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Science Forum Index » Optics Forum » point diffraction interferometer
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| Neal Shepard |
 Posted: Mon Nov 10, 2003 11:03 am |
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Has anyone used the PDI?
How does one make the pinhole in the semitransparent material and would the
inverse ie. a black dot instead of pinhole work?
thanks
NS |
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| Elliot Burke |
| Posted: Mon Nov 10, 2003 6:37 pm |
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The observation has been made that a bad filter coating is a PDI.
You want a neutral density coating, light tranmitted through it is the
object beam.
Light diffracted by the pinhole is the reference beam.
If you are lucky enough to know a coater there may be some scrap that you
can look through.
Otherwise small particles are dusted over a substrate, coated over, and
brushed off to make pinholes.
With some care a capacitive discharge can make a spark that will burn a
hole.
Modern lithographic techniques can do a good job too, and are a lot more
predictable than other techniques.
The size of the required pinhole depends on the f/# of the beam being
interfered.
It is hard to see how a black dot would make a reference beam.
regards-
Elliot Burke
"Neal Shepard" <Neal@nospam.com> wrote in message
news:vqvdmppbsmu1d0@corp.supernews.com...
Quote: Has anyone used the PDI?
How does one make the pinhole in the semitransparent material and would
the
inverse ie. a black dot instead of pinhole work?
thanks
NS |
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| AES/newspost |
| Posted: Mon Nov 10, 2003 9:07 pm |
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Guest
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In article <3fb0214e$1@newsfeed.netlojix.com>,
"Elliot Burke" <elliot@nospamhitide.com> wrote:
Quote: Has anyone used the PDI?
How does one make the pinhole in the semitransparent material and would
the
inverse ie. a black dot instead of pinhole work?
I believe this is also called a Smartt interferometer. Used one a long
time ago to look at the phase front of a collimated laser beam, and I
believe a small enough black dot would work as well as a pinhole -- or a
tiny glass bead, or dust particle, or anything that would serve as a
small enough "point" scatterer. |
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| Elliot Burke |
| Posted: Tue Nov 11, 2003 1:04 am |
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I spoke too soon, you are right.
If the black dot was small enough compared with the Airy disk this should
work, but the diffracted energy would be smaller than from the pinhole, so
the semitransparent film would have to be denser to give good match of beam
strengths, resulting in dimmer interferograms.
Like I said, a (spark) capacitive discharge through a metal film can be
adjusted to give a usable pinhole. Sorry, I don't remember the circuit
values. If you aren't comfortable working with high voltages, don't try
this.
Smartt rediscovered the PDI, the original paper was Russian from the 1930's,
I think.
An issue with PDI's is that if you need much fringe tilt, the contrast goes
down: the pinhole isn't at the center of the Airy disk anymore, and the
reference beam intensity goes way down.
If you are making one lithographically, its a good idea to have a range of
pinhole sizes to be able to adjust beam intensity.
regards-
Elliot
re
"AES/newspost" <siegman@stanford.edu> wrote in message
news:siegman-159A72.18074810112003@news.stanford.edu...
Quote: In article <3fb0214e$1@newsfeed.netlojix.com>,
"Elliot Burke" <elliot@nospamhitide.com> wrote:
Has anyone used the PDI?
How does one make the pinhole in the semitransparent material and
would
the
inverse ie. a black dot instead of pinhole work?
I believe this is also called a Smartt interferometer. Used one a long
time ago to look at the phase front of a collimated laser beam, and I
believe a small enough black dot would work as well as a pinhole -- or a
tiny glass bead, or dust particle, or anything that would serve as a
small enough "point" scatterer. |
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| Neal Shepard |
| Posted: Tue Nov 11, 2003 1:28 am |
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Guest
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Thanks for verifying my intuition.
I got a neon transformer that produces good sparks.
Aluminized mylar for solar viewing is probably too dense to use as PDI.
Looking for a friend in the lithography industry.
NS
"Elliot Burke" <elliot@nospamhitide.com> wrote in
news:3fb07c0d$1@newsfeed.netlojix.com:
Quote: I spoke too soon, you are right.
If the black dot was small enough compared with the Airy disk this
should work, but the diffracted energy would be smaller than from the
pinhole, so the semitransparent film would have to be denser to give
good match of beam strengths, resulting in dimmer interferograms.
Like I said, a (spark) capacitive discharge through a metal film can
be adjusted to give a usable pinhole. Sorry, I don't remember the
circuit values. If you aren't comfortable working with high voltages,
don't try this.
Smartt rediscovered the PDI, the original paper was Russian from the
1930's, I think.
An issue with PDI's is that if you need much fringe tilt, the contrast
goes down: the pinhole isn't at the center of the Airy disk anymore,
and the reference beam intensity goes way down.
If you are making one lithographically, its a good idea to have a
range of pinhole sizes to be able to adjust beam intensity.
regards-
Elliot
re
"AES/newspost" <siegman@stanford.edu> wrote in message
news:siegman-159A72.18074810112003@news.stanford.edu...
In article <3fb0214e$1@newsfeed.netlojix.com>,
"Elliot Burke" <elliot@nospamhitide.com> wrote:
Has anyone used the PDI?
How does one make the pinhole in the semitransparent material and
would
the
inverse ie. a black dot instead of pinhole work?
I believe this is also called a Smartt interferometer. Used one a
long time ago to look at the phase front of a collimated laser beam,
and I believe a small enough black dot would work as well as a
pinhole -- or a tiny glass bead, or dust particle, or anything that
would serve as a small enough "point" scatterer.
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