b 
glass plate was placed in the ordinary posi- 
tion of the diapositive. The "camera" axis 
was horizontal and oriented parallel with the 
l m centerline of the grid. The height of the 
lens above the grid plane was 0.2 m and the 
distance between the lens and the closest 
1.6 m line of the grid was l m (Figure 2). 
l'uLTIPLEX-PROJECTOR 
  
  
  
  
0.2m 
  
  
  
1m LINE-GRID 
Pig 2 
The perspective reproduction of the line-grid 
on the photographic glass plate is shown in 
Figure 3. When placed in the projector in the 
same position again, the photograph offers an 
accurate reprojection of the rays from the 
instant of photography according to the prin- 
ciple of Porro-Koppe. With the projector and 
the surface to be tested in the same relative 
position as for the photographing of the ori- 
ginal plane surface, a height deviation of 
the surface to be tested causes a displace- 
ment of the projected lines in the direction 
of the projector-axis. This displacement has 
a magnitude of five to ten times the height 
deviation and can easily be observed since 
the 1.6 m grid lines appear as curved lines. 
Thus the height deviation can be estimated 
and also measured. 
Because a mould surface is extremely dark and 
light-absorbing there were some problems to 
get enough light intensity during the projec- 
tion. The ordinary lamp of the projector had 
to be substituted by a 100 Watt halogen lamp 
and still the light was insufficient for 
photographic documentation with short expo- 
sure time. However, photographing the projec- 
ted line-grid using a polaroid camera and an 
exposure time of approximately two minutes, 
makes evaluation possible with sufficient 
Speed and accuracy. 
Further research is planned to increase the 
practical utilization of the method. 
The principle of the method will possibly find 
other applications where the problem is to 
make a rapid determination of the lack of 
flatness of surfaces. 
  
  
  
  
  
  
Fig 3 
123