Fig. 1: imaging of a target (left side) to a target image (right side) 
blurred by geometric optical aberrations 
In multi-media photogrammetry a minimization of 
blur for a special project by optical design considering 
the additional media is possible. But this is only real- 
istic, if the arrangement of all optical components - 
camera and additional media - is constant for each 
exposure of the project. This case is called bundle in- 
variant (Kotowski 1988). An example for a bundle in- 
variant multi-media application is the underwater 
photogrammetry with cameras emedded and fixed in 
watertight and pressure-resistant housings. 
If the shape and positions of the refracting surfaces 
between camera and object are constant in relation to 
the object, an image blur minimization is impossible. 
Exposures of an object placed behind a glass plate 
taken with convergent optical axes may serve as an 
example for this case of an object invariant image 
block. 
To avoid a major accuracy reduction in those images 
the extension of classical comparator measurement 
methods to an object based method is presented. 
2.Principles of Determining Comparator 
Coordinates 
The determination of comparator coordinates can be 
devided into two steps: the definition of image points 
and their identification. 
2.1.Classical Measurement in Images of 
Sufficient Sharpness 
The classical methods of determining comparator 
coordinates, visual measurement and edge based 
digital measurement define image points geometrical- 
ly: the center of an object target, defined as the object 
point, is assumed to be imaged as the center of densi- 
ty of the sufficiently sharply contured target image, 
because the basic imaging model used in photogram- 
metry is central perspective, possibly disturbed by 
distortion only. Object- and image point correspond 
by the perspective ray. So, the target image center is 
defined as the image point. 
In geometrical optics the principle ray is equivalent 
to the central perspective ray. This ray can be as- 
sumed to be identical for all wavelenghtes, if the 
imaging is sufficiently perfect (Hofmann 1980, 
pp.91f). Only in this case the just given image point 
definition makes sure, that the center of the target 
image is identical with the intersection point of the 
principle ray with the image plane. 
2.2.Extension to Object Based Determination 
If the aberrations are increased by an inproper ar- 
rangement of the optical components, the principle 
rays of different wavelengthes now diverge; so their 
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