Central positions of light stripe are important for 3D surface measurement based on light-sectioning method. This paper presents a method to extract the central positions of light stripe in sub-pixel. The gray level of image is considered as a 2D function of the coordinates. The directional derivative and the gradient of the image of light stripe are calculated

and the pixel gradient represents the direction of the change in gray level

which is assumed as the normal direction of the light stripe curvilinear. Pixels at the positions of light stripe can be extracted by setting a threshold for both absolute value of the gradient and the gray level. The sub-pixel central positions of the light stripe are equal to the mean value of the gray-level-weighed positions of the pixels in the stripe’s adjacent region along the gradient direction. With this method

the normal direction of the stripe can be simplified to be the gray level gradient

avoiding thus complicated steps to calculate a Hessian matrix to get normal directions

and ensuring enough accuracy with less computation.