A Slide Rule for Calculating the Size of an Object from the Size of its Shadow on an X-ray Film
G. S. Innes · British Journal of Radiology · 1953
Fig. 1 shows a slide rule designed to give the true size of any object visible on an X-ray film taken at an F.F.D. of 33 in. and a film to table top distance of 1·5 in., providing the distance of the object above the table top is known and the tube is over the table. The rule is shown set ready to calculate the true size represented by a 15·5 cm shadow on a film. To do this the adjustable central slide has been set with its arrow opposite the figure 15·5 on the bottom scale.