SEM 3d/toe in

[T3D Edward Gosfield III, M.D. <gosfield@xxxxxxxxxxxxxxxxx>]

As a practical matter, i agree with George that stage tilt is the standard 
method of creating 3d SEM pairs.  At least that is what one is taught as the 
proper method.  I was taught to use 6 degrees of tilt. In the textbooks of 
SEM that i have seen (sorry-don't have them here) there was no particular 
justification given for the technique--or even much discussion.

I think George has done more total SEM work than I, and I get the impression 
that most of this was of fairly 'flat' specimens--coatings, semi-conductor 
surfaces, etc.  My work has been on tiny biological specimens (3 day chick 
embryos) looking at individual cells' behavior, at mags to about 15000.  In 
these shots i saw no obvious geometric distortion.  But for amusement i did a 
lot of shots of insects, mushroom spores, butterfly wing scales, etc.  

In these much larger specimens, at much lower magnifications, (in the range 
of dozens to hundreds, as opposed to thousands, of diameters mag) there was 
often considerable 'stretch' in the stereo image, and exaggeration of 
apparent depth--as one would expect with tilt/toe-in or in this case actually 
subject rotation.

A quick review of Ferwerda didn't reveal much specifically discussing 
microscopy/SEM except a mention of the extreme depth of field of SEM compared 
to standard microscopy and macro work.  In view of the stretch i have seen, 
and even some 'close-up misery', i think the combination of magnification, 
working distance, specimen size, front to back depth, and degree of 
convergence, can create a final result with similar problems to 
'infinity-to-close-up' normal scale 3d photos. Looking over a couple of my 
SEM pairs, the window seems set fairly far back from the 'nearest' specimen 
surface.  My pairs are not precisely mounted, since i didn't even know what 
mounting was, in those days.

So when one refers to the shots being like those done at infinity, are you 
referring to the window, or the geometric description of the ray path from 
the specimen to the virtual observer, or to the perspective created?

As a practical matter, i think the degree of translation possible in small 
(micro range) SEM specimens is too limiting for the depiction of depth, and 
specimen tilt is necessary.

Can anyone provide citations for actual discussions of stereo SEM?  I 
remember someone inquiring on the microscopy news group, and even on P3d, but 
i don't recall any answers.  There must be some discussions in the materials 
science literature.

BTW--i am not sure how long the (sshhhh) secrecy of this list will last, 
since Alta Vista and ?other search engines seem to point to lots of postings 
on listgroups, and a search done on our various discussion topics will reveal 
our existence to the curious.  Meanwhile we can enjoy our secret clubhood.

thanks for the list, John

ted g


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