McMaster1 has quoted derivations showing the optimum magnification required to be used in screen fluoroscopy for a given set of focal spot diameter and screen unsharpness. These formulae show that magnification of over 50 can be afforded using commercial screens and microfocus x-ray unit. This shows that at the lower magnification like 10X Ug term can be neglected. With this substitution, the formulae degenerate to a simple expression and the minium detectable defect can be expressed by

Using the screen RTR set up at our laboratory having low light level camera and microfocus x-ray unit (lesser than 35 micron), unsharpness of two types of fluoroscopy screens was determined with the help of wire-meshes and resolution pattern at different projective magnifications. This gives more accurate value for screen unsharpness. Minimum detectable defects were also determined for different magnifications. 7 lp/mm (67 microns) could be resolved at 10X. Effect of x-ray target material and zooming of image are also discussed in the paper. REFERENCE

1. R. C. McMaster (Ed), "Fluoroscopy and X-ray Image Devices" in Non-destructive Testing Handbook, Ronald Press Company, 1959.