From: "James Lerch" 
To: "Dale Eason" , "ATM List"

Cc: 
Reply-To: "James Lerch" 


----- Original Message -----
From: "Dale Eason" 


> James there is another possibility.  I was looking at
> your picture of the grid.  I had done the same with my
> setup.  I had a white board with black lines made by a
> black sharpie.  It had high contrast.  When looking at
> it with the camera on the tester the contrast was much
> lower.  It appears very low in you image as well.  I
> am pretty sure that this is caused by flare in the
> lens system.
>
> The flare mixes the light to where it should not be.
> This could cause bad readings.

Hi Dale,

The loss of contrast could be a result of lens flare, or internal reflections.

However, keep in mind my "lens system" is just a single dual
convex lens with a 35mm focal length and fabricated from a single piece of
unknown glass.

The lens is 1 of three lenses included with a Surplus Shed eyepiece lens
set I purchased some time ago.

I had 'assumed' the loss of contrast was a result of:

#1  Using a broad band white light source (compact fluorescent bulb)
#2  A non-acromat lens
#3  A color CCD sensor that is also sensitive to near IR

I figured the three items combined would make for a terribly low contrast
image as the lens is by no means "color corrected"!

I've also been thinking about how reflections or 'flare' might interfere in
the Foucault test, and here's my thoughts:

#1 the light from the Foucault test passes by the knife edge and thru the lens.
#2 the lens focuses the light onto the CCD
#3 some of this light would reflect off the CCD glass cover back to the lens
#4 some of this light would reflect off the backside of the lens (which is
convex)
#5 the light reflected off the backside of the convex lens would make it back
to
the CCD

The question is, how much light would make it back to the CCD after 2
internal reflections, and how would it appear on the CCD?

James

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