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[OM] Re: Anomalies in E-1/OM Images -- Update

Subject: [OM] Re: Anomalies in E-1/OM Images -- Update
From: Andrew Gullen <andrew.gullen@xxxxxxxxxxxx>
Date: Sun, 16 May 2004 22:42:40 -0400
on 2004/05/15 11:15 AM, Joe Gwinn at joegwinn@xxxxxxxxxxx wrote:
>> Date: Thu, 13 May 2004 22:09:22 -0400
>> From: Andrew Gullen <andrew.gullen@xxxxxxxxxxxx>
>> 
>> on 2004/05/13 8:20 PM, Cyrtolite@xxxxxxx at Cyrtolite@xxxxxxx wrote:
>> 
>>> The spots are about perfectly circular and the larger ones are
>>> composed of concentric rings.
>> 
>> They aren't Newton rings are they? Or have we discussed this already?
> 
> No, they aren't Newton Rings, which arise when two pieces of glass
> (say two lenses) get too close together, close enough that
> interference  becomes noticeable.
> 
> They are diffraction (Airy) patterns in the shadows of particles of
> dust on the glass cover of the image sensor.

You could be right - how far (how many diameters?) does the object have to
be in front of the sensor to see these instead of a normal shadow?

Newton rings would imply non-uniform thickness of a layer in front of the
sensor, or an air gap, of dimension (I think) lambda/2 to a few lambdas or
so - correct me if I'm wrong; it's been a while. However, they probably
wouldn't be perfect circles unless the flaw was circularly symmetric, so as
I said, you're probably right. Which, for Scott's sake, is good. And now
that I think about it wouldn't Newton rings have to be darker, not lighter?

Andrew


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