The tool is placed on a rotating table so that by turning 
the cutting edges can be put in a position parallel to the 
reseau grids and the cameras. Otherwise some edges could 
be hidden by the tool. As measurements with an accuracy 
of a few microns in the object space are influenced by 
a variation in temperature a device for the measuring of 
the temperature with six single sensors was also installed. 
Fig.2 shows the laboratory layout (without the PC). 
  
Fig.2: photograph of the laboratory layout 
The arrangement of the beam-splitters and the course of 
the light rays is explained in Fig.5 and Fig.6. 
3. Orientation of the Cameras 
The layout shown in Fig.2 was mainly chosen because of 
the very small angular aperture of the CCD-cameras. The 
combination of a 1/2" -CCD-array with a principal distance 
of 120 mm gives an angular aperture of only 4 gons. Under 
these conditions the orientation parameters are highly cor- 
related and the orientation is unstable. So the original idea 
of placing the cameras between the tool and the reseau 
grid as shown in Fig.3 had to be rejected. 
  
  
tool cameras reseau 
Fig.3 : rejected layout 
The position of the cameras can only be computed with 
an accuracy of a few millimeters which does not matter, if 
the unknown object points lie in the object space surroun- 
ded by control points. But if there is an extrapolation like 
in Fig. 3 the accuracy of the projection centres is essen- 
tial and the computation of the object point coordinates 
difficult. 
  
      
   
   
    
  
   
   
  
   
    
  
  
  
  
  
  
  
   
   
    
   
   
  
  
  
  
   
    
Therefore several different configurations were tested. Two 
of them are presented with theoretical and empirical re- 
sults. 
In order to render the orientation parameters more sta- 
ble, the angular aperture of the cameras was artificially 
widened by adding another beam splitter to each camera 
pointing to another reseau grid. Fig. 4 and Fig. 5 show 
these layouts, in one case the tool opposite to the reseau 
grids and in the other case the tool between them. Fig. 5 
corresponds to the finally chosen layout of Fig. 2. 
  
  
  
  
  
tool cameras reseau grids 
Fig.4 : layout 1 
  
  
  
1 
reseau grids + tool 
  
  
cameras 
Fig.5 : layout 2