Re: [photo-3d] X-ray stereos

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

----- Original Message -----
From: "Peter Abrahams" <telscope@xxxxxxxxxx>
To: <photo-3d@xxxxxxxxxxxxxxx>
Sent: Friday, February 23, 2001 8:42 PM
Subject: [photo-3d] X-ray stereos


> >But the funny thing about stereo-x-rays is that both work with
> >parallel as well as with cross-eyed viewing...

> ...You might think that either way - correct or reversed - would do, but
the
> orthoscopic view looked better & was easier to view, than the pseudo view.
> This might be because the foreground was slightly more magnified than the
> background; also we used a very large disparity between L & R (after all,
> some shells are almost spherical).  But I believe there was one other
> reason why there was a subtle difference: almost all shells corkscrew in
> the same direction, like a 'dextral' corkscrew -- not like a 'sinister'
> corkscrew.  If I remember correctly, pseudo mounting causes the spiral to
> reverse -- but sometimes even that was hard to tell...

> Peter Abrahams   telscope@xxxxxxxxxx   The history of the telescope &
>    the binocular:   http://www.europa.com/~telscope/binotele.htm

I doubt that the direction of the spiral would be much of an identifier,
since your visual impression could be that of looking at the spiral from
either point of view.
Sort of like looking at an X-ray of a human head, where you cannot tell if
you are looking at it from the front or the back without an identifier.

And size of parts in themselves is not a great clue, since we don't really
know if one particular part is really closer or larger.  However, the
material of the part can sometimes be a clue, as it is often (but not
always!) of more consistent thickness than variations in the size of the
parts themselves.

However, experienced X-ray readers use other (often extremely subtle) clues.
One of these involves such great experience with a particular subject that
they "know" which direction that they are probably looking at by common
practice in subject orientation procedures and X-ray "gun" position.  A
medical lab will always image a particular part of the body from the same
"standard" point of view, unless the doctor requests otherwise.

When this happens, a "descriptive" accompanies the radiogram, specifying
this unusual POV (and the subject position), as well, very often, as the
placing of a metal position identifier in a noncritical part of the image
area.

The identification becomes slightly easier (very slightly) if you know some
things about how X-rays work.  An X-ray source radiates 360 degrees equally
in all directions.  X-rays cannot be focused by a conventional lens.
Therefore control (sometimes referred to, not quite accurately as focusing)
is primarily with shielding.  You surround the source with a lead box, with
a small hole in one side, which is the exit of the "gun".  Although it is
possible to have a slight effect electromagnetically (like in a CRT), this
is usually not all that useful in imaging applications, except in so-called
X-ray "scanners".

The further from the source, the less penetration.  Now things get
interesting.  The far side of an X-rayed object of equal density will appear
very slightly more dense than the side closest to the source.  Unless the
object is fairly large, this difference is hard to detect.  At least from
the standpoint of being able to glance at an X-ray image and say "yeah, that
must be the far side, because I can see it is denser".

Actually, you probably do see it.  But, it is so subtle, that you are not
aware that you are seeing it by direct observation.  Rather, the brain
processes this very subtle variation in opacity and you "perceive" or "feel"
that you are looking at it from the correct (or incorrect) side.

It would be interesting if someone were to do a "double-blind" test of this
concept.  I have a feeling that you would be right more often than you were
wrong.  Any takers?

JR



 

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