The imaging principle and coordinate system construction of optical image measuring instruments are the core foundation for achieving high-precision measurements.

In terms of imaging principles, optical image measuring instruments are based on machine vision technology, which projects the image of the measured object onto a CCD sensor through an optical lens to form a digital image. This process involves optical magnification, using an optical microscope to perform high magnification optical magnification imaging of the object under test, allowing tiny details to be clearly presented. Subsequently, the CCD camera system sends the magnified image of the object to the computer, which processes the image through specialized measurement software, including image enhancement, filtering, edge detection, segmentation, and other operations, in order to extract the feature element information of the object, such as edges, contours, etc.

The construction of coordinate system is to accurately describe the position and orientation of the measured object in three-dimensional space. Optical image measuring instruments typically involve four coordinate systems: world coordinate system, camera coordinate system, imaging plane coordinate system, and computer image coordinate system. The world coordinate system is a real reference coordinate system that can be defined by users and set at any position in space. The camera coordinate system takes the focal center of the pinhole camera model as the origin, with the z-axis coinciding with the camera optical axis, and the X and Y axes parallel to the horizontal and vertical directions of the CCD imaging unit arrangement, respectively. In the imaging plane coordinate system, f is the focal length of the lens, the plane is perpendicular to the lens optical axis, and the x and y axes are parallel to the X and y axes of the camera coordinate system, respectively. The computer image coordinate system is a two-dimensional coordinate system in pixels in a computer, with the origin located in the upper left corner of the image and the u and v axes parallel to the X and Y axes of the image plane coordinate system, respectively. By establishing these coordinate systems and determining their conversion relationships, optical imaging measuring instruments can achieve accurate measurement of parameters such as size, shape, and position of the measured object.