RE: Infrared & Water

[Otto Giesenfeld <otto.giesenfeld@xxxxxxxxxxxx>]

--On den 20 mars 2000, 19:52 +0000 Rolland Elliott
<rolland_elliott@xxxxxxxxxxx> wrote:

> Just took a look at a glass of water with an IR camcorder.  Looks,
> crystal  clear to me, just like it does under regular visible light.  The
> main reason  water looks dark in many IR photos is because it is
> reflecting the sky, NOT  because it is a good absorber of IR light.
> Before suggesting explanations  to a phenomenom it would be beneficial to
> test!  Peace Rolland
> 

According to the book "Moderne Infrarot- und UV-Fotografie" by Günther
Spitzing (Augustus Verlag), which I bought recently for 20 DEM from Brenner
Foto-Versand (they might still have copies left), 10 m of clear sea water
decreases the intensity of 800 nm infrared light 1000-fold.

Thus, one meter of sea water would only cut the intensity by half. The
attenuation caused by a glass of water should therefore for most purposes
be negligible. Still, the author claims that because the absorption is much
higher in the farther infrared range, water in a glass tube is useful as a
heat-absorbing filter.

According to the same source, water is supposed to have its minimum
absorption at 470 nm, where it takes 300 m of water for a 1000-fold
decrease to occur.

The effect of water surfaces appearing black on infrared photos is also
commented on. The author says it happens because of the absorption, but
adds (my translation): However, an important role is also played by the
fact that reflections of the blue sky are pictured as black in an infrared
photograph. Personally, I still tend to believe that the latter effect is
the more important one... But it surely becomes complicated when scattering
(which occurs at a higher degree for shorter wavelengths) is taken into
account, so I may very well be wrong.

Best regards,

Otto Giesenfeld
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