308 
upper jaw 
  
        
    
  
Camera E 
E T 
A A e ER 
I, (T 
     
         
   
upper jaw 
Camera 
EE a 
lower jaw 
Fig. 5: Image acquisition for upper jaw Fig. 6: Image acquisition for lower jaw 
On the backside of the mirror 11 crosses are engraved, so that they are visible from the front. The crosses cannot be engraved on 
the frontside, since they would be mirrored themselves and would appear twice in the images. They are used as control points for 
the relative and absolute orientation of the images. Therefore they have a special arrangement, so that crosses are not superimposed 
on the teeth in the images. The reference-coordinates of the crosses were determined with a mono-comperator with an accuracy 
better than 20 microns. This corresponds to 1/15th of a pixel in image space and seems to be sufficient. Unflatness of the mirror is 
not taken into consideration at the moment. Systematic error due to unflatness of the mirror have to be investigated in the future. 
The following figure 8 shows the mirror and the arrangement of the crosses. Figure 7 shows the camera arrangement for image 
acquisition seen from above. 
  
+ + + +) 
mirror with teeth = + 
crosses 
+ 
  
  
> ~ 100 mm — € + + 
Camera I +II —ey— 
Fig. 7: Camera arrangement seen from above. Fig.8: Mirror with crosses 
Until now images were taken from a dental test model and from living test objects. Images from the model were digitized directly 
with the frame grabber of a standard video board. Images from living test objects were intermediately recorded on a VHS 
recorder. These images were used to investigate the feasibility of taking images with the mirror directly from the human mouth. 
Figure 9 and 10 show the camera arrangement during the image acquisition process in reality. Figure 11 and 12 show the 
corresponding images that were taken with the cameras. No crosses are visibles since a different mirror was used. These images 
were not used for measurement purposes. Both cameras are placed in front of the mouth acquiring the images reflected by the 
mirror. The lips are stretched using special equipment, so that the mouth can be opened widely while images are acquired. 
  
Fig. 9 and Fig. 10: Camera arrangement during image acquisition. 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995