P3D X-Ray photographs in 3-D

[Peter Abrahams <telscope@xxxxxxxxxx>]

Here's an excerpt from an unpublished article I wrote:
	Stereo x-rays are a standard technique, used by radiographers to locate
bullets, tumors, and other items of interest inside a body.  In an article
written in 1896, Elihu Thomson described an experiment with stereo pairs of
radiographs, “The effect is very curious...two heavily insulated wires
twisted together...the wires alone are seen, standing apart in space.”  He
also suggested stereo fluoroscopy (motion pictures,) and his paper was
followed by others on the topic in both English and French Journals.  The
discoverer of x-rays, Wilhelm Roentgen, took at least one stereo
photograph, a self portrait with his wife.  
	Stereo fluoroscopy was tried at the Mayo clinic in the 1950s, using an
x-ray tube with two foci and spinning polarizing filters in front of the
viewer; and switching LCD glasses are used in modern systems.  At a Naval
laboratory, the effect on the spine of impacts from ejection seats is
studied using stereo x-rays.  In England, the technique has been developed
for scanning luggage at airports.  Radiographic pairs of flowers were
featured at a recent NSA convention and in Stereo World magazine.  
...........I made a series of stereo x-rays of seashells.......There were
many technical puzzles to be solved, since shells are of widely varying
densities and thicknesses, and to capture the thin edge along with the many
layers of material in the center was a constant challenge.  Penetration by
x-rays involves a balance between kilovolts of power and quantities of
‘mass’, and a rough analogy with light is that kilovolts corresponds to
color (frequency or wavelength) and mass to brightness.  To preserve
contrast in the exposure, some materials must be penetrated with higher
kilovolts and others with higher mass.  Although the film is not too
expensive, lab time is at a great premium, and to our delight the very
first stereo we took was successful.  The inner layers of the shell could
be seen to arch up from the film surface, each layer demarcated from the
next, a truly complex and inspiring construction.  Some of the most
beautiful shells had an uninteresting inner structure, and the most
beautiful radiographs were of less glamorous shells.  Many had to be imaged
using a ‘mask’, a thick wedge of plasticene clay that blocked more
radiation on one side of the image, and allowed the thin lip of the shell
to appear on the image.  We now have about 70 successful pairs, the
beginning of an ongoing project.
	Sharing these radiographs with the public has posed some problems.  These
are 8 x 10 transparencies, and few can freeview such dimensions.
Radiologists use Wheatstone style viewers with large mirrors, and public
use of one inevitably results in fingerprints on the first surface mirrors.
 (A very unusual stereoscopic antique is the General Electric
Orthostereoscope, a radiograph viewer that uses two large rhomboid prisms
to effectively widen the interocular distance.)  Another problem is
backlighting these exposures.  An enormous lightbox would be the ideal
solution, and sheets of frosted mylar taped to windows have been an
inadequate substitute.
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I have a longer file of text on this subject I could send.
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx


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