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


Hello Again :)

In an effort to actually calculate reflectivity I Created a Dark Frame
image (http://lerch.no-ip.com/atm/Flat.zip (76KB))

Wrote some code to Find all non zero RGB pixel values for each of the 4
images.  This code then spit out the Average pixel value for each image,
results below

Dark Frame = 1.8
UnCoated = 4.6
OLD Silver = 44.1
Aluminum = 54.7

Next I "Solved" for the only possible known value I had, (the
uncoated mirror being equal to 4% reflectivity) by using the folowing
equation

(AvPixel - DarkFrame)*x = 4%

So for the Uncoated mirror image, this would mean

(4.6-1.8)X = 4 or X = 1.43

If this is correct, solving for both the Silvered and Aluminium coated
images, I get the following Results:

SILVER = (44.1 - 1.8) * 1.43 = 60.5% Reflectivity AL = (54.7 - 1.8) * 1.43
= 75.6% Reflectivity

Do these seem like reasonable numbers?  (I think they do... :)  However, I
see two procedural items that could improve the results:

#1 Set Camera White Balance to Manual!!  (I forgot and left this setting to
Automatic, Consequently who knows what effect this had on the resulting images!)

#2 A little longer exposure would have been nice! (I'm concerned the
uncoated image is to faint, Consequently its value is suspect as its so
close to the noise floor.  Had I gone with just a slightly longer exposure,
I'd feel more confident in my numbers)

In any event, this has been an entertaining excersise ;)

As always, comments and suggestions are welcome!

Take Care,
James Lerch
http://lerch.no-ip.com/atm

--- BBBS/NT v4.00 MP