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There is a debate about whether the gray level of coaxial keyhole image reflects keyhole depth and weld penetration or not in the coaxial vision sensing of laser welding.For continuous wave Nd:YAG laser deep penetration welding,the coaxial vision monitoring system was founded with industrial standard CCD camera by selecting the proper filter to impress the interference of laser induced plasma,and the keyhole coaxial image was acquired and analyzed.The keyhole in acquired image has two edges,which indicates the radial shape and size of keyhole upside and bottom,respectively.The characteristic parameters of keyhole were defined and obtained by data fitting after smoothing filter,image enhancement and extraction of keyhole edges.The analyzed results showed that the gray level of keyhole coaxial image in these experiments can indicate the keyhole radial size and shape rather than its depth.The results also show that the radial keyhole size is larger than that of laser spot at different laser power and welding speed,and the inclination of the keyhole in vertical direction decreases as laser power increases and welding speed decreases.