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DISCUSSION ON X-RAY PHOTOGRAMMETRY 27 
leading results. In particular in the application 
of X-ray photogrammetry in medicine it seems 
most important that the accuracy conditions of 
the measurements can be estirnated with high 
reliability. This necessarily requires that the X- 
ray instruments to be used are carefully tested, 
and that methods are applied which can give the 
best possible information about the accuracy 
which can be expected in each individual case. 
First, everybody usually assumes the X-ray 
photograph to be a correct central projection 
of the object. The X-ray plotting methods up to 
now are as a matter of fact founded upon this 
assumption. Therefore it may be of interest to 
show the results of an investigation of the radial 
distortion of an X-ray photograph from an in- 
strument which is used in practical medicine. 
Sec Fig. 2, 
The test of the instrument was founded upon 
the grid method. A regular grid, consisting of 
small holes, drilled in lead plates on a flat plexi- 
glass surface was imaged on film with the X-ray 
instrument in question. The planes of the grid 
dr‘ mm 
  
  
s = 0,07 0,05 0,04 0,04 9,8 mm 
x 
o 
Jan. 1960 0,06% 0,07 
x Data from nine points 
Fig. 2. X-Ray Neg. Nr 2:7; Film. Radial distortion 
curve. Apparatus 2; z = 1706.9 mm; c = 1817.4 mm. 
and of the film were approximately parallel and 
the center of the grid coincided approximately 
with the principal point. The coordinates of the 
grid points and of the corresponding image 
points in the film were measured with high 
precision and the usual procedure for the 
computations was applied. The method is 
described in details in the paper: The Basic 
Geometric Principles of X-ray Photogrammetry. 
Transactions of the R. Institute of Technology, 
Stockholm. Nr 123, 1958. From Fig. 2 it is 
evident that a considerable radial distortion was 
present in the actual photograph. In many other 
similar tests typical radial distortion effects have 
been found. 
Further, the demonstrated standard error of 
unit weight of the image coordinate measure- 
ments is of great interest since it gives us im- 
portant information about the basic accuracy of 
the X-ray measurements under the actual con- 
ditions. Evidently, it is most desirable to make 
corresponding determinations of the accuracy of 
the measurements in the practical application of 
X-ray photogrammetry. For such purposes y- 
parallax measurements in actual stereoscopical 
models usually give very interesting information, 
not only concerning the photogrammetric pro- 
cedure but also about the identification problem. 
The procedure for such y-parallax tests is very 
similar to the corresponding procedure in aerial 
photogrammetry. In the above mentioned paper 
also the principles of y-parallax tests in X-ray 
photogrammetry are demonstrated. 
For the complete determination of the in- 
terior orientation of X-ray instruments and 
photographs special devices have been con- 
structed and have successfully been applied to 
instruments in practice. 
In summary, for reliable measurements in 
X-ray photographs as well as in all kinds of 
photographs it is necessary that the geometrical 
qualities of the photographs (the actual imaging 
device) be carefully investigated under real 
working conditions. This is a very important 
condition for the development of photogram- 
metry in general. 
Herr Professor BURKHARDT: Ich danke Herrn 
Kollegen Hallert und darf nun Herrn Kollegen 
Kóhnle bitten, vielleicht noch etwas Ergánzen- 
des zu dieser Frage der Róntgenmedizin zu 
sagen. 
Herr KóHNLE: Wenn zwei dasselbe tun, ist 
es nicht dasselbe. Wenn der Photogrammeter ein 
Bild mit sichtbarem Licht ausmisst, und wenn 
der Arzt ein Róntgenraumbild ausmisst, ist es 
nicht dasselbe. Das beruht einerseits auf den 
physikalischen Unterschieden, die wie wir eben 
in den schönen Vorträgen gehört haben, bei der 
Röntgenabbildung gewisse Ähnlichkeit haben 
mit der Elektronenabbildung. Auch wir haben 
ein Durchdringungsbild, das uns Schatten gibt, 
die aber in ihrer Art etwas anderes bedeuten als 
der Begriff Schatten im sichtbaren Licht. Das 
sind die physikalischen Unterschiede, aber dazu 
kommen die psychologischen: der Arzt ist ge- 
wöhnt, sein Bild met den Augen im sichtbaren 
Licht zu sehen, den Patienten, das anatomische 
Präparat, und nun muss er das übersetzen in die 
Schattenbilder, die das Röntgenbild liefert. 
Diese Unterschiede sind sehr wesentlich und 
dazu kommt das Psychologische. Er sieht die 
Bilder in Verbindung zu dem, was er im sicht- 
baren Licht zu sehen gewöhnt ist. Der Ortho- 
und Pseudo-Effekt, der im sichtbaren Licht uns