The characters and factors were analyzed when the laser probe measurement was used in inside screw surface. According to the noise distribution of the laser speckle on the measured plane

a synthesis filter algorithm combined the median filter and the wavelet threshold denoising method was designed to eliminate noise of the image

in which median filter can effectively wipe off the impulse noise in the speckle image

and the threshold denoising algorithm based on wavelet transform can eliminate Gaussian noise of the image. The algorithm is able to remove the noise influence on measurement accuracy and maintain the image individual character. According to the intensity distribution of the laser speckle image on the declining object surface

the circle-fitting algorithm was proposes to overcome the affect of the laser speckle center shift caused by the slope surface to locate the center of the laser speckle image at the sub-pixel level. The algorithm has the features of high efficiency

less calculation compared with the traditional ones and is more suitable for testing on line. Laser probe scanning measurement base on coordinate machine using the above techniques was designed to measure an internal screw standard gauge. The results show that the internal screw-pitch measurement accuracy

pitch diameter of thread and the thread angle accuracy increase 0.48 mm

0.44 mm and 0.57°

respectively.