# X-Ray versus Light Field centres difference

In the example figures above, the witdh is vertical because the test object is rotated by 90°

From the positions of the two fields (X-Ray and Light) the differences of centres on width and height directions are calculated. The difference has a sign: positive values mean that the X-Ray field centre is on the right/lower than the light field centre.

Centres difference on width = :CWXCWL$CW_X - CW_L$

Centres difference on height = :CHXCHL$CH_X - CH_L$

where:

CWX = Centre on width of the X-Ray field
CWL = Centre on width of the light field
CHX = Centre on height of the X-Ray field
CHL = Centre on height of the light field