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Stewart: technique. There are other techniques, as you are aware, which
SR
can provide off-center information, but we are not using those techniques.
Gates: I think perhaps I could make a contribution here, if I could have
my slides please. (Sections 8-10 of Gates' paper) These, Fig. 21 and 22,
are just pictures of X-ray reflection, testing the flatness of a surface.
I won't take time on that. But I want to move on to these diagrams, Fig.
23, and Fig. 24, which show the difference between contact microradiography
and projection microradiography. These are pictures from the paper which
is in the Proceedings in which the possibility exists of varying the scale
of the image on the photographic plate by changing the distance 'b'. The
source of the X-rays is formed by the focussed electron beam on the thin
sheet of foil. The scale of the record is given by the ratio b/a. In the
technique of stereomicroradiography in which this was used the aim of the
work was in determing the location of microscopic inclusions in metal alloys
and things of that kind. Techniques to provide stereoscopic facility in-
cluded several possibilities. One of these is to move the electron beam
across the foil so that the X-ray source is acting as à perspective center
and can be moved across successively to give parallax projection of the
inclusions on to the photographic plate. There is then a simple geometric
problem to relate the scale of the subsequent stereo photographs, knowing
the microscope's geometrical parameters, to determine the size and position
of the particle within the alloy.
Another technique, as I believe Dr. Ghosh has also used, is to tilt the
specimen. One of the contributions (Section 10) in my paper makes a mention
of the use of a tilting stage in the transmission electron microscope. When
the stage is tilted the particles appear in different relative positions,
and the parallax provides the measure of height differences. A diffraction
grating, possibly of the same sort that Dr. Ghosh has used, was employed as
a spacing standard. Further, the technique was used to determine the groove
depths (typically only 5-50 nm) on X-ray gratings themselves, as shown in
Fig. 26. Perhaps the most extreme claim for sensitivity is that made in the
work on the surface topography of evaporated gold films, as shown in Fig. 25.
Height determination to about 1 nm are made within areas on the surface only
20 nm square.
Gates: I think we have had a fair airing of the points which have arisen
from these papers, and we will close the discussion of the papers in the
second half-session, on X-ray and related techniques.
I wonder if we now could go back to the first half-session of papers on
holography. Are there any points arising from those where people would
like to make general or particular comments about the function of holography
as a constituent part of measurement?
Vigneron: A point of information only. The MATRA Company in collaboration
2m the French organization ETCA, has conducted tests of three-dimensional
measurements using holography. The tests were made on both real and virtual
images. The results and information from these tests are available from the
MATRA Company.
Gates: I think we should now conclude discussion of details in the papers
presented and move on to the final part of the session, to discuss in
broader terms the relevance of the topics raised.
