Re: X-Ray and Colour?
Elton N. Kaufmann - Cycloid Fathom Group (cycloid@cycloid-fathom.com)
Wed, 29 Oct 1997 23:11:22 -0600 (CST)
Color is a meaningless concept outside the range of human vision. It's only
a perception. So the question may be, "can images using x-rays be recorded
from a single object with differing contrast from one part of the image to
another?" The answer is yes, indeed! There are many variations, some of
which have already been mentioned in other postings. The x-ray source will
have a broad distribution of wavelengths up to a maximum that depends on the
voltage of the tube. The source spectrum will also have "characteristic"
lines (sharp spectral features) of great intensity that are at wavelengths
determined by the x-ray tube anode (copper or molybdenum or tungsten, etc.).
The image can be formed by reflection from an object (very weak process), by
transmission (which the shadowgraph analog), or by scattering (diffuse
usually) and or even diffraction from the atoms in the material. There is
also the possibility of exciting florescence of new xrays that would be
emitted in all directions form the object and contribute to the image. What
one gets depends on the relative intensities of all these effects for a
given case.
Interposing materials between film (or digitally compatible detectors like a
CCD chip) and the object can change the contrast in different ways. As
pointed out in another posting, it can simply absorb like a neural density
filter. But absorption lengths (that is the thickness needed to reduce
intensity by about x3) depend on wavelength. So if the source is broad band
or has several characteristic lines, these different wavelengths will be
absorbed more or less (the high energy, short wavelength always gets
absorbed less). The most interesting case is when the material has a
characteristic line (or when the object has one or more of these in
transmission mode). Then big contrast differences will occur when the
incident x-ray wavelength is shorter versus longer than the line in the
absorber. This is exactly how x-ray tomography is used to image different
chemical elements in the presence of others. Eg., one could easily "see" a
blob of gold inside a block of feldspar(for you prospectors out there).
In the end, once you have a bunch of alternative contrast images of the same
object, you could arbitrarily assign different visible colors to these and
by computer or by printing with color filters onto color paper or film make
a really weird rendition.
Sorry for the length of this message. All of this info is straight
out of elementary physics and chemistry texts related to chemical and/or
materials analysis in the lab.
Good luck with this,
El
ton K. (cycloid), a physicist in photographers clothing.
At 07:26 PM 10/27/97 +0100, you wrote:
>
> Hello,
>
> If you take 3 sonsecutive photographs through a red, a blue and a
>green filter, the resulting pictures can be used to reproduce the original
>colours of the subject. Taht's the principle of colour separation as it's
>used in the offset procedure, for example.
>
> Now I wonder what happened if you took 2 or 3 X-Ray photographs
>through a same object, interposing successively 2 or 3 sheet of different
>opaque materials. Could these radiographs be used to elaborate a coloured
>picture, which would permit the optical distinction of different materials?
>
> Anybody has proved that? Is there some literature about it?
>
> And another question: If it does'nt work, why not?
>
>
>Kind Regards
>
>---
>
>
>
>
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