Fig.1 Double projector (Duplex) 
developed by Zeiss of Ober- 
kochen in the early fifties. Fig 
   
„2 7E 
2. Design of the instrument 
At first sight, the DP-1 Double Projector (Fig. 2) appears to be of relatively conventional 
design, two plotting cameras arranged side by side projecting a pair of photographs onto a screen, for 
example that of a plotting table. The following design features of the instrument are, however, no:e- 
worthy: 
a) The overlapping portion of each photograph is projected all the time by means of a suitable 
projection system. This eliminates the need for spot-light illumination (static instead of dynamic 
solution). The stereocompiler sees the entire model at a glance and need not necessarily use a 
special screen (e. g. a plotting table) to scan the image. 
b) The z- motion is transmitted by means of a pedal disk to the plotting cameras instead 
of to the plotting table. This leaves both hands free for guiding the plotting pencil and above all 
makes it possible to draw the manuscript directly on the instrument table — an advantage which is 
particularly welcome for map revision. 
c) The plotting cameras are designed to take unreduced aerial photographs. 
Examining the designs represented in the foregoing table, we find that the combination described 
under a) to c) has already been used three times. Two other features will, however, define the position of 
the DP-1 Double Projector more clearly. 
d) The use of Fresnel lenses results in a light-weight illumination system that is easy to manu- 
facture. 
e) Due to the fact that the film portion lying outside the individual model is bent so that it can be 
accommodated between the two projectors (Fig. 3), the DP-1 may be used for magnifications down 
to approx. 1.6x while preserving the original negative size. The addition of reflecting mirrors 
provided in other designs has thus been avoided, which again is a plus for simplicity. 
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