Re: Thomas Duplex Sodium Vapor Safelight

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From: Phillip Murphy (pmurf@bellsouth.net)
Date: 04/06/03-04:02:53 PM Z


Hi Jack,

"Flashing" the light-sensitive salts brings the exposure closer up the
characteristic curve to the "straight-line" portion which is more
reactive to exposure than the "toe" of the curve. This will allow one
to have a better visual reference with the coin trick. If the material
is left "un-flashed", most of the exposure is in the "toe" of the
curve, thereby, given the brief time for the test, a visual difference
may
only be slight. This leads to a false conclusion about the "safety" of
the ambient light because of the long "toe" line's unresponsiveness.

Here's a poor analogy:

Let's look at giving a kid a penny on the first day of the month for
her/his allowance, then doubled the amount the next day, and doubled it
again the next day.... etc. until the end of the month. The first two
weeks is like the "toe" of the characteristic curve. Not much
"change".. heh... over time. By the end of two weeks , maybe enough for
a bag of groceries. However, if you look at the
fourth week, the rate of change between days is substantial. By the end
of the month, maybe enough for a down payment
on a house in San Francisco!

( btw... thanks for the article Tom.... )

-Phillip

Jack Fulton wrote:

>
>
> Something to keep in mind when doing a coin test for "safe"
> lights, be sure that you "flash" the paper or film first to
> the
> inertia point of the material before beginning the coin
> test. Leaving this step out is a common mistake.
>
> -Phillip
>
> Phillip . . . this is interesting. I can see one doing this
> but don't quite see the need to do so. Yes, to 'trigger off'
> the theoretically 'edge' of what constitutes an exposure to
> create density could propel or initiate a more sensitive
> reaction to a worn out or incorrect safe-light. But why do
> you say this is necessary?
> Jack
>


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