Re: SEM follow-on question

[T3D John Bercovitz <bercov@xxxxxxxxxx>]

>> OK, so now we know George is right and you don't get
>> distortion in an SEM pair by using a tilt table.
 
> WHOA!  Hang on here.  
>
> Am I right in assuming that a SEM images from the top of the specimen? 
> and the tilt table is nominally sitting in the horizontal plane?
> and the tilt axis is parallel to the SEM camera's imaging plane?
> Am I also right in assuming that a SEM images in a similar manner to
> the way a conventional camera operates (and not for example like
> the way an Xray is taken)?
 
I don't know - Maybe George or Ted or... could help us?  I was 
understanding that it was like an X-ray is taken but I may well 
have got it wrong.  Not that it's transmission, but that somehow 
in some other way it simulates transmission with a collimated 
beam.  Did I hear you incorrectly, George?
 
The way I understood it, the rays coming to the image plane 
were parallel to each other.  So it would be like illuminating 
a slide transparency with a perfectly collimated light source.  
If you do that, anywhere you intercept the beam, you get an 
image.
 
Let me try an example.  (Which Andrew doesn't need.)  Let's draw a 
square on a piece of paper and lay the paper on a table.  We take 
two shots of the paper with a toe in of six degrees inclusive.  
Say the square is 50 mm on a side or some such and you take the 
picture from 100 or 200 mm.  When the camera is on the right, the 
right side of the square subtends a larger angle at the camera 
than the left side of the square does.  When the camera is on the 
left, vice versa.  So now you have two pictures and neither one of 
them is a square - they're both trapezoids but in one, the left 
side is the long side, and in the other, the right side is the 
long side.  If we view the pair, it bulges out at us.
 
Now we haul the camera back to 10 meters and put on an appropriate 
tele so that the square occupies somewhat the same space on the 
film.  We again shoot toed in with six degrees inclusive.  This 
time we see the trapezoids are nearly squares and when we view 
them, they barely bulge out at us at all.
 
If I understood George correctly, the SEM makes it as if we were 
standing infinitely far away when we took these toed in pairs.  In 
that case, both images will be square.  So when we view the pair, 
we see a flat square.
 
George, are the words I've attributed to you correct?  I may have 
misunderstood you in which case, I take it _all_ back.  888-)  I 
may also have completely blown this analysis in which case I'd 
greatly appreciate being shown where I've erred.
 
Thanks,
John


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