Digitizing X-radiographs

David Harvey <topladave [at] gmail__com> writes

>Rachel Danzing <rachel.danzing [at] brooklynmuseum__org> writes
>
>>We are interested if anyone has digitized their x-radiographs and
>>what their experiences were.  Were any equipment or special
>>techniques found to be useful?
>
>In examining radiographs of various objects from archaeological
>materials to architectural metals I have often found that the film
>captures significant details that can be only seen through a
>binocular microscope ...
>...
>Also, do the new digital industrial x-ray units possess the same
>resolution and clarity that film has?

It depends on how fine a grain film you are using, factors relating
to your X-ray unit and how you are using this equipment.

The resolution of a film image is not just controlled by the film
grain but by the X-ray set-up you are using.  Your actual resolution
will be limited by X-ray scatter generated by all the materials
through which the X-rays pass, including the object itself and the
film, and geometric factors relating to the X-ray equipment, such as
the effective focal spot size of the X-ray beam, the X-ray source to
film distance (SFD) and the object to film distance.  Because of
these factors, it may not be, for instance, possible to appreciate
the difference between a fine grain film and an ultra fine grain
film when using a cabinets type unit with restricted SFD.  As a
result the actual resolution of the film may not be that different
to that of the digitized image.

The 50 micron pixel size we get with the industrial X-ray scanner is
excellent for digitising whole radiographs which can be examined at
over 10 times magnification (linear) before pixelisation becomes
much of an issue. Scanning 16 bit greyscale images at this
resolution produces large archive files.  430 mm x 350 mm films
produce uncompressed archive files of over 120 (JPEG copies with
minimum compression are about 20 to 25 MB).  If the pixel size was
any smaller these files would become unmanageable for most users.

On the rare occasion that I have needed to record a detail which
could not be adequately captured with the X-ray scanner I have taken
an additional close-up photograph with the film on a lightbox, using
a relatively high resolution digital camera.  These images can be
magnified much further with out pixelisation.  However because the
camera has a lower Dmax and narrower dynamic range than the
radiographic film, and conversion to a greyscale image reduces the
bit depth, the captured image is not very useful for exploring by
DIP.

Sonia O'Connor
Research Fellow in Conservation
Department of Archaeological Sciences
University of Bradford
Bradford
West Yorkshire BD7 1DP
+44 1274 236498 (office)
+44 1274 235210 (lab.)
Fax: +44 1274 235190


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                  Conservation DistList Instance 19:48
                  Distributed: Friday, March 31, 2006
                       Message Id: cdl-19-48-003
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