CCD camera 
   
     
   
    
      
   
  
  
  
diffuse diffuse 
illumination fibre optic illumination 
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Figure 5 Image acquisition arrangement for testing 
retro-reflective B 
targets Eu 
  
  
  
  
     
   
8 x zoom of edge 
Figure 6 CCD frame with knife edge 
4.2 Illumination 
The chosen lighting conditions have a decisive influence 
on the image quality. An optimal brightness and contrast 
can only be reached by a good illumination. Bad lighting 
can fade features into the background, make edges fuzzy 
and produce pseudo edges by shadows. The lighting 
depends on the surface shape, surface texture, surface 
reflectance and surface colour of the object, furthermore 
on the physical constraints imposed by the manufacturing 
environment, the light level and the light character (e.g. 
polarised light). The problems of surface texture, 
reflectance and colour can be avoided by a special 
surface treatment. The knife edge has a dazzle-free edge 
and the nozzle a special surface treatment. 
In the accuracy test there are two objects to be measured: 
the object and the targets of the calibration field. The 
illumination must be chosen such that both are well 
illuminated. To reach this aim a different illumination for 
each of the two objects has been used. The retro- 
reflective targets need only a weak illumination. They are 
illuminated with a specially designed fibre-optic lighting 
system arranged around the camera lens. The fibre-optic 
system is adjustable and thus an optimal target 
illumination can be selected. 
The use of diffuse lighting allows a uniform illumination 
of the object and produces little shadows and reflections. 
The diffuse lighting is achieved by a white curtain around 
the whole set-up and an illumination with high frequency 
fluorescent tubes. 
43 TEST RESULTS 
All tasks of image acquisition, target location, bundle 
block adjustment with sclf calibration and the edge 
matching were performed with the program DEDIP.