Interview with Walter Clark

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

Hi gang,

Here the promised translation, extracted from the German book
"Moderne Infrarot und UV-photography", written by Guenter Spitzing, 
printed in 1992. Note that this interview is much older, it also is 
included in his 1978 edition (I have the Dutch translation of that). 
He also wrote a book in 1967, which I hope to find one day....;-))


xxxxxxx The Infrared Aerial Photograph xxxxxxx

Walter Clark of Kodak Research Laboratories in Rochester, USA, 
courteously allowed me to interview him on the subject of aerial 
photography. Focus of our conversation was the topical infrared color 
material, which is not only produced as 35mm, but mainly in aerial 
photography formats, under the name Aero Ektachrome Infrared Film.

Question:
   Mr. Clark, you have been involved in the development of infrared film 
from the very beginning. At what time did you discover the fact that 
haze can be penetrated with IR-photography?

Walter Clark:
   Real IR-photography started in 1931. At that time Kodak 
started with the production of near-IR sensitive materials, with the same 
overall sensitivity as normal films. With self-sensitized (sp?) 
emulsions there were much earlier experiments, in astronomy and 
spectrography. Even in 1907, in the person of Wood, it was known that 
materials sensitive beyond 680nm could penetrate dust and record 
foliage as white. 

Question:
   Already in 1940, 5 years after the launch of the first color film, 
you consided the development of color-IR film. Why did this material 
look so promising for aerial photography?

Walter Clark:
   Modern color photography started in 1935 with the launch of 
Kodachrome. In 1937 we, and more importantly the US Air Force, 
performed aerial photograph experiments. In 1938, Brad Washburn and 
I, Walter Clark, shot many aerial color photos in Alaska on rolls of 9.5" 
aero film. In 1940, US Air Forces demanded Kodachrome films for aerial 
use. At that time, we explained that the processing of these films 
was too complicated for military use. But only one year later, in 
1941, our experiments with build-in color-couplers performed so well 
that a much easier to process film could be produced. So for the US 
Air Forces, we released the so called Kodachrome-Aero-Reversal-Film.
   Also in these days we researched the aerial photography on the 
potential of camouflage recognition. We considered the possibility of 
combining IR-characteristics with color contrasts. I believe that the 
first thoughts about a two-color separation camera working according 
the Technicolor principes originates from D.A. Spencer and A. 
Marriage at Kodak in London. This idea was then lifted to Kodak 
Research Laboratories in the US, in the person of Mannes, Jelly, 
Wilder, Copstaff and me. We then considered the use of the multiple 
layer color film, in which one layer should be infrared-sensitive. 
Many combinations were tested, in particular a two-layer/two-color 
system and two three-layer/three-color films.
   One of these three-layer films was then selected, and released to 
the US Air Forces as Kodacolor Aero-Reversal-Film for camouflage 
recognition. The modern vision of this is the Ektrachrome 
Aero-Infrared-Film.
   Color-IR is more usefull than black&white-IR, since color contrast 
is easier detected than grey-scale contrast.

Question:
   The b&w IR films that are used in aerial photography are sensitive 
for the very near infrared. Do you exlude films between 1000 and 
1500nm mainly because of stock problems, long exposure times and 
such? Or does this range of IR not bring any additional information?

Walter Clark:
   The practical limit is around 1340nm. Films with a sensitivity of 
more than 950nm are to slow for aerial photography. Such materials 
are only recommended for spectroscopy.

Question:
   Again a question about the color-IR material! Is this special film 
type always considered better than normal color film?

Walter Clark:
   No!, certainly not. The color-IR film should only be considered a 
better solution if strong IR-reflections are present.

Question:
   How is this question answered for aerial photography if we compare 
b&w-IR with color-IR?

Walter Clark: 
   This kind of questions are still being investigated currently. In the
UK there is an experiment right now, to assess whether the harvest
time of field fruits is better detected with color- or b&w-IR. 
   As a last remark I would like to comment that - even apart from
aerial photography - several tests are underway which should tell us
where color-IR can have advantages over b&w-IR.


[final alinea from Guenter Spitzing] 

   After these words from the mouth of an expert, I like to add a few
comments: 
   The Crazy-Color film acts, as mentioned before, very contrasty. This
is explained with its original application: aerial photography. The
by haze scattered light between airplane and surface of the earth
decreases contrast strongly. The circumstances under which most of
these aerial photo missions take place remind us of copying
topographic carts by hand, drawing on a diffuse transparant piece of
paper. The outlines pour through, but the contrast is gone. Like
within transparent paper, light scatters on dust particles and water
molecules, drifting around in the air. The higher the plane, the
stronger this lack of contrast. The haze penetration of IR-rays, but
also the very contrasty acting color-IR film compensate this loss of
contrast. Normal Aero-films from Kodak - even ordinary non-IR films -
also tend to result in large contrasts. 
   Apart from IR-photo's there are currently also thermographic images
being taken from air planes. For nightly aerial images artificial
light - for example in the form of 'christmas trees' - is not
neccessary anymore. Very recently the US Air Force ordered the
development of a special UV-camera for air reconnaisance at Perkin
Elmer Corporation. This camera is used together with a set of
interference filters, which only let pass a small spectral band of
light. How UV-reflection photography is related to especially aerial
photography....only Lord and the Air Force knows. 
   UV is possibly involved because UV-sensitive films with enormous
resolution can be used.

 
--
Bye,

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      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 3