of a photomap, 
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reproduction 
ssible result 
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li 
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Lipa Kontrast 
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the photo- 
  
  
for best results 
jse for the pro- 
lensity of 70-80 
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v through print- 
line glass screen 
(lass screen does 
production. To 
contrast in the 
to the lith£ilm, 
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e light as result 
egative plane. To 
5 contact screens 
tinuous tone images 
e the magenta and 
rtunity, as Sweer- 
ver the contact 
  
Screens give a better reproduction of the details because of the fact 
that here is no diffusion of the light as was between the glass screen 
and the lith film. Another important factor is that contact screens 
open the possibility of more standardization in the method of production. 
Disadvantages are: (a) dustfree work is absolutely essential in this 
contact method 
(b) the life of a screen that one uses in contact 
is shorter than that of glass screens 
Concerning the last point however we can remark that the price of a 
contact screen is much lower than that of a glass screen. 
Sweerman (Ref.: Sweerman, 7-13 1960) discusses which contact Screen is 
ideal for tone reproduction. 
Tone reproduction is ideal when the density of every part of the original 
image is the same as the density of the same part in the reproduction, 
both measured with the densitometer. 
About 1960 the Dutch Institute for Graphical Techniques (IGT) developed 
in cooperation with the English Patra-institute the "IGT-Patra" contact 
Screen. This was especially made to produce ideal screened negatives 
from black and white originals (positives).  Sweerman indicates that 
in fact it is impossible to make one type of screen that is good for the 
screening of positives as well as negatives. He and Noemer (Ref.: 
Noemer 1971) give the same conclusion that 
(a) for the Screening of a continuous tone negative, to get a screened 
positive, one has to use a positive screen and 
(b) for the screening of a continuous tone positive, to get a screened 
  
negative, one has to use a negative screen. The difference be- 
tween a negative and a positive screen is in the structure of the 
Screen dot. This can be seen in the printed screen dots with the 
following examples.  Noemer compares the results of a positive 
print produced with a negative contact screen. He and Sweerman 
both explain that a good screened negative should not have much 
detail in the shadows but considerable contrast in the light parts. 
In general screened negatives made from a positive original with 
a negative screen give the impression of being "hard" and as 
Noemer says "even good camermen are inclined to choose the incorrect 
negative because it seems to show much more detail in the darker 
parts." However, these dots in the dark parts and especially in 
the middle tone areas have so much build-up and are so dense that 
these important tones print far too light. Reading this in the 
graphical reproduction techniques periodicals, it seemed worthwhile 
to apply this to aerial photo imagery. 
  
  
  
The results of this experiment are shown here. 
These are screened images of two aerial photographs taken by K.L.M. 
Aerocarto N.V., the Netherlands for the Topographic Service of the 
Netherlands at Delft. The photos are treated with 50 and 54 line/cm 
Screens because 70 line screens in positive and negative were not avail- 
able in the organization which conducted the work. This has of course 
no influence on the result other than the fact that the dot pattern is 
2]6--