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Transactions of the Symposium on Photo Interpretation

by S. A. Hempenius
International Training Centre for Aerial Survey, Delft, The Netherlands
Abstract Specifications for mirror stereoscopes are given for the properties connected with
image formation, image recording, image quality and operational, mechanical, and econom
ical aspects. The measurements are explained briefly and the results for a number of stereo
scopes are discussed. No single figure or formula for the overall performance is given; the
user is asked to weigh the various aspects and qualifications according to his needs.
Résumé Les particularités des stéréoscopes à miroirs touchant à la formation, à la percep
tion et à la qualité des images font l’objet de spécifications qui couvrent aussi le système
mécanique, le mode d’opération et l’économie d’emploi. Des explications sont données au
sujet des mesures. Les résultats concernant divers stéréoscopes sont discutés, mais aucune
tentative n’est faite de représenter la valeur pratique globale de chaque instrument à l’aide
d’un nombre unique ou d’une formule. Il est laissé à l’utilisateur d’étudier séparément les
divers aspects du problème, de voir lesquels lui importent vraiment, et de trouver ensuite
quel stéréoscope répond le mieux à ses besoins particuliers.
Zusammenfassung Nähere Angaben über Spiegel-Stereoskope und deren Eigenschaften
bzgl. Bildformung, Bild-Registrierung, Bild-Qualität, sowie operative, mechanische und
ökonomische Gesichtspunkte werden besprochen. Die Messungen werden kurz erläutert und
die für eine Anzahl von Spiegel-Stereoskopen gültigen Ergebnisse werden gemeldet. Es wer
den keine einzelnen Ziffern oder Formeln bzgl. der allgemeinen Leistung gegeben; der
Benützer wird angewiesen, die verschiedenen Gesichtspunkte und Qualifikationen seinen
Bedürfnissen entsprechend gegeneinander abzuwägen.
Why are specifications necessary? As the interpretation of aerial photography
becomes more important, more attention must be paid to the stereoscope, in
order to obtain a better idea of the possibilities and limitations of the inter
pretation of aerial photographs.
Specifications for Whom? For interpreters who need a better understanding
of their key-instrument. Therefore we will consider the performance of stereo
scopes from the user’s point of view.
Specifications of What? Of Optical and Operational Properties. We will only
measure how the instrument influences the observation of the object (paper-
print) and whether it is convenient in use. We are not concerned with how it
is designed, what special solutions and materials are applied and how it is
How to obtain Specifications? Considering the stereoscope as a “black box”,
we do not need an exploded view of the optical and mechanical parts. The
only way to arrive at specifications is to establish the properties of interest to
the user, then to point out a way of measuring them and finally to judge the
results (fig. 1).