ower 
lus 
he 
een 
nge 
eng'ths 
ge of 
urvey 
ld be 
sed 
pho- 
manu= 
Lt to 
or 
rent 
which 
| Pise 
Ye 
ment- 
using 
wave radiation from green included. The other dye-stuff is also 
transparent to green but shall absorb radiation from longer wave- 
lengths as orange and red radiation. Unfortunately it is common 
that these dye-stuffs transmit a more or less narrow wavelength 
range in the upper red range. This is the case with this filter. 
We see that the absorption in red reaches a maximum at about 6500 
À and then decreases again towards infrared. This transparence in 
the farthermost part of red is, however, not seen oculariy owing 
to the very small sensitivity of the human eye for the longer 
wavelenghts in red. But as the sensitivity curve of the film is 
extremely high within this range, which is seen from fig. 3, whe- 
re the ordinate values are given logarithmically, we have here a 
very Serious Source of error, especially as also the leaves and 
needles have a very high remission power at these wavelengths, 
It may once more be emphasized that it is impossible to decide 
by ocular inspection whether a green filter to be had in the mar- 
ket transmits light containing red or not. It is necessary to 
control this with a spectrophotometer, especially at the red end 
of the visible spectrum. 
According to First Cartographer E.Welander the above mente 
ioned green filter (fig. 4) notwithstanding gave somewhat better 
results at photography for forestry purposes than the earlier 
used orange one. A still better differentiating of the tree spe- 
cies was obtained if according to my directions, an orthochromat- 
ic film material and a strong yellow filter were used at aerial 
Photography. The sensitivity range for the Orthofilm is seen 
from fig. 5. It extends from 4000 Á to about 5800 A. Then only 
one filter-dye is needed, which is opaque to blue light but trans- 
nits all the light above 7200 A. The yellow filter worked out by 
us (fig. 6) is based on the dye-stuff tartrazine, in gelatin, 
Cast with 10 grammes per Square meter on each filter gless which 
    
     
  
  
  
   
    
  
  
  
   
    
   
  
   
    
   
    
  
  
  
NT STE AN us 
Buh iid ww Lo > o PM A p ie Rio i lor.