Re: compensating zone plate

----- Original Message -----
From: "Gordon J. Holtslander" <holtsg@duke.usask.ca>

> A question for all the mathematicians and physicists. Is it possible to
> make a zone plate that compensates for fall-off.

I am no mathematician or physicist, just a plain guy that sort of remember
some of the math I studied more than 25 years ago.

> Is it possible to
> make a zone plate that compensates for fall-off.
> Is there a way of altering the line pattern and or line thickness of a
> zone plate so that the edges of the picture get more illumination?

This are some thoughts:

A ZP behaves just as a pinhole as far as fall-off is concerned. You could
elongate the ZP so when you see it off-axis it resembless more a circle than
an elipsis. But the only "edges" that will see the ZP more as a circle will
be those toward which the ZP has been elongated. Also, you can only
elongate vertically or horizontally (if you do both, you are effectively
just making the ZP bigger and not good for the focal length in use,
anymore). Fall-off is a relative measurement, the difference between the
center and the edges, by elongating the ZP you will be getting not only more
light at the edges but also at the center of the image, so the end result
may not give us less fall-off than before.

> And just to make it interesting, if this is possible, is there a way of
> calibrating or predicting the difference in exposure from one region of
> the picture to another?

Thanks G*D is not possible (IMO) otherwise I'd have to give you an answer!!

Having said the above, the way to calculate it would be a Cos^4 law modified
by the effect of the eliptical shaped aperture.

> The things I end up talking about over lunch with another pinholer!

If you were in Quebec I'd probably ask you: "what were you guys smoking",
but probably in Saskatoon as well as in Toronto, eateries are smoke free
places by law. :-)

Guillermo
Received on Tue Aug 21 08:33:47 2001