Water absorbance (was Stigmata)

["Lee Tarpley" <tarpley@xxxxxxxxxxxxxx>]

> yes water absorbs IR, and a lot of other wavelengths up the red end of the
> spectrum too, just have a look at any underwater photo.  They are all
blue!

> >> and as anyone who has taken a photo
> >> across a body of water will attest, water absorbs the wavelengths of IR
> >> that IR film records and thus looks dark!
> >>
> >
> >Is this really the explanation? If the rendering of a water surface in
> >general photography is due to reflections of the sky, then the reason for
> >dark water in IR photography might be the dark sky... Anyone knows for
> sure?

Water transmits most of the radiation in the spectral range in which IR
films are sensitive. For example, green leaves are typically translucent
when viewed in the just-beyond-visible range (the very near infrared), yet
we know that the water content of these leaves is high. There is still some
absorbance that becomes apparent when underwater, but the extent of
absorbance by water in the visible/very near infrared is very small compared
to that in certain regions of the short wave infrared (beyond film range)
where no radiance even penetrates to the earth's surface because of the
atmospheric water absorbance.

Lee Tarpley


*
****
*******
******************************************************
*  To remove yourself from this list, send:          *
*         UNSUBSCRIBE INFRARED                       *
*       to                                           *
*         MAJORDOMO@xxxxx                            *
*----------------------------------------------------*
*   For the IR-FAQ, IR-Gallery and heaps of links:   *
*  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
******************************************************