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

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

----- Original Message -----
From: <abram.klooswyk@xxxxxx>
To: <photo-3d@xxxxxxxxxxxxxxx>
Sent: Sunday, February 25, 2001 12:45 PM
Subject: [photo-3d] Re: X-ray stereos


> ...For shells are _asymmetrical_ objects, mirroring makes them
> pseudo... > ...In the same way a stereo x-ray of a left hand would appear
as
> a right hand when the images are interchanged or reversed...

Exactly the point.  In the absence of clues other than asymmetry, how do you
tell by left/right orientation alone whether you are looking at a normal
view from one side or a pseudostereo from the opposite side?  In the example
of the shell, a clockwise spiral from one side would appear as a
counter-clockwise spiral from the other side, and if you did not know if the
first or the second image were pseudo, how would you tell which one was
correct, even if you knew the correct direction that the spiral was on the
real subject?  You would have to know not only the correct direction, but
also from which side it was imaged.

>  >...The far side of an X-rayed object of
> >equal density will appear very slightly more dense than the
> >side closest to the source.
>
> This not often of much help, but sometimes definition is.
> Because details closer to the tube are slightly more magnified,
> they also become slightly less sharp. But this only works when
> object size is relatively large compared to tube-film distance.
> This is sometimes the case in lateral skull x-rays.

With smaller objects, your tube-film distance can also be smaller, as with
dental X-rays.

> Being off topic, a comment on  Ralph Johnston's friend who
> said on the flower stereo x-rays:
> >"It requires very low energy (kilovoltage) X rays and high
> >resolution, fine grain industrial X ray film, and manual
> >processing.  It is just the opposite of what's done these days
> >in medical X ray where you want high energy, very rapid
> >exposures and fast film to reduce radiation exposure to the
> >patient. (...)"
>
> ...This is not always true, and especially not when you compare
> it with flower X-rays, which is quite comparable to a special
> medical x-ray technique...
> ...In fact dedicated mammography x-ray machines can be used
> for objects like flowers, onions, grapes...

Certainly medical X-ray techniques can be used.   But, for inanimate objects
such as flowers and seashells, where the total amount of radiation is not a
factor, it is possible to use both smaller apertures, and slower films.  The
necessary exposure can be attained either through higher voltages or through
longer exposure times.  It should be noted that changing the voltage of a
particular source will also affect the frequency (and wavelength) of the
radiation, which will affect the apparent gamma of the image on the film
(high opacity to low opacity ratio).  This is what radiologists mean by
"hard" X-rays and "soft" X-rays.

Changing the aperture size mainly affects the apparent sharpness, as a
larger source area tends to make edges less distinct.  Increasing the
subject-to-film distance also tends to soften the image, in addition to
creating an apparent magnification in parts of the subject closer to the
source (farther from the film).  This magnification is not an optical
enlargement, as with a lens, but rather is a matter of the relationship of
the cone angle of the radiation to the subject and the film.
ile the source-to-film distance affects the amount of radiation available.

> I have also seen stereo x-rays of female breasts. This are
> cases of accidental stereo, for taking two x-rays of the same
> body part on the same day is not justified, but sometimes two
> x-rays one year apart form a reasonable stereopair.
>
> Abram Klooswyk

Yes, also you can ask your dentist to give you your dental X-rays after
he/she is through with them.  Keep them until the next time your teeth are
X-rayed, and compare them stereoscopically.  Chances are, there will be
enough difference in the positioning to allow for stereo.  I have done this,
and although there was not much stereo depth in the opaque teeth (they
produce essentially shadowgrams), the surrounding soft tissue was
interesting.  Blood vessels produce an intricate, multidimensional, lacy
pattern.

JR


 

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