mE E 
NT EN 
ed ten OST Set at e 
e E Mouse crane Se Save 
for forestry purposes, the transparence range of the film must 
be chosen in such a way that the greatest differentiating power 
for different tree species is obtained, An orange filter thus 
is inappropriate, and also the green filters to be had in the 
market cannot be considered well balanced. A very narrow green 
filter would be needed, transmitting only the wavelength range 
2200 to 5800 À but preventing higher as well as lower wavelengths 
to reach the film. The greater part of the sensitivity range of 
the film thus would be of no use. But as the Geographical Survey 
Office was of the opinion that for practical reasons it would be 
of great value if the same kind of panchromatic film were used 
for photography for forestry purposes as well as for aerial pho- 
tography for other purposes, we made an attempt in 1954 to manu- 
facture a suitable green filter (fig. 4). Now it is difficult to 
make a colour filter for a relatively narrow transparence range 
in green. The borders of this range do not become sharply defin- 
ed and the remaining absorption within the range becomes rather 
high. The latter is very unsuitable, as it would prevent a com- 
plete exposure; a compromise must be made, and the colour densi- 
ty thus will not be as great as would have been desirable for 
other reasons. It must also be pointed out that the development 
towards more and more fast aircraft for aerial photography, which 
may be valuable from the point of view of saving time, gives rise 
to further difficulties from the point of view of photography. 
Shorter and shorter exposures are made necessary if the movement- 
unclearness shall be avoided, and in this way the difficulties 
are increased for obtaining correctly exposed pictures when using 
narrow filters, 
The green filter according to fig. 4 is built up by two 
dye-stuffs, Tartrazine 5g/m® and Filter Blue-Green 0.35g/m*. 
The formed is opaque to blue radiation but transparent for long- 
    
  
  
  
    
   
  
  
  
   
    
   
   
     
  
  
  
   
     
     
wave radia‘ 
transparent 
lengths as 
that these 
range in th 
We see that 
À and then 
the farther 
to the very 
wave lenghts 
extremely h 
re the ordi 
very seriou 
needles hav: 
It may once 
by ocular i 
ket transmi 
control thi. 
of the visi! 
Accorc 
ioned green 
results at ; 
used orange 
cies was obi 
ie film mate 
photography. 
from fig. 5. 
one filter-c 
mits all the 
us (fig. 6) 
Cast with 1C