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3. Image Stability

The fiducial framework is a physical part of the camera. Film does not have the dimensional stability of the camera so the medium can expand or shrink with temperature change. A sensor array, being part of the camera, does not have this problem. Whether film or sensor, creating a physical positive image requires another medium. Depending on the medium composition and method of positive image creation, differential expansion or shrinkage may occur.

The only way to determine the amount of expansion or shrinkage is by comparing measurement distances between fiducial marks to calibrated values. Any change represents systematic behavior so a correction can be applied if it is sufficiently significant.

Image coordinates can be corrected by:

a. Applying shrinkage/expansion to measured coordinates

Figure G-7

Measured coordinates are corrected using Equations G-1 and G-2.

Equation G-1
Equation G-2
x'p, y'p : corrected coordinates
xp, yp: measured coordinates
dxc, dyc: calibrated fiducial distances
dxm, dym: measured fiducual distances

b. Measure from fiducial marks

Using the reference frame correction of Figure G-6 will also compensate shrinkage/expansion error.