most digital cameras use CCD sensors for capturing the
image instead of the emulsion material, film deformation will
not be present in digital cameras. Nevertheless, there still may
exist a certain deformation for the CCD sensor plane. On the
other hand, the image coordinate determination process is
not error free. The above affinity parameters A and B, are
therefore kept in the calibration procedure in order to
investigate these effects.
Thus, seven parameters, i.e., K,, k,, k;, p,, p, , A and B are
included to model the image distortion, which together with
the basic interior and exterior orientation parameters forms
the functional model of the UNBASC2 self-calibrating bundle
adjustment.
3. CALIBRATION EXPERIMENT WITH THE FUJIX DS-
100 DIGITAL CAMERA
3.1 The Fujix DS-100 Digital Camera
The Fujix DS-100 Digital Camera is an all purpose, portable,
compact digital camera manufactured by Fuji Photo Film with
a slide-in memory card to store images. Its relevant technical
data are as follows:
Table 1. Technical Data for the Fujix DS -100 Digital Camera
CCD array size 720 x 488
Pixel size 9.7 um x 8.5 um
Lens type Autofocus, zoom f=8--24mm
Aperture setting f/2--f/11
Minimum focus distance 30 mm
Shutter speed 1/4--1/750 second
The image data is resampled internally to an array of 640 x
488 pixels of 9.7 um x 9.7 um square size for storage on the
slide-in memory card. The captured images can then be
transferred to a computer for further processing by using a
DP-100 Card Processor available as an option.
3.2 Calibration Experiment
The objective of the designed experiment was to evaluate the
geometric quality of the Fujix DS-100 digital camera using the
existing self-calibration program UNBASC2. Two simple
models were prepared for the calibration purpose. The image
coordinates were determined with the help of a Digital Video
Plotter (DVP) from Leica.
3.2.1 Model Preparation. Two models were prepared for
the experiment. Model | is a metal grid plate with dimensions
of 17 cm x 17 cm, containing 36 intersections engraved on
the plate surface and 25 bolts with different heights (ranging
from 12 mm to 37 mm) fixed perpendicularly to the plate (see
Fig. 1). Model Il is a roof model measuring 95 cm x 63 cm x
25 cm. There are 21 targeted points distributed uniformly
throughout the three-dimensional model space. 17 and 11
fine metal strings are stretched across the model from the
four sides of the model, respectively (see Fig. 2). Thus, 17 x
11 intersections are formed by these strings. This model is
also used as a hanging and standing roof structure for digital
industrial photogrammetry research (Faig et al, 1996). The
62
three dimensional coordinates of all the targeted points in
both models were determined by an Electronic Coordinate
Determination System (ECDS) (Wilkins et al, 1988). The
resulting accuracies are 0.03 mm, 0.04 mm and 0.03 mm in
X, Y and Z directions, respectively.
Fig. 1 Model I for the Calibration Test
Fig. 2 Model Il for the Calibration Test
3.2.2 Digital Image Acquisition. For each model, digital
images were taken directly from three different camera
stations, which form a multi-station convergent imaging
configuration. For the model | case, the digital camera was
fixed at the support frame, while the plate was rotated along
its central axis into three orientations, i.e., backwards, level
and forwards, which is equivalent to the situation that the
model is kept stil and the camera is placed at three
corresponding locations providing convergent coverage. For
model Il, three camera stations were distributed uniformly
around the model while the model was left untouched during
the imaging process.
As the Fujix DS-100 digital camera is equipped with an
automatic zoom lens, the interior orientation will alter with the
focussing process (Faig et al, 1990). It seems therefore
necessary to investigate the performance of the digital
camera for different object distances. Thus, two distinct
distances 0.4 m and 2 m were adopted for models I and II,
respectively.
Several sets of digital images were captured under different
imaging conditions from each camera station to assess the
image appearance. The best one was selected from each set
for the subsequent evaluation. It is well known that the
illumination has an important influence upon the digital
image's quality. However, it is worth to note here that no
special illumination was adopted during the image taking,
only the normal indoor illumination with ceiling fluorescent
light was applied. To avoid shadows on the image, the flash
of the digital camera was disabled when the images were
captured.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
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