Re: [ATM] lurie-houghton progress

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

Perfect! Thanks! Will post a photo this weekend, I
hope.
Guy

--- Mike Lockwood <melockwo@uiuc.edu> wrote:

> Guy,
> 
> Guy Brandenburg wrote:
> > Yes, convex on concave! ;-) I thought that from
> one
> > side to the other (7.5 inches) you might cut 2 or
> 3
> > fringes if you followed a straight line that would
> cut
> > as few as possible, which is what I think you
> mean.
> 
> OK, so the surfaces agree to 1.5 waves or so.
> 
> > With the float/plate glass, all you need is 2
> crossed
> > polarized plates to see if they are free of
> > inhomogeneities (if I spelt that correkly) (;-))
> and
> > strain and such.
> 
> That will let you check for strain, but not
> inhomogeneity, I believe. 
>   Hopefully the glass melt was well mixed.
> 
> > suppose that after we pushed a little bit and
> released
> > the pressure, we saw lines curving slightly like
> the
> > lines of wood in a quarter-sawn tree. (as shown
> here:
> >
>
http://www.woodfloorsonline.com/techtalk/properties.html)
> > Then what would one conclude, and what action
> would
> > one take?
> 
> I would conclude that the surfaces did not match
> exactly.  Rotate both 
> pieces so that the fringes are vertical from your
> viewpoint.  Put a 
> ruler across the pieces, aligned with the ends of
> the fringe that is 
> nearest to the center of the pieces (not at the left
> edge, not at the 
> right edge - right down the middle).
> 
> Move to a viewpoint at least times the diameter of
> the pieces under 
> test away from the test apparatus (70" for your 7"
> pieces).  Count the 
> number of fringes the ruler crosses at the center of
> the disk.  Let's 
> say the fringe bends enough that the ruler crosses
> one fringe and just 
> touches another at the center.  This means that the
> curvature is two 
> fringes, and the difference between the surfaces is
> one wavelength.
> 
> To determine what kind of mismatch is present, use
> one finger to press 
> on the center of the top piece and observe the
> direction that the 
> fringes move in.  The fringes move towards the high
> side, which means 
> a larger air gap.  Release the pressure.
> 
> Make the fringes shift slightly to the RIGHT when
> you press on the top 
> piece.  (Rotate the two pieces 180 degrees in the
> tester if this is 
> not the case.)  Now the air gap is larger on the
> right.  Now,
> 
>    1) If the fringes curve like (, the air gap at
> the center is larger 
> than the air gap at the edges, and the convex piece
> is not convex enough.
> 
>    2) If the fringes curve to the left like ), then
> the air gap at the 
> edges is larger, and the convex piece is too convex.
> 
> So, we can also say that wherever the fringes bend
> to the right 
> (compared to a straight line), the convex piece has
> a high spot.  For 
> 1), it's at the edge, for 2), it's at the center.
> 
> > Would one conclude that one of the pieces of
> > glass has astigmatic cylinder?
> 
> It's possible, but not likely.  If you get straight
> fringes that are 
> not parallel, then you definitely have astigmatism. 
> FringeXP can also 
> be used to analyze the difference between the
> pieces.
> 
> What would be good is to take a photo of the pieces
> under test, with 
> the fringes aligned as mentioned above, and so that
> they shift to the 
> right when you put pressure on the top piece.  Post
> the image and we 
> can tell you what's going on.
> 
> 	Mike Lockwood
> 
> 
> 


Guy  Brandenburg
Washington, DC
My home page:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
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