MARUYASU, NISHIO—ON THE STUDY AND APPLICATION OF INFRA-RED AERIAL PHOTOGRAPHY 5
Japan and we are obliged to depend solely upon import for its supply. The shortcoming of
the emulsion for infrared films is that its effective preservation period is limited. So the timing
of photographing and time of import is a matter of great importance.
3-2 EXTENSION OF FOCAL DISTANCE OF LENS
In ordinary aerial photos, photos are always taken from high altitude and this necessi-
tates the use of a yellow filter to eliminate the effect of haze, and the wave lengths that are
actually sensitive are within the range of from around 450 to 700 mt. In infrared photos,
however, only the light with a wave length longer than that is used, so it becomes necessary
to extend more or less the focal plane. The extension of this focal plane varies according to
the type of lens, focal length of lens and difference in the refractive index toward infrared
ray. But, as the range of wavelength used is wide, in practice, amount of extension is de-
termined by color sensitivity of the film and filter used. Generally, one might extend the
focal plane within the range of from 1/200 to 1/400 of the focal length.
In ordinary miniature cameras, focusing is not so difficult because they are marked with
degrees for the convenience of taking infrared photos. However, in the case of aerial camera,
focal length must be predetermined with utmost accuracy. So we are not allowed to freely
change the focal length, that is, the interval between the lens and film.
Fortunately, however, Dr. Bertare has recently contrived a lens for the exclusive use of
infrared photos and it is marketed under the trade name of "Infragon". This lens can be
fitted to Wild fully automatic aerial camera RC-5a and RC-8.
In the past infrared photos were used merely for interpretation purposes, but with the
emergence of Infragon, infrared photos have come to be used for stereoplotting purposes, too.
In the first-stage test we used a miniature camera and ordinary aerial camera, but we
could pursue our studies by using Infragon lens from the middle of our work.
3-3 SELECTION OF FILTERS
Although filters produced for 100
aerial photos vary more or less with
the makers, each maker seems to be 20
manufacturing several varieties of ". : Js
filters manufactured by Wild Co., 1. wild's filters
Zeiss Aerotopograph and Eastman 100
Kodak now in use in Japan are as
shown in Fig 3.
Of these filters, those used for
infrared photos are red filters which
absorb a wave length of less than
670 mg. Since we did not conduct
an experiment with the so-called
Modified Infrared Method which is
being used mainly in the United
States, no mention is made on it in
this report. However, it is one of
our tasks that must be tackled in
the future for comparison's sake.
50
0
2. Zeiss's filters
100
ZA.
7
50 LE
0
3. Wratten filters
T T T T T 1
300 1,00 500 600 700 800
Wave Length (mu)
Fig. 3 Comparison of the Performance of filters.
