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Re: [ATM] How do I test a convex surface?
On Tue, 27 Mar 2007, Peter John Smith wrote:
> I found Johnson's baby Oil much lower than suggested here which is the main
> reason I am replying.
>
> Johnsons Baby Oil 1.458
Hi Peter!
I would never argue with you as I recognize you as an expert. You
can find a number of websites where people are using Johnson's baby
oil and others because they (are claimed to) have an RI same/close
to amber or pyrex, claiming values from 1.5 to 1.54 for both. For
instance:
http://baltictreasure.com/tests.html
http://www.physics.gatech.edu/demopage/6/6a4030.htm
It is perfectly clear that some pretty irritating chemicals have
convenient RI (xylene, urea formaldehyde). It is also clear most
common oils are below 1.5. Some older ATMing texts specifically
described using mixtures of chemicals with different RI to get a
match, and this matching procedure I can speak of as a common
laboratory practice. I am not sure oils are so hopeless.
This is all temperature and wavelength dependent, and there is a
lot of variability in glass types, so there is room to play. The
trend is for RI to decline with increased wavelength, so red
light could help. For low iron plate glass, I have plotted RI vs
wavelength (numbers provided by manufacturer to construct this),
and these values are not so favorable compared to the RI values
for liquids you provided (although these also must have wavelength
dependencies):
http://www.canit.se/~dlwebb/catadioptric/310schmidt/optiwhit.jpg
On the other hand, Pyrex is listed by some sources as 1.473 @ 589.3 nm.
I have seen a similar number for car headlights (also pyrex?). This
value looks quite compatible with sugar water and many of the oils you
just listed, and note that one source I list at the top of this email
tested oil against Pyrex, claiming both to have RI around 1.5. Compare
to this....
http://www.valleydesign.com/pr16.htm#sodalime
http://www.valleydesign.com/pr16.htm#7740
Given the above, I would test directly and see what happens (I
have gotten some close matches). I wish to end this with a question
for you, Peter. In your opinion, does this optical test really
require an exact RI match anyway?
Dominic-Luc Webb
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