Re: Infrared and Water -- A robust debate -- <SMACK!>

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

Steve Hodges wrote:

>
> > Guess what?  If water attenuates Near IR by 10% each centimeter, how many
> > centimeters of water does the light need to pass through until 50% of the near IR
> > is attenuated..?
>
> Lets see T = t^n
>
> where T is the total transmission,
>       t is the transmission per unit
>       n is the number of units.
>
> and A = 1 - T
>
> where A = total absorption
>
> so A = 1 - t^n
>    0.5 = 1 - 0.9^n
>    0.5 = 0.9^n
>    ln 0.5 = n ln 0.9
>    n = ln 0.5 / ln 0.9
>      = -0.6931 / -0.1054
>      = 6.58 cm
>
> >
> > <Final Jeopardy Music Playing>
> >
> > Approximately 6.2 cm, according to YOUR book..
>
> Poor maths in that book...
>

Steve, Thanks for the correction...

>
> > Last time I checked, that means at about 15.75 inches, 50% of the available IR
> > light will be absorbed...
>

Yeouch! Guess who reversed his correction from 2.54 cm/inch to 2.54 inches/cm... Bonehead
mistake on my part... Just like the guys who transposed meters and feet at NASA crashing a
Mars probe in the process..  Hmmm. Maybe I can get a job at NASA...

>
> Ooooh!  Last time _I_ checked, 6.58 cm was approximately 2.6 inches.  So
> after a round trip of 2.6 inches (1.3 inches of water - the light has to
> go both ways) you lose a stop.
>

YUP!

I had forgotten about that return trip too...

Goes to prove I should not stay up all night and then do math...

>
>
> How about, you lose a stop for each 1.3 inches of water depth (by these
> figures).
>
>

Can't argue with you there...


> p.s.  Note that I am just correcting sloppy maths,

I would have been lucky to get any credit at all for those problems in school.  My
instructors are spinning in their graves, assuming arguendo that they are dead...  And I got
a 740 on the Math section of my SAT!  Talk about being red-faced...  Excuse me while I join
some penitents flagellating themselves in celebration of Lent!

> not making any
> judgment on the accuracy of the quoted figures for absorbency.  But I do
> tend to believe what I see in the graphs.
>

As do I...

>
> Also I note that my swimming pool is almost totally black when viewed
> through a filter that permits only the very near IR (just visible)
> light.

Now if we could just get Rolland a ticket to Australia to see your pool.... LOL

Actually, that reminded me... Geez what an idiot I can be sometimes...

I have some NEAR-IR HIE water polo shots from the Goodwill Games!  They looked like crap
because of how the water rendered, but the negs should be here (as opposed to at the
office)... Will try to find one that is usable and run it through the scanner...

Again, Steve, thanks for the corrections...

"Ozzie, Ozzie, Ozzie, Oi!"

The preceding was sponsored by the University of Wallamaloo...  ;-)

Keith

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