Date sent:        Thu, 24 Oct 1996 21:47:29 +0100
Send reply to:    infrared-photography@a1.nl
From:             "Willem-Jan Markerink" 
To:               w.j.markerink@a1.nl
Subject:          Re: Exposure and development control

On 24 Oct 96 at 12:58, TAMMY L. SULLIVAN wrote:

> Willem,
> 
> Glad you've read the book ... I'm still thumbing thru it.  Weird how they took
> photographs with IR of the oddest things.  Yuk.
> 
> Tammy

"The explanation of the white appearance will be clear from a consideration of Figure 55, which is taken from the work of Willstaetter and Stoll, and which shows how light is reflected internally in a leaf. The light passes through the epidermis and the palisade cells, but is scattered diffusely and even reflected back towards its source in the spongy parenchyma, the interstices between which are filled with air. Mecke and Baldwin pointed out he similarity of behaviour between the leaf and freshly fallen snow (!!!), the bright reflection of which is due to the presence of air between the small ice crystals. If snow is pressed or tamped, it becomes darker and transparent. The same occurs in case of a leaf. If the air is removed by the leaf being placed in a vacuum, and if it then placed in water to fill the air spaces, no difference results which is visible to the eye, since the chlorophyll absorbs most of the light. In the infrared, however, to which chlorophyll is transparent, the bright reflection of the leaf is almost completely lost, and the leaf becoms relatively transparent."

There you have it. The reflection of snow and foliage are based on exactly the same principle!

Flabbergasting....8-))

--
Bye,

       _/      _/       _/_/_/_/_/       _/_/_/_/_/
     _/  _/  _/               _/       _/  _/  _/
     _/  _/ illem    _/     _/ an    _/  _/  _/ arkerink
                     _/_/_/  



      The desire to understand 
is sometimes far less intelligent than
     the inability to understand







From:             "George L Smyth" 
To:               
Subject:          RE: Wood effect
Date sent:        Sun, 24 Oct 1999 19:20:41 -0400
Send reply to:    infrared@a1.nl

> -----Original Message-----
> the Wood-effect was first reported in 1910 by a guy named Wood when
> shooting with a filter of 720nm. The effect is caused by the structure of the
> chlorophyll containing leafs, which absorb most radiation below infrared. Only
> green gets (mostly) emitted after beeing reflected in the parenchym layer of
> the leaf. Wavelengths above 680nm are reflected there with no absorbtion at
> all thus producing the nice light spots on the IR film.

Nope, chlorophyll has nothing to do with it, with the exception that it
allows infrared radiation to pass right through.  What happens is that
radiation gets bounced around within the spaces between the cells, and is
reflected back.  This is the same sort of thing that happens in newly fallen
snow.  It appears whiter because of the spaces between the flakes, which allow
light to bounce around and be reflected back more strongly than when the flakes
are close to one another.

george

---
Handmade Photographic Images
http://www2.ari.net/glsmyth/






Date sent:        Wed, 27 Oct 1999 21:40:57 +0200
To:               infrared@a1.nl
From:             Jesko-Henning Tanke 
Subject:          RE: Wood effect
Send reply to:    infrared@a1.nl

>> -----Original Message-----
>> the Wood-effect was first reported in 1910 by a guy named Wood when
>> shooting with a filter of 720nm. The effect is caused by the structure of the
>> chlorophyll containing leafs, which absorb most radiation below infrared.
>> Only green gets (mostly) emitted after beeing reflected in the parenchym
>> layer of the leaf. Wavelengths above 680nm are reflected there with no
>> absorbtion at all thus producing the nice light spots on the IR film.
>
>Nope, chlorophyll has nothing to do with it, with the exception that it
>allows infrared radiation to pass right through.  What happens is that
>radiation gets bounced around within the spaces between the cells, and is
>reflected back.  This is the same sort of thing that happens in newly fallen
>snow.  It appears whiter because of the spaces between the flakes, which allow
>light to bounce around and be reflected back more strongly than when the flakes
>are close to one another.
>
>george
>
For the reflection of the infrared the chlorophyll does not play a roll
because the IR radiation passes it anyway, back and forth. BTW, there is
now way for the trees but letting the warm IR rays pass through; otherwise
their leafs would heat up too much thus violating the biochem reaction in
the chlorophyll layer.

What makes the chlorophyll a point of interest for foto application is the
fact that for the human eye the leafs appear darker (absorbtion...) than
for IR sensors. This looks strange and fluffy and thats basically all:
simply a new perspective for the human eye.

I donīt know at which stage of decay during fall the parenchym layer gets
harmed and the leaf stops "reflecting" as described above. But it happens
*after* the chlorophyll was deactivated to allow the tree retracting
valuable minerals. When you shoot dryed hay with IR and Pan material you
will see no difference while living grass appears like leafs (what it is).
(I checked the bio details with a forester...)


Have a nice time
Jesko

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