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<-> System/camera calibration
E» Image acquisition
i
Detection and extraction of data
—— Analysis of data
«t——», Decisions and actions based on analysis
= P
Self diagnosis
&
quality report
fig1 Basic system design
41 System /Camera Calibration
The calibration of the cameras and systems are vital
parts for the system performance. They may be done
simultaneously in a combined adjustment of the
camera and system calibration parameters or as
separated procedures.
41.1 Camera calibration The calibration of
electronical cameras have been thoroughly
investigated (e.g. Bósemann, 1990). The calibration
must not only take into account the optical system of
the camera but also the electronic parts. The
traditional photogrammetric optical calibration
adjust data to a mathematical model based on
physical assumptions. This leads to e.g. the familiar
polynomial equations for the radial distortion.
Another approach is to calculate the deviations, or
errors, for each pixel. From the deviations a look-up
table is created. This method is not concerned with
the physical background of the errors. It is fast and
easy to implement and especially suited together with
the Direct Linear Transformation, DLT. The two
systems which are not developed by photo-
grammetrists are using this approach (fig 2).
O ms
@ MacReflex
e a L1 MapVision
e O L1 BB TrackEye
1 1 1 2
No
Factory Self
Calibration Calibration Calibration
fig2 Camera Calibration
4.34.2 System calibration The system calibration,
or the outer orientation, is computed using either the
DLT or the bundle adjustment (fig 3). The advantages
of the DLT is, at least initially, its easy imple-
mentation and the simplicity of the outer orien-
tation. The advantages of the bundle adjustment is its
flexibility in the control, e.g. 1-3D points, distances,
plumb lines, and its theoretical superiority and error
propagation capabilities compared to the DLT. For a
more comprehensive study of the differences
between the DLT and the bundle approach see
(Edgardh, 1992).
O Ms
@ MacReflex
B L] MapVision
e O r1 Bg TrackEye
1 1 1 Ba
DLT Bundle with
Adjustment self-calib.
fig3 System Calibration
Comments The camera and system calibration is a
part in the measuring process where the differences
between photogrammetrists and other engineers are
visible.
In the camera calibration both of the systems which
are not developed by photogrammetrists are using a
factory calibration. This is motivated by the stability
of the CCD cameras. Both the system measure on
laser spots or on reflective targets which partly
reduces the need of re-focusing the cameras or
changing the aperture, an argument often brought
against factory calibrations. It may however not only
be a question of precision but also of reliability as one
of the photogrammetrists expressed "...never rely on
a previous calibration. The system should be
calibrated after the installation and it must be fairly
easy to re-calibrate both the interior and exterior
orientation...".
The choice between DLT or bundle adjustment can be
difficult in some applications. An advantage using
the bundle adjustment together with CCD cameras
compared to analogue cameras is the possibility of
making several measurements after each other. Two
systems, #2 @ and #3 Ll utilize this technique to
calibrate the systems. A known distance is moved
around in the measuring volume until a satisfactory
number of observations are made. This is also used
for the self-calibration of one of the systems, #3 Cl
This greatly reduces the problem of calibration of
both the cameras and the system compared to the
traditional test field calibration.
4.2 Image acquisition
In close range applications, the typical image
acquisition part consists of standard video-rate CCD-
cameras. A good reason keeping to this standard is
the large number of fairly cheap electronic
components, ranging from the CCD-cameras over
frame grabbers to hard ware implementations of basic
image processing functions.
