Abstract: In a radiographic imaging system, the created light along the thickness direction of the scintillator is focused by a lens to form a blurred image, which is a limiting factor for the spatial resolution of the imaging system. Geometric optics models have been developed to describe the imaging process. To parameterize the spatial resolution, the standard deviation of the blurred image has been expressed by the parameters of the imaging system. A method is found to optimize the position of a thick scintillator with respect to the object focal plane of the lens, in which the deviation of the blurred image will be minimized.