Date sent:        Sun, 15 Sep 1996 16:18:53 +0100
Send reply to:    infrared-photography@a1.nl
From:             "Willem-Jan Markerink" 
To:               w.j.markerink@a1.nl
Subject:          Interview with Walter Clark

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....;-))

The Infrared Aerial Photograph

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 transferred to Kodak Research Laboratories in the US, the persons involved were 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]

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 transparant 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@a1.nl)
[note: 'a-one' & 'en-el'!]

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