This paper discusses a physics-based imaging model of scanning electron microscopes (SEM). The purpose is to accurately examine the imaging process of a SEM, which has to be necessary to realize novel applications of the SEM images, such as 3D shape reconstruction from image brightness. Its brightness is determined by the total energy of the secondary electrons derived by the incidence of accelerated electron beam to a surface point and then captured by the SEM detector. There are several simple imaging models. But, they are pointed out that the actual brightness cannot be dealt with. We then develop a better imaging model that precisely describes the physical process of the emergence of the secondary electron, their reflections and detections.