re: InfraRed and Water- A comprehensive view - Part 2

["Editor - P.O.V. Image Service" <editor@xxxxxxxxxxxxxxx>]

part 2...

When we left our story...

Rolland Elliott wrote:


>
> 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

Wrong.. The camcorder records a significant range of light even in IR
mode..  On
the other hand at near infra-red wavelengths (those for IR film) water
is IR
absorptive...  Doubt me?

OK, here, check this cite:
"GUYOT, G., 1990., Optical properties of vegetation canopies., in,
Applications
of remote sensing in agriculture, editors, Steven, M, D., Clark, J, A.,
Butterworths."


> and the quantity of water was appropriate in my opinion.

NOT! And you had clear water with no suspended particulate matter. That
suspended
matter found in almost all naturally occurring water (and particularly
sea water)
multiplies the water's absorptive effects in the near IR..

You need to go scuba diving my friend, where RED items become black.. 
The
reason, as others have tried to explain is that there is no RED light to
reflect,
as one slowly rises, you can watch the red color return to the items as
one nears
the surface (and please don't tell me it has something to do with water
pressure
or salt -- I will be driven to hit something...)

Lemme tell you a secret: the reason this happens is that shorter blue
wavelengths
of light pass more readily through aggregated water molecules.   UV
passes
through water and water vapor easily... That's why you need a UV
filter... The UV
filter  increases the apparent blueness of a sky or water by filtering
out the
near UV that would otherwise partially overwhelm blue portion of the
film and end
up shifting the blues to white... As we proceed to longer and longer
wavelengths,
they are absorbed more and more readily by Water... Near IR is pretty
long...

Another example is Fog, Fog will generally render as such in NEAR IR...
This is
to be differentiated from haze or smog, which are generally IR
transparent...
Again, water makes the difference.

>
> 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.

Not true, it makes a huge difference in how plants appear in IR!

"The anatomical structure of a plant has a significant effect in the
near infra
red due to plants cell structures. A drought adapted plant will have a
much
higher near infra red signature, while one with hairy leaves will have a
higher
visible and mid infra red signature. As a leaf ages and enters
senescence, its
spectral properties change due to both the changing pigmentation of the
leaf, and
its drying which affects its near infra red signature. Water content has
a direct
effect on the mid infra red and an indirect effect on the visible and
near infra
red. As water content decreases the reflectance increases, however,
extreme
levels of water stress are needed before this effect is measurable. The
effects
of mineral deficiency and pests or disease also can be see in a plants
spectral
reflectance curve, again due to changes in pigmentation affecting the
visible
range, or plant photosynthesis and transpiration rates effecting the
near and
middle infra red (Guyot., 1990). "

BTW: You just proved the absorptive quality of water yourself... You
look into a
lake and it is dark in the lake even though you can see the sky
reflected in
it... Why?  The coincidence of the reflectivity of the surface of the
water with
the light absorptive quantities of masses of water.

In fact this is a stunning effect to take advantage of with IR imagery..
Unfortunately my own example of this is not on my website anymore..  I
had a
model laying alongside a shallow pond..  The pond is black as night, but
you can
see her reflection in it nonetheless..

There is one sense in which you are right, there is ALSO the reflective
effect
produced on the surface of the water where a blue sky is overhead...
That will
increase the dark rendering of the water, IF you use a blue blocking
filter.. You
can enhance this more greatly with the addition of a polarizing
filter... This
effect does not depend on the absorptive quality of water except in
explaining
why the sky is blue in the first place... Water vapor... No atmospheric
water on
Mars = No Blue skies...

>
> 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.

Actually, underwater strobes emit excessive red and IR.. They are
purposely
designed that way SPECIFICALLY to overcome the short wavelength
absorptive
qualities of water... As the water absorbs the RED end of the spectrum
more
readily than it does the blue end, you need to emit more red light to
compensate...

The other choice is to use an emulsion like the Kodak UW (Underwater)
E-6
emulsions, which have the vast majority of their Blue absorption
masked...


to be completed in part 3....

Keith Krebs

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