Re: [ATM] How to test a mirror blank?

[Aaron L Taylor <lists@subgeniuskitty.com>]

For reference, the polarizers in the set of photos are 8.5"x11" in size.

I tried the idea suggested by Jarvis and Tom of using an LCD screen as 
the backlight for the mirror. This time, there was no evidence of the 
strong stress that the previous test showed. Instead there was one 
stress arc that seems very mild. I was also able to see every piece of 
dust on my LCD. :-)

I was pretty sure that what I saw in the first test was stress in the 
mirror. Previously, when I had rotated the mirror, the lines had rotated 
with it. In order to verify this, I duplicated the test and saw the 
lines. I then rotated the mirror and the lines did not rotate. The only 
thing I had changed was the first polarizer. Originally I had taped it 
to the mirror as I was rapidly running out of hands to test with. This 
time I had my girlfriend holding the polarizer in place when the mirror 
was rotated.

It turns out that the horrid stress marks I saw originally were from the 
protective sheet on the polarizer. After removing a thin plastic sheet 
from the polarizers surface, the lines went away and I saw the same 
image as when I used the LCD.

Here are the updated images:

http://www.atmlist.net/contrib/lists-at-subgeniuskitty-dot-com/mirror_strain/mirror_strain_lcd_01.jpg
http://www.atmlist.net/contrib/lists-at-subgeniuskitty-dot-com/mirror_strain/mirror_strain_lcd_02.jpg

The mirror is rotated 180 degrees between the two images.

Does that level of strain in the glass look acceptable?

Thanks,
   -Aaron Taylor



Jarvis Krumbein wrote:
> It's hard to tell from your pictures as to what you've got there.  How
> big are your polarizers?  If you have a lap top or an LCD display for
> your computer, use that with a blank white screen for your light source. 
> It's already polarized and you can see the whole mirror at one time. 
> This will give you a much better idea of the strains.
>
> Jarvis
>   

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