Role of Optical Transfer Function for Optimum Magnification in Enlargement Radiography

The over-all OTF in enlargement radiography is composed of the OTFs of the focal spot of the X-ray tube and the imaging device. The general equation giving an optimum condition which leads to the maximum over-all OTF multiplied by two elements is obtained by applying a simplified model of the system. The OTFs of the focal spot of the X-ray tube and the image intensifier are approximated by Gaussian distributions, and the OTF of the screen-film system by an exponential distribution. The over-all OTF curves are calculated by using these approximated distributions in relation to the speed and magnification of the system. The Gaussian-Gaussian approximation has a sharp optimum magnification that depends on the spatial frequecy. An optimum-free magnification system is shown to the exponential-exponential system.