Re: Problem with Wisner 4x5 camera probably solved!

[Jud Hofmann <hofmann@xxxxxxxxxxxxxxxxxxxxxx>]

>> In the early 1950s there was a riteway holder that fogged
>> film. The problem was the dark slide. When the holder was
>> hit by sunlight the slide was said to absorb energy. After you made a
>> picture, you turned the slide around when putting it back
>> to indicate an exposure was made. The energized side of the
>> slide was then put close to the film and fogging occured,
>> we are told.

>I'm trying to make sense of this and am failing miserably.  I'm
>wondering what kind of energy could be stored on one side of the slide
>that was not transferred to the other.  

One mechanism that could do this fogging is flourescence:  when the dark
slide was exposed to the visible and UV from the sun, it flouresces -
emitting visible light.  In many materials, this emission continues for a
short time after the light that caused it is removed.  When you turned the
dark slide around to face the film, the flourescence fogged the film.

So do we now have a light slide instead of a dark slide?  Gotta be a use for
such a thing, but it escapes me at the moment.

Jud




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End of INFRARED-PHOTOGRAPHY Digest 221
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