From: Jerry Hudson 
To: atm{at}shore.net
Reply-To: Jerry Hudson 


Kevin,

I use this jig for testing convex curves:

      near UV
    o Lamp (Hg) o Eye

               === Green filter







     ================
         Fresnel magnifier


        ---------
        |       |  Concave master
        ---------
        ---------
        |       |  Convex surface being tested
        ---------

The mercury lamp is just a bug-catcher lamp with the high voltage supply
carefully taped off. The Fresnel magnifier comes from Office Max (I'm sure
other companies like Staples have them), and is roughly 8 X 10 inches,
plastic.  I built a jig to hold the lens and lamp at various distances off
the table.  Adjust until you see nice even illumination coming from the
interface, and use wedges of tissue paper betw. surfs., pressing down until
you get fringes.  I've used this rig on everything from flats to 39 inches
convex radius.  No limit implied; you could easily add another magnifier if
needed; they're cheap. You could also turn the concave surface upward and
dispense with the lens, but I think the working distance isn't so
convenient.  Depends on the radius of course.  Texereau describes this
setup in his book.

A neon lamp will work, too, but it is harder to catch the returning rays
with your eye from such a small source.  (Don't need a filter in that
case.)  For a filter, I've been using just a green piece of plastic.

This rig is called a Fizeau interferometer.

- Jerry Hudson

--- BBBS/NT v4.00 MP