The Focal Length Meter Digital Measurement Technology Research

The traditional method of measuring focal length exists several shortcomings,such as poor efficiency, unstable precision and high requests for operator. The application of high sensitivity and low noise CCD camera and the development of digital image processing technique make automatic measurement of the optical system focal length digitization become true. Digital image processing technology has been applied to the focal length measurement and made a digitized transformation to the traditional focometer, yield high practical value.Based on the image definition function, an improved square function of the gradient vector is proposed in this paper, which is applicable to both bright background and dark background Porro reticule image proved by experiment. The focusing accuracy reaches0.010mm, which is superior to the visually focusing accuracy-0.051mm under the same conditions.Firstly, the multi-irnage average method for image preprocessing is chosen to eliminate some of the noise in the definite images. Then, using the Canny algorithm and two morphological algorithms respectively to get the edge of the division lines, which is then identified and marked by means of Hough transform, the results show that the expansion-based morphology algorithm is the best. Finally, with the comparison and analysis of several commonly used locating methods, the locating precision of the barycentre method is the highest, thereby obtain the precise spacing of the division line pairs.Compared with the results of visual method, the accuracy, repeatability and practicality has got an obvious improvement.