to be made, 
It might be of interest to note that in many cases stereo- 
metric cameras have been used without correcting for lens dis- 
tortion. Obviously if absolute object space coordinates were 
the final result desired, lens distortion would be a prime 
error producing factor which has to be taken care of, if the 
highest possible accuracy is to be expected. In experimental 
mechanics, however, one is mainly interested in displacements, 
or differences in X, Y, and Z of the undeflected and deflected 
object under testing. This subtraction process cancels out, to 
an appreciable extent, the effect of the uncompensated lens 
distortion, provided, of course, that the position of the 
camera relative to the object remains essentially unaltered 
from one exposure to the other. 
2.2.5.3.6. Object Contrast 
  
In order to be able to undertake the photogrammetric 
measurements, one relies either on the object texture or on 
targeted points at selected locations. The size, shape, 
material, and color of targets are matters which may vary 
considerably from case to case, and have to be decided upon 
according to the prevailing conditions in each particular 
situation. Of the numerous targets reported on in the litera- 
ture, the following are mentioned here just as an example: 
white ink dots on black paper ribbon, numbered on ribbon (Doyle, 
1967); intersection of marked lines of white chalk (Konecny, 1965), 
circular gummed reinforcements (Section 3.2 of this paper). 
Several approaches have been reported on in the literature 
offering solutions for cases where the object cannot be targeted, 
for one reason or another, and/or for cases where the problem at 
hand deals with areas and contour lines rather than discrete