ee
Se of
ies
c har-
l, for
ible
nin
ifra-
last
ater,
AVE =
max-
point. Thus in the figure several of the curves practically run
into each other at 5100 À, and several curves intersect at 5800
to 5900 À. Because of this the transparence range of the filter
downwards ought to be limited at about 5200 À already on account
of the tone separation, but there exists a further cause to draw
this limit rather high, namely the influence from the haze in the
air, In the other direction we see that the upper limit of the
transparence range ought if possible to be put at about 5800 A,
The nearer to each other the curves will flow within the trans-
parence range of the filter, and in particular if they intersect,
the more the differentiation is blotted out.
The wavelength range between 6800 and 7800 ought better not
at all be represented within the spectral range used at photogra-
phy for forestry purposes. Here the remission curves increase
very rapidly And they often intersect. As further the remission
values are very high this means that an equalization of the dif-
ferences between the tree species will have a great importance
here and may destroy the differentiation carried out by other
wavelength ranges, for instance the green one.
Unfortunately it appears that the filters to be had in the
market are not well fitted for the requirements of photography
for forestry purposes. Also regarding the film material a certain
one-sidedness has hitherto prevailed in aerial photography, only
one kind of panchromatic film (fig. 3) having been used, which
Was considered suited for the main part of the reproduced ob-
Jects, and which was used together with different standard fil-
ters from the different camera manufacturers, Thereby the fil-
ters were chosen according to the prevailing haze conditions and
have for short wavelengths cut off a greater or smaller range of
the film sensitivity. But if aerial photography is made purely
T
EIER PP PU Ww : d
UNE at a ee
Pa
