Re: Safelight safety

[Tom Campbell <tcphoto@xxxxxxxxxxxxx>]

Tom none Jarana wrote:
> 
> No need to worry about the safelight. Test it.
> 
> 1..In total darkness cut a small piece of paper say 3x3 inches.
> 2..Plasce it on the table beneath your safelight.
> 3..Place a large coin on top of the paper.
> 4..Turn on the safelight for 5 minutes.
> 5..Turn off the safelight.
> 6..Develop and fix the small piece of paper in total darkness.
> 7..Turn on the white lights.
> 8..Examine the paper, if you can see the outline of the coin, your safelight isn't safe.
> Solution: Install a dimmer on the safelight, or a lower wattage bulb, and test again.
> 
> --Tom

This test has one flaw as it relates to the real world.  Since we don't
(purposely!) develop unexposed paper, and the fog potential affects
threshold values primarily (high values with prints) you must first give
the paper the maximum exposure that will produce no visible density when
processed.  After this is done, your test will be fine.

Without this "sub-threshold" exposure, areas of the print that receive
slight exposure may show fog in an otherwise "safe" environment, due to
the cumulative effect.

We don't really want to know the fog potential for the unexposed paper,
but for the values in an actual print.  Actually, we *may* want to know
the former, but if the latter is safe, so will be the unexposed paper. 
The other direction can give you some surprises.

Tom Campbell
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End of Infrared-Digest V0 #544
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