From: "James Lerch" 
To: "ATM List" 
Reply-To: "James Lerch" 


Thanks Jerry, Very good suggestions!

Only thing to add is the original 'Goal' of this test was to see what the
difference between old Silver and Fresh Aluminum turned out to be.  The
resulting 60% for silver and 75% for fresh Al, while not real numbers, the
difference between the two seems reasonable.

At the moment, I'm on the border line between pursuing this further and
"Who Cares, It works pretty damn well as is!" :)

Thanks again,
James


----- Original Message -----
From: "Jerald F. Wright" 
To: "ATM List" 
Sent: Saturday, January 11, 2003 12:45 PM Subject: Re: ATM Reflectivity
Measurment Part2


>
>
>
> I'm  not sure what specific reflectivity you are looking to find.
>
> Are you looking for reflectivity throughout the visual range?
>
> I may be thinking wrong here but it seems to me to get an accurate
determination
> for some range such as visual range for example that you would need to
take
> readings at many wavelengths through the range.  The resulting plot of
> reflectivity may be sufficiently linear that discrete R G B will give a
good
> result.  But I am not sure.
>
> I have seen in a number of sources a number of something like 88% visual
> reflectance for aluminum.  New of course. So your number could be quite
good if
> the aluminum is not new.  Seems to me the only way to know would be a
calibrated
> light source.
>
> It may be a good idea that rather than compare two reflective surfaces,
use just
> one surface and place a filter of known absorption in the light path and
> calibrate by the known loss through the filter and without the filter.  Or
you
> might image your light source directly without reflection on to the ccd
and then
> using the same imaging system (lens) reflect off the mirror. I would think
that
> it would be better to have the f number of your source to be higher than
the COC
> f number of the mirror so that you know all of the light from the source
is
> reflected back into the ccd.  In fact a defocused image on the ccd may be
a way
> of comparing.  What size defocused image achieves the same pixel values
after
> reflection as some certain size defocused image size without reflection.
Then
> the ratio of the number of pixels involved in each image would
"reflect"
the
> reflectivity.
>
> Another Idea may be to reflect the light source off an uncoated flat and
vary
> the angle of the reflection off that flat there by varying the percentage
of
> reflection.  Compare angles of reflection that achieve equal pixel values
after
> reflection off the surface in question and the calibration is a physical
> property of the test and not to some possibly arbitrary external
reference.
>
> Just a few ideas without having taken time to think of what could be wrong
with
> my thinking.
>
> Jerry
>
>
>
>
>
>
>

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