Re: TECH3D digest 7

[T3D Edward Gosfield III, MD <gosfield@xxxxxxxxxxxxxxxxx>]

I like George's explanation and analogies, and i find them intuitively
convincing as well.

"---snip----
> ... the key point is the magnification!
>If the magnification is low and the working distance small, then you 
>cannot assume orthogonal projection.  Tilt will introduce keystone
>distortion just a a regular tilt with a camera.

This is completely consistent with the results i have seen, in which some of
my larger insect specimens, in stereo pairs done at low mag., produced
images with stretch, etc. 

>However, at higher magnifications, only a tiny area is being "imaged"
>(scanned).  In this case, you can safely assume orthogonal projection.

and my pairs done at higher mag, of e.g. spores, looked much more natural.

I agree that the 6 degree tilt is only conventional, or historical, or what
everyone gets told to use, but not necessarily the best point of view.  I
had good results with less tilt in some cases, but worse results with more
tilt, and also got worse depth of field with more tilt. (Remember that depth
of field in SEM is inversely proportional to tilt.)

I also agree with John Vala that it isn't very helpful to think of the EM as
any kind of optical lens--at least from a 'geometric optics' point of view. 

The way i think of the set up sometimes is like a table top full of
ping-pong balls.  I am standing above the table with a powerful garden hose
(no nasty jokes, now)  sweeping it in a raster pattern across the table top.
As it hits the balls its tiny stream is reflected off in various directions,
which are determined by the angle of incidence of the stream with the ball's
surface. 

You have to imagine here that the ping pong balls are the emission source of
the reflected splash to make the analogy work.  It is the splashed off
droplets, which are collected by some kind of counting device above the
table, which provide a signal whose instantaneous variation creates the
'map' of the balls and surface of the table.  That is where the lens analogy
breaks down. 

I think George's analogy of 'long' focal length and 'narrow' point of view
with ref. to magnification, and specimen size is correct.  Once the beam
hits the specimen i don't feel comfortable with the optical analogy any more.

I don't think the beam characteristics affect much of the image
characteristics other than ultimate resolution.  Field of view, specimen
size and depth, tilt, working distance, and final mag are all the important
factors.

( i am not here considering scanning electron microspectrography, in which
xrays or gamma rays are emitted by the specimen, which has been bombarded by
very energetic beam electrons, and the emmitted rays are measured and mapped
by the collecting system, providing a map of the elements in the specimen.
George may have done a bunch of this, but i know almost nothing about it.)

ted g


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