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Re: [OM] OM1 Viewfinder Black Cloud

Subject: Re: [OM] OM1 Viewfinder Black Cloud
From: "O.C" <oben@xxxxxxxxx>
Date: Thu, 19 Jul 2001 23:08:21 +1000

I think to be one hundred percent accurate I should say that the prism doesn't require a silver mirror coating to work. It works on the principle of total internal reflection; ie when rays of light meet a glass to air interface at an angle greater than the critical angle for that particular glass (related to the refractive index) they reflect totally rather than refract through the interface. Prisms are the most efficient mirrors as they reflect 1000f the incident light unlike coated surfaces. They also don't accumulate dust etc like a mirror would. This is also how Porro and Dach (roof) prisms in binoculars work and why they are used rather than mirrors.

The problem described that occurs with the foam and prism is that the foam rots and becomes bound to the prism surface. When stuck on the glass it alters the glass to air interface that is required for the total internal reflection to occur. What you see instead is the foam through the glass. Thats why cleaning the prism of the gunk and restoring the original glass air interface will restore things.

Just a technical thing....

Oben

At 10:47 PM Thursday 19/07/2001, you wrote:
That's the prism coating being eaten away by foam that is pressed against it
under the top cover.  For best results, replace pri$m.  Otherwise "most" of
the black stain can be cleaned off.

John  Hermanson


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