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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