Re: ATM Lateral Wire Test

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Bob Walker wrote:


>   I tried using the LWT, but the images were so dim and
> indistinct that I think I must have done something wrong.  The way I
set up
> the test apparatus was by using my Foucault/Ronchi tester.  It has
moving
> source, a 3/16" diameter blue LED with a very simple clip in front
of it
> into which I can slip either a Ronchi grating or a carefully lapped
utility
> knife blade for a slitless Foucault tester.  What I did was grind
away the
> edge of a utility knife blade until it was shaped sort of like a C,
and glue
> a very thin (.004") wire across where the cutting edge used to be.
I then
> slipped this into my tester with the lower part of the wire
extending across
> the center of the LED and the upper part rising above it where I
ccould look
> through it.  Just like a single line on a Ronchi grating.  The
results were
> much harder to see and much more ambiguous than using a Couder
screen.  Did
> I do something wrong, or is this test just tough to perform?

I believe it would be a lot easier if you use a pinhole or slit for a
light source - a slit has the advantage of lessening the influence of
corneal irregularities. I don't think the width is very critical. I
have used a laser diode run at low current - it is as close to a
point-like light source as you could possibly wish. I expect you have
thought of it, but a tester for the LWT needs a precision measurement
of the sideways movement (but not to-from the mirror - this is
normally fixed) but a dingbat that works for the Gaviola test should
work for this (most Foucault testers don't do this job). Another
idea - for a wire, I have made a thin scratch or rather cut with a
sharp utility knife on a piece of clear acrylic - by varying the
pressure, I can select a "wire" that looks good - it's not that
critical.

Nils Olof