，但光强的剧烈变化会导致计算时间增加1~3倍以及25 % 的计算点匹配失败。结合图像质量评估指标，验证了实验结果的准确性。为了获得高质量的图像，本文建立了绿光带通滤波成像系统，该系统在外界环境光剧烈变化条件下依然能够保持图像的亮度与对比度基本不变，应变误差与计算效率均接近于最优光强。相对于传统2D-DIC测量系统，绿光带通滤波成像系统可较好地应用到室外环境测量。
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关键词:
- 数字图像相关技术 /
- 混凝土表面应变场 /
- 光强 /
- 图像质量 /
- 绿光带通滤波成像系统
Abstract: When the surface strain field of concrete structure is measured by digital image correlation(DIC)technology under complex light conditions, the decorrelation effect of the pattern will be caused by the variation in light intensity. For this problem, the optimal light intensity suitable for DIC calculating and analyzing is studied, and the influence of light intensity variation on the measurement accuracy and calculation efficiency of DIC is explored by the traditional 2D-DIC measurement system. The experimental result shows that the average gray level of 190 is the optimal light intensity. The picture is used as the reference image at the optimal light intensity. The strain error increases exponentially when the light intensity increases, and the strain error increases linearly when the light intensity decreases. The computation time and the matching rate of calculation points are insensitive to the slight range fluctuation of light intensity, while the computation time of DIC will increase 1 to 3 times, and about 25 % of calculation points will match failed because of the extreme variation of light intensity. The accuracy of experimental results is verified by Combining them with the image quality assessment index. In order to get high-quality images, the green band-pass filter imaging system is established in this paper. It can stabilize the image's brightness and contrast when the external light varies seriously. The strain error and calculation efficiency are all close to the optimal light intensity when external ambient light changes seriously. Compared with the traditional 2D-DIC measurement system, The green band-pass filter imaging system can be better applied to measure in the outdoor environment. -
图 4 基于平均灰度的光强参数表征流程
Figure 4. The flow of light intensity parameter representation based on average gray
图 5 不同光强下x方向的应变误差云图
Figure 5. The strain error nephogram in x direction with different light intensity
图 6 不同光强下x、y、xy方向上的应变测量误差与计算效率
Figure 6. The strain measuring error in x, y and xy direction and calculation efficiency with different light intensity
图 7 光强剧烈波动时x方向的应变误差云图
Figure 7. Strain error nephogram in x direction when light intensity fluctuates violently
图 8 光强波动时x、y、xy方向上的应变测量误差与计算效率
Figure 8. The strain measuring error in x, y and xy direction and calculation efficiency of light intensity fluctuation
图 9 平均灰度梯度随光强变化的曲线
Figure 9. The curve of average gray gradient varying with different light intensity
图 11 光学带通滤波片的透射光谱曲线及实物
Figure 11. Transmission spectrum curve and picture of the optical band-pass filter
图 12 Prosilica GT3400相机量子效率曲线
Figure 12. Quantum efficiency curve of Prosilica GT3400 camera
图 14 光强变化时传统2D-DIC测量系统与绿光带通滤波成像系统x方向的应变误差云图
Figure 14. Strain error nephogram in x direction between traditional 2D-DIC measurement system and green band-pass filter imaging system when light intensity changes
表 1 光强变化时传统2D-DIC测量系统与绿光带通滤波成像系统应变场测量误差
Table 1. Comparison of strain measuring error between traditional 2D-DIC measurement system and green band-pass filter imaging system when light intensity changes
应变测量误差 低照度 最优光强 高照度 传统2D-DIC测量系统 绿光带通滤波成像系统 传统2D-DIC测量系统 绿光带通滤波成像系统 传统2D-DIC测量系统 绿光带通滤波成像系统 x方向应变测量误差/ 10-6 66 63 25 45 58 993 79 y方向应变测量误差/ 10-6 56 44 22 21 57 894 68 xy方向应变测量误差/ 10-6 44 44 19 21 23 583 49 
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表 2 光强变化时传统2D-DIC测量系统与绿光带通滤波成像系统所采集的图像质量
Table 2. Comparison of the quality of the pattern collected by traditional 2D-DIC measurement system and the green band-pass filter imaging system when light intensity changes
图像质量 低照度 最优光强 高照度 传统2D-DIC测量系统 绿光带通滤波成像系统 传统2D-DIC测量系统 绿光带通滤波成像系统 传统2D-DIC测量系统 绿光带通滤波成像系统 平均灰度 20.89 173.59 190.41 192.77 254.78 208.59 平均灰度梯度 3.21 39.48 27.76 39.85 0.3 37.42
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表 3 光强变化时传统2D-DIC测量系统与绿光带通滤波成像系统的计算效率
Table 3. Comparison of the efficiency of calculation between traditional 2D-DIC measurement system and green band-pass filter imaging system when light intensity changes
计算效率 低照度 最优光强 高照度 传统2D-DIC测量系统 绿光带通滤波成像系统 传统2D-DIC测量系统 绿光带通滤波成像系统 传统2D-DIC测量系统 绿光带通滤波成像系统 计算时间/ s 0.297 0.125 0.124 0.125 0.488 0.126 计算点匹配率/% 100 100 100 100 75 100
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