[ATM] RE: [atm_free] zone one challenge

["Koehler, Steve" <Steve_Koehler@securecomputing.com>]

Jerry,

> If you look at the images closely you will see that what you are seeing is
> something similar to the classical "doughnut"  in each mask opening...
> separately.  A doughnut, except that it is an arc. Your simulation seems
> to be showing "nulls" either side of center in each mask opening.
> Therefore the center of each of the openings is not the same as the
> average intensity. The average is located a little to the left and right
> of center in both mask openings.

This may be an optical illusion.  I updated the page to add a gradient profile
plot, so you can see better where the average intensity falls within the
opening.

> I'm thinking that the doughnut made into an arc is an artifact of the
> simulation method. Maybe, maybe not, but the shadows are showing a
> doughnut profile in each opening. So I'm thinking an opening was simulated
> on its own and somehow a doughnut like profile generated. Then that
> profile extended around the arc of the zone opening.

The simulation is a full wave-optics simulation of both openings at once.  I
believe the simulation is very close to reality.  I got started on this path by
examining some data collected by Dale Eason with his robo platform about a year
ago.  I didn't show this in my report, but it clearly shows something very
strange is happening in the brightness profiles in the vicinity of zone 1 (of 9
zones, in this case).  The simulations duplicate this very well.

> But the left and right openings do have a symmetry in their appearance.
> Look at simulation J, The right opening  has a central area (center of the
> mask opening) that is light bounded on both sides by darker shades and on
> the left it is dark in the center and lighter to either side. Now the
> central band in each opening maybe is not the same intensity, but the
> center of symmetry in each opening is at the center of the mask openings.
> If that is not an artifact it may say something about the method of
> looking for simultaneous change in intensity in left and right openings
> when jogging the knife laterally across the slit image.

Actually, I disagree.  The central bands in the two openings do have the same
intensity (for images I and J), except that the null is displaced a tiny bit to
the outside for J.  This is because I use the equal area formula for the center
of the zone, and derived the zero point for the simulation from that.  Perhaps
this demonstrates that the arithmetic mean of the inside and outside zone radii
is better for the first zone.

> I suspect that the shadow contrast in the simulation is exaggerated. That
> can be hard to simulate when you don't know how I have my monitor
> adjusted. What is the radius on the mask of the outer edge of the opening?

Actually, I thought the contrast was a little low.  It turns out that the
contrast increases a lot with a narrower slit, and gets worse for a wider slit.
I chose to use an example where the contrast was a little sub-optimal.  I
believe a bright source with a very narrow slit can help to get zone 1 right.

Here are the zone boundaries I used (in mm):

[1]  33.75000  85.58507 116.23488 140.34347 160.87893 179.07477 195.58506
[8] 210.80621 225.00000

-- Steve Koehler
   steve_koehler@securecomputing.com

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