where R is the radius of the input phosphor screen curvature, and da is the d0 distance projected onto the input screen at a 0 angle.
Note that, because of the manufacturing process, there is no one single radius of curvature for II tubes, yet for simplicity we consider only a single radius.
The X-ray beam may also fall at an arbitrary angle to an object. This leads to a generalization of the equations just derived. From the geometry of Fig. 13,
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