显微镜非均匀光照对煤岩显微图像的影响及校正方法

Influence of uneven lighting of microscope on petrographic images and its calibration methods

镜质体反射率自动测试及煤岩显微组分智能识别需结合图像分析和深度学习算法,而图像分析一般在较大视域下采集图像,以获得全面的图像信息。利用蓝宝石(L)、钇铝石榴石(YAG)和钆镓石榴石(GGG)3个光学各向同性的标准物质,研究煤岩显微镜光照对大视域下采集显微图像特征的影响,分别假设显微镜光源为点光源和面光源对其进行理论分析,采用基于低通滤波和顶帽变换的数字图像处理技术对非均匀光照产生的影响进行校正,并通过实际样品测试结果进行验证。结果表明:蓝宝石、钇铝石榴石、钆镓石榴石3个标样图像的灰度(反射光强)近似为以图像中心点为圆心的同心圆分布,灰度从图像中心到边缘逐渐降低;图像中心区域的灰度梯度降低较小,而远离图像中心位置的灰度梯度降低急剧增大;标样灰度分布特征随显微镜光轴中心变化而发生改变。理论分析表明,无论煤岩显微镜光源为点光源还是面光源,均会产生非均匀光照现象,煤岩光片表面的反射光被显微相机采集后形成图像阴影。使用低通滤波和顶帽变换均可校正非均匀光照形成的图像阴影,校正后的标样图像灰度均值大幅提高,标准差显著降低,1/4分位~3/4分位的灰度差可控制在2个灰度以内;采用顶帽变换校正后的标样图像灰度标准差和灰度均匀性均低于低通滤波,而细节分辨率高于后者。单煤和混煤样品的测试结果表明,使用低通滤波和顶帽变换均可校正图像,采用顶帽变换的效果优于低通滤波。

The combination of digital image analysis and deep learning algorithm is essential for automatic determination of vitrinite reflectance and intelligent recognition of coal macerals.Generally speaking,the images are collected in a large field of view to obtain comprehensive information.Three optically isotropic standard samples,i.e.,sapphire,yttrium aluminum garnet(YAG)and gadolinium gallium garnet(GGG),are selected to study the influence of uneven lighting of microscope on the characteristics of petrographic images,as the images of standard samples are collected in large view of field.Theoretical analyses are conducted as the illumination of petrographic microscope are point light and area light.Finally,the digital image processing technologies based on lowpass filter and top-hat transformation are used to calibrate the uneven-lighted images.The coal samples are used to verify the calibration effect.The distribution of gray level(reflection intensity)in sapphire,yttrium aluminum garnet(YAG)and gadolinium gallium garnet(GGG)are similar to concentric circles as the center point is located in the center of the image.The gray value decreases from the center to the edge of the image.The gradient of gray value in the center of the image decreases slightly,while that of the point far away from the center decreases dramatically.The distribution of gray value in the standard samples changed with the optical axis of the microscope.The uneven lighting is sure to occur whether the light source of the petrographic microscope is point light or area light according to the theoretical analysis.The reflected light on the surface of blocks is captured by the CCD camera and some shadows will be formed in the digital image.The shadow image caused by uneven lighting can be calibrated by lowpass filter and top-hat transform.The mean gray value of the calibrated standard image is greatly improved while the standard deviation is significantly reduced.The gap between the first and third quartile is within 2 gray value.The standard deviation and uniformity of the gray value in the standard images after top-hat transform are lower than those of lowpass filter,while the resolution capability of pixel difference after top-hat transform is better than that of lowpass filter.The petrographic images can be calibrated by both lowpass filter and top-hat transform as shown by the results of single coal and blending coal.The top-hat transform is preferred in petrographic image calibration.