Re: Infrared & Water

["Rolland Elliott" <rolland_elliott@xxxxxxxxxxx>]

I wrote:
<<Just took a look at a glass of water with an IR camcorder.  Looks, crystal 
clear to me...  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.>>

Stan Wrote:
"...the question refers to whether water absorbs IR from light being
reflected from it. Your test would answer the question of whether water 
absorbs IR from light being transmitted through it. ......Your test showed 
no effect because your instrumentation was far too
insensitive and/or the amount of water was far too small."

Actually this whole long discussion is the result of incorrect information 
that was posted trying to explain why blood appears dark in IR photos:
"blood IS composed largely of water 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!"....Which is WRONG!

The real reason blood appears dark is quoted below from Kodak's Medical 
Infrared Photography Book:  "Still left unexlained is the blueness in the 
color IR photographic record of veins at the normal distance under the skin. 
  A clue was provided by the fact that blue sky records blue with 
camoflage-detection film  Visually this blue is the result of minute 
atmospheric particles.  Both the visual blueness of superficial veins and 
the color infrared rendering can be explained on the basis of the veins 
being embedded in the translucent layers of the body tissues. To demonstrate 
scattering under these circumstances, tubes of venous and arterial blood 
were photographed partly under a thin scattering of milk & water.  Plate 
II-3 shows that the immersed portions of the tubes do record blue, whereas 
the free portions record as expected.  Plate II-4 & 5 further demonstrate 
the effect of scattering layers. Veins in the prepuce recorded blue through 
the external skin, and the same veins appeared reddish brown through the 
internal, transitional, more transparent epithelium that presents itself 
when the prepuce is retracted."

To answer the question "Does  water absorbs IR from light being
reflected from it." The answer is NOT ANYMORE SIGNIFICANTLY THAN VISIBLE 
LIGHT.  I just looked at the reflection of my hand in a big bowl of water 
with an IR camcorder. The IR reflection is about as intense as the visible 
light relfection.  Besides I've seen IR photo's of ponds that reflect the 
trees around them quite nicely.  See Laurie's White's book on pages 83 & 84 
for examples.

As far as the comment that "your instrumentation was far too
insensitive and/or the amount of water was far too small" My camcorder is 
not insensitive and the quantity of water was appropriate in my opinion.  
Like you said, the only time water absorbes/scatters enough IR light is when 
you have several meters of it, like when you are swimming underwater.  
However, I think I've seen ONE IR photo in my entire life taken under water. 
In addition most Underwater Photography is done with strobes (that emit lots 
of IR light) and at fairly close distances.  Given these facts it is very 
unlikely that any photographer shooting IR film will ever have to be 
concerned or compensate their exposure due to the IR absorbence of water 
unless they are doing underwater IR photography (with distant subjects) 
which is virtually non exsistant.

Whew, Rolland.
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