Re: Whorls continued


Wayde Allen (wallen@boulder.nist.gov)
Wed, 21 Apr 1999 17:02:46 -0600 (MDT)


On Tue, 20 Apr 1999, Chris Stone wrote:

> Desperately seeking more advice about them whorls (Newton's rings?),
> which appear to be Newton's rings, that are appearing when I sandwhich
> 4x5 negative film, emulsion to emulsion, to make interpositives. I've
> tried flipping the negatives and exposing through opposite sides, but
> had a distinct loss of sharpness.

That is correct, and it won't do anything about the Newton ring problem.

Placing two very smooth surfaces in close proximity results in a a very
thin film of air trapped between the two materials. The result is three
regions of differing indices of refraction which cause internal
reflections. I've tried to diagram this below. (It might be readable if
you use fixed fonts.)

                               reflection reflection
                incident from from
               light beam film/air air/film boundary
                       \ boundary /
                        \ / /
   ----------------------\------/-------/------------------------
                          \ / /
      Sheet of film \ / /
                            \/ /
   --------------------------\------/---------------------------
                              \ /
      Air space \ /
                                \/
   ------------------------------\----------------------------
                                  \
      Sheet of Film \

   ----------------------------------------------------------

If the air space is on the order of the wavelength of light, you get
constructive and destructive interference of the wave fronts reflected
from the film/air boundary with the reflection from the bottom air/film
boundary. Flipping one of the sheets of film over really doesn't change
this condition.

> I've also tried coating the negs with
> a very light poof of fine talc. That appeared to work,

Yes, since that scatters the reflections.

> I don't know if the
> problem is orginating between the sheet glass and the original negative,
> or between the two negatives.

It could occur at both places.

> Is it worth the investment to replace the
> sheet glass in the contact frame with anti-newton glass?

The anti-newton glass will have a bit of texture to help scatter the
reflections, and might help some. It won't fix the problem if it is
primarilly occuring between the sheets of film. You could probably test
this by carefully powdering the film-to-film interface and leaving the
glass to film interface clean. If that fixes the problem you probably
don't need anti-newton ring glass. On the other hand, it probably can't
hurt.

> Or would it be
> helpful to slip a sheet of mylar between the negatives?

The mylar would separate the two sheets of film, but you'd lose some
resolution due to the added separation. You would also get two new air
spaces to worry about. Depending on how the internal reflections end up
working out it might fix it, or it might make things worse. Powdering the
negatives is the simplest solution.

- Wayde
  (wallen@boulder.nist.gov)



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