Product inspection of industrial parts is the most important link to ensure that the quality of the parts is qualified. The traditional contact detection method is difficult to meet the requirements of high efficiency and high precision in the industrial field. The image measurement system based on machine vision has been widely used to detect the geometric parameters of the product. A lot of work has been done on the measurement of high-contrast images formed by non-transparent objects, and relatively little researches have been done on low-contrast images formed by transparent objects. Right-angle prisms were used as the object, and an image measurement system for measuring the size of low-contrast products was developed. First, the images collected under different light intensities are averaged to obtain a smooth image. Then the contrast-limited histogram equalization algorithm is used to enhance the contrast of the image and the Zernike moment edge detection algorithm is used to determine the precise sub-pixel edge. Several comparative experiments have verified the rationality and superiority of the improved algorithm. The average error of prism thickness is less than 0.003mm, and the standard deviation is less than 0.0015mm. The solution proposed provides a feasible solution for the high-precision measurement of relatively transparent objects.