re: InfraRed and Water - A comprehensive response - Part 1

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

Rolland Elliott wrote:

> 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."
>

I normally would chop this more, but to explain, I cannot do so...

You are misreading this Rolland if you think that the venous tissue here
is
reflecting skylight or atmospheric light in a manner similar to what you
posit
for water, and that said reflection is causing the blue color..  Were
that the
case a simple change in the colors of the roof and walls surrounding the
mount
upon which the veins are held, would demonstrate a marked difference
from what
one might see in a room with white walls and ceiling..  That's not the
factor
that was altered here though.. Instead, what is demonstrated is
differential
scattering.  The difference in scattering and absorption of different
wavelengths
of light by intermediary substances between the venous tissue and the
observer or
IR recording medium.. At the simplest level it can be related to the
optical
discontinuity created by differing refractive properties of air and
water, when
you place a spoon in a glass full of water..  The spoon appears
discontinuous at
the air-water margin...  Well, this same difference of refraction when
combined
with scattering particles (not necessarily atmospheric - and in the case
of veins
it is the scattering differential and absorptive properties of
intervening
tissue)  causes the visible rendering of the veins..

>
> 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.
>

Reflected light is axiomatically not absorbed, I will give you that
much.. But
your logic from thereon is just trash.. No other word for it..

Want obvious proof?

You've seen a black car, correct?

We can all agree that a black car absorbs more visible light than a
silvered
mirror does, correct?

And if you look at all the black cars I have viewed in IR, they still
look
black...

OK, now can you see a reflection in that car (assuming it has been
polished)?

Why...?

Again, reflection and refraction!  Sure the black car absorbs more light
across
the spectrum than a white car, but you can see a reflection in both
surfaces.
Substances behave strangely at the margin where substance A meets
substance B.
That behavior is NOT INTUITIVE!

The fact that at one incident angle one finds reflection and at another
one finds
refraction and absorption, etc. is a basic fact of material transitions
in the
optical arena.. The absorptive and scattering qualities of any
particular media
are entirely distinct from it's reflective qualities at the margin...

In fact... That is why your lenses are multi-coated... To allow light to
make the
air/glass transition more efficiently...

Another example of darkness not having a direct relationship to
reflectivity can
be found in the following... Check out some mirrored sunglasses... Many
of them
admit more light through than do simple dark glass.. And lets leave
Polaroid
Lenses or Polarizing filters out of the equation for now...

Funny, how counterintuitive this is... That the mirrored lenses reflect
more
light AND admit more light... Again this is the internal absorption and
scattering of light within the glass  that is causing this phenomenon...
The
darker glass is actually absorbing more photons and converting them to
heat...

to be continued...



Keith Krebs

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