Praca Dyplomowa - Photogrammetry and Remote Sensing - AGH

THEORETICAL BACKGROUND
To make camera calibration, pictures of special calibration chart should be taken with fixed
settings: fixed focal length, focus, principle distance, aperture; and it is more secure to at least
fix the lens with a tape to prevent eventual moving. They should be taken also on the same
day with the same conditions, e.g. temperature and humidity. The test field may differ. Most
of software companies developed their own calibration pattern; however it is usually possible
to create a new one. Using pattern that the company recommends to use – calibration
computation is automatic, otherwise mainly all the points have to be measured manually or at
least semi-automatically. Every software has his own recommendations how to take pictures
(from which angles etc.) and how many. It is also possible to make self – calibration on the
field, but the results may not be as accurate as using test field. Without camera calibration a
slightly different equations are used and it is very hard to find the error when it occurs.
No matter what kind of software is used the same features, listed below are computed. The
algorithms may be slightly different but the principle is the same – to get the most accurate
results – with Least Square Method (LSM).
Perspective centre and distortion
Mathematically, the perspective centre is defined by the point of central perspective that is the
point through which all straight lines from all image rays pass. Perspective centre depends on
chosen aperture. This point may be defined both external and internal of objective. In the ideal
case these points are equal. Similarly in ideal case, principle distance c is equal to the image
distance a’, still there is always some error. What is more, lens distortions, which will always
occur, also change interior geometry. Geometric basics of perspective centre and distance
shows Figure 2.2.The shift of the point on the image may be computed:
Where:
τ – angle of incidence
τ’- exit angle
r’ – image radius of an image point
∆r‟- radial shift due to distortion
Figure 2.3 Perspective centre and principle distance (Luhmann et al., 2006)
12
(2.2)