ide it possible 
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the calibration 
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to explain DC- 
pability and the 
In this test we 
1aeological ruin. 
LIBRATION 
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nage for digital 
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the system for 
DI-1000 in our 
  
    
Figure 2. Camera network 
To operate the DC-1000 system, we first print out a flat sheet 
(Figure 2) by a printer or plotter or display the same target 
pattern on the Liquid Crystal Display (LCD monitor: Figure 3). 
But if we use LCD monitor we can even work out the 
calibration single-handedly with only a digital camera and a 
notebook computer at hand. 
For the present experimentation, we used a digital camera of 
Minolta Dimage7, which has 4,950,000 pixels as in Figure 4 
and its specifications as in Table 1. This time since we used a 
digital camera with wide angle, we worked on a flat sheet of A1 
size. But with the recent development of larger screen of LCD 
monitor, such restriction will soon be gone. 
  
Figure 4. Dimage7 
Figure 3. Target on LCD monitor 
  
  
  
  
  
CCD 2/3-type interline primary-color CCD 
Number of pixels 4.95 million pixels (2568x1928) 
Lens construction 16 elements in 13 groups; include 2 AD 
glass elements and 2 aspheric elements 
Maximum aperture f/2.8 — f/3.5 
  
  
  
  
Focal length 7.2mm ~ 50.8mm (equivalent to 28- 
200mm in 35 mm format) ~ 
Price About $1,200 ~ 
  
  
Table 1. Dimage7 Specifications 
2.1 Analysis by Flat Sheet 
When we make calibration not by the 3D Target-field measured 
with precision but by a flat sheet, we can obtain the lens- 
distortion value without difficulty but not the precise focal 
length necessary for 3D measurement. 
Tan C (Central point) AD C BE 
i 
n Camera distance iu 
sf. 
= 
: 
comes position 2 
(a) (b) (c) (d) 
  
Figure 5. Flat Sheet as being photographed 
So, as explained below, we take picture of the sheet from 5 
different positions, each with the difference of the angle of 15 
degree. 
As there are 5 points each highlighted by a square (or circle or 
anything) on a sheet, first we take a picture of the sheet in such 
a way that the entire sheet would just fill the entire screen of the 
camera with the central point right at the center of the screen 
(Figure 5(a)). 
Hereby the distance between camera and object (hereafter: 
camera distance) H is determined in function with the focal 
length. 
Second, we take a picture of each of the other four points in 
each corner in the following way. 
Suppose we choose the point *A" in upper left corner of the 
sheet. Keeping the same camera distance H vis-a-vis the flat 
sheet (or on the imaginary plane extending parallel to the sheet), 
we move the camera from the position at which we have taken 
the first picture towards the point *A" about 1/3 of the camera 
distance “H”, if the H is more than 1m (Figure 5(b)). If the H is 
less than 1m, we move the camera from the first position (which 
is right above “C”) towards “A” till it gets right above the point 
«AT. 
Next aiming at "A" right in the center of the screen, and 
keeping the camera at the same position, we turn the camera 
towards "C" (Figure 5(c)) and now put *C" in the center. Then 
always aiming at "C" right in the center of the screen, we 
approach the camera towards the sheet until the entire flat sheet 
would just fill.the entire screen of the camera (Figure 5(d)). 
And click the shutter. 
We do the same procedure for “B”, “D” and *E". 
On this sheet there are 145 control points. We can calculate the 
position of all of them through the projective transformation 
from the highlighted points (A, B, C, D, E). 
And then we can obtain the position of all control points with 
sub-pixel precision through automatic measurement. 
This enables us to establish the camera's interior orientation 
parameters (principal point, focal length and lens distortion) by 
self-calibrating bundle adjustment. This even enables us to 
visualize the distortion before and after the correction on the 
matrix of correction and ultimate ideal form. 
—55—