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Any SEM data can be processed rigorously provided that central projection geometry 
is applied. Depending on image magnification and required accuracy, it may be 
treated as an approximate parallel projection. The higher the magnification the 
more the geometry tends to parallel projection. 
The SEM stage plate containing the object generally has five degrees of freedom - a 
uniaxial tilt about the Y-axis of the stage so as to generate x-parallaxes, a rotation 
around the general direction of the principal electron axis (corresponding to Z) and 
translations in the directions of X, Y, and Z. Stereoscopic coverage of two 
micrographs is obtained if the object is tilted by a certain angle between the 
recordings. This results in convergent image pairs which are processed as if the 
recording system had been tilted and translated. 
At the BAM a modified Japan Electro Optical Laboratories system (JEOL JCXA- 
733) is used (fig. 3). To keep geometrical distortions as small as possible, the image 
data are recorded by digitizing of the SEM scanning signal directly in a computer 
controlled scanning process. With conventional SEM devices the SE beam would 
control the intensity of an electron beam recorded on CRT and stored on 
photographs - a procedure that causes several serious distortions (4). 
JEOL JCXA-733 
  
  
Electron Beam | Deflection Coil 
X/Y - Coordinates Scan 
- Generator 
  
RE - Detector | 
  
  
  
  
Converter 
  
    
  
  
  
  
  
SE - A 
Detector 
  
, | Scan Area 
  
  
  
  
  
  
  
  
Monitor 
  
  
  
Digitized Image Data 
Fig. 3: Sampling of SEM data (schematically sketched) 
PREPROCESSING AND ORIENTATION 
Due to the 8-bit representation of image data on common display systems, 256 gray 
levels are available for each pixel. In most cases SEM images do not use the full 
range of gray values. To enhance image information, contrast stretch algorithms are 
available. These operations are performed almost in real-time by the system. 
In addition several filter algorithms for restauration of degraded images are 
implemented and could be applied on the SEM data. The side effects of these 
operations on the image correlation are studied at present time. 
133