Calibration is the basis of accurate vision measurement in an imaging system, and its purpose is to establish the mapping relationship between object points in three-dimensional space and image points on the sensing plane. Different from the traditional pinhole projective model, a ray model for the calibration and measurement of imaging system is introduced in this paper. The model assumes that each pixel point of the imaging system in focus state corresponds to a virtual primary ray in space. By determining the parameters of ray equations corresponding to all pixels, calibration and imaging characterization can be achieved, thereby avoiding the structural analysis for complex imaging systems and modeling. Complex imaging systems include special imaging systems such as light field camera, large distortion lens, and telecentric lens. This paper reviews the basic principles of the ray model and the development of its calibration methods, and presents some of the progresses made by our team in the 3D measurement with fringe structured light based on the ray model, showing that the ray model can be used for high accuracy measurement of various complex structural imaging systems. It is an effective model for calibrating non-pinhole projective imaging systems.