Siemens ARCADIS Varic VB 13C Quick Manual page 58

Postprocessing
Distances cannot be measured directly in
the images, since the position of the object
in the beam path and thus the magnification
effect is not clear. A calibration must be
performed first before a distance can be
clearly determined.
(1)
(2)
(3)
The calibration object must be
perpendicular to the central ray in the area
of the structure to be measured (generally
in the center of the measuring field to avoid
I.I. distortion).
(4)
(5)
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