Re: chlorophyll

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 30 Jul 96 at 22:53, George L Smyth wrote:

> Adam-HALPID Klyce wrote:
> > 
> >      ...The impressive special "wood-effect" (very bright to white
> >      reproduction of chlorophyll in black and white positive images)
> >      appears the strongest at low sun angles. 
> 
> The white you see is not due to chlorophyll.  Chlorophyll is transparent
> to infrared radiation.  What you are seeing is the reflection from the
> cells, which is enhanced by air spaces within the cellular structure.

Let me add some more odd's and ends to this, from Guenter Spitzing 
book: 
First, the effect is not wood-effect (from forest), but Wood-effect, 
named after Prof. R.W. Wood, who discovered this phenomena in 1910, 
exposed on self-sensibilized IR material, and also explored the 
creative use of this effect. [does anyone know what part of 
science he was teaching?]
As an aside, my Kodak Wratten catalogue from 1928 states with the 88 
filter: "88 Infrared, as used by Prof. R.W. Wood".
Guenter Spitzing he mentiones three theories for this Wood effect over time:
1) Reflectance of IR on chlorophyll
2) Fluorescence of chlorophyll
3) Transparence of chlorophyll

- It seems as if chlorophyll can be excited by visible light to 
fluorescence in the red and dark red spectrum, but this effect is too 
small to be recorded in an IR image

- Chlorophyll doesn't reflect IR

- The bottom of the leaves - in particular the with air bubbles 
filled so called sponge-parenchym - reflects IR as well as visible. 
So leaves would appear white even to the human eye, if they weren't 
embedded in chlorophyll. Chlorophyll absorbs light up to 680nm; 
therefor the visible part of the light will never penetrate the leave 
to the level at which it can be reflected. Light above 680nm can 
penetrate the chlorophyll easily, and will then be strongly reflected 
by the sponge-parenchym. [note wj: I think this also accounts for the fact 
that most leaves are more visibly white on the bottom than on top, since 
sponge-parenchym sits on the low half of the leaf]

--
Bye,

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The difference 
between men and boys
is the price of their toys

<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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Topic No. 22