A method for on-line rapid determination of the diameter of metallic cylinder is introduced in this paper. Under the
radiation of diffuse light, there is a bright area close to the margin of metallic cylinder, and the method of this paper is
based on the intensity distribution of the bright area. In this paper, with the radiation by a diffuse plane light with special
shape, we present the relation expression of the distance between the peak point and the real edge of the cylinder and the
distance between the diffuse light and the pinhole aperture of the camera. With the expression, the diameter of the
cylinder to be measured can be calculated. In the experiments, monochromatic LED uniting with ground glass forms the
diffuse light source, then the light irradiates the tested cylinder. After the cylinder, we use a lens with a front pinhole stop
to choose the light into CMOS, then a computer is used to analyze images and export the measurement results. The
measuring system using this method is very easily implemented, so it can realize the on-line rapid measurement.
Experimental results are presented for six metallic cylinders with the diameter in 5~18mm range and roughness in Ra-
0.02um, and the precision reaches 3um.