Hi folks
Some people regard the safelight coin test as inadequate. Paper(and film) has a
threshhold exposure beneath which no density appears. Duringexposure and
processing, the paper is subject to enlarger + safelight exposure.Although the
safelight exposure may not be sufficient on its own to cause visibledensity, it
may be sufficient to lift the paper to just below its threshhold,thus lowering
the contrast achieved by the enlarger exposure. The filmequivalent is a
pre-flash, used deliberately to reduce contrast. Such below-threshholdexposure
will not be revealed by the coin test.
A more exacting safelight test is as follows:
1. With the safelight off, determine the enlarger exposure requiredto produce a
light grey print density. This means processing the paper in totaldarkness.
2. With the safelight off, expose a full sheet of paper for thedetermined
exposure.
3. Place a board over the developer tray (assuming thatthis is where the
safelight is brighest), place the exposed paper on the board, covera quarter of
the paper with an opaque card, then turn on the safelight. After 1minute, move
the opaque card to cover half of the paper; after another 2minutes, move the
opaque card to cover three quarters of the paper; afteranother 4 minutes, turn
off the safelight and process the paper in totaldarkness.
Thus the paper has been given a threshhold enlarger exposuresufficient to give
an overall light grey density, one area that has received no additional
safelight exposure, and three areas that have received increasing amounts of
additional safelight exposure. If the safelight issafe, nothing will be visible
except the overall light grey density. However,if any of the areas that
received the safelight exposure are visible, then there’s a problem.
Regards - Brian
On Sunday, 10 October 2021, 07:19:22 am NZDT, Thierry ARNAUD
<aristark@xxxxxxxxx> wrote:
Many thanks for all your explanations! They are very interesting, as usual.
Best regards,
Thierry
-----Message d'origine-----
De : pure-silver-bounce@xxxxxxxxxxxxx [mailto:pure-silver-bounce@xxxxxxxxxxxxx] ;
De la part de `Richard Knoppow
Envoyé : vendredi 8 octobre 2021 23:12
À : pure-silver@xxxxxxxxxxxxx
Objet : [pure-silver] Re: [pure-silver] Re : [pure-silver] Re: Light_in_the_Lab
I don't know if there is a definite limit for paper (or film
either). The usual technique is to test for fogging with what is
called a coin test, where the paper is exposed to the safelight
with part covered for a length of time equal to the time it will
be exposed to the safelight during use. Completely process it,
including fixing and washing and examine it for fogging. Since it
has been partially covered there should be a noticeable contrast
between the covered and exposed part. If there is none you are OK.
You can probably get the spectral sensitivity of the paper
from the manufacturer (but I have not tried) and the output of
the LED's again from the manufacturer (but again I have not
tried). Now, a complication: what you want to do is to minimize
the amount of what used to be called actinic light the paper is
exposed to. Actinic meaning light that the emulsion responds to.
So, you need a compromise between the minimum sensitivity of the
paper and the maximum sensitivity of the eye so that the amount
of actinic light is minimized. I wrote about the use of green
light for panchromatic materials. There the emulsion is sensitive
to all visible colors so the safelight is chosen to have the
highest visual brightness, which turns out to be green for a dark
adapted eye. The light is used in a way that further minimizes
the amount the emulsion is exposed to. For paper one has a
greater choice but the same factors apply, the minimum light the
emulsion is sensitive to. In the case of blue sensitive (graded)
paper, which is not very sensitive to yellow or red the use of a
yellowish green (OC) safelight has about the highest visual
brightness for a band of colors the paper is not very sensitive
to. Variable contrast papers, however, have some orthochromatic
sensitivity, that is they are sensitive to some green and yellow
as well as blue light. The OC or yellowish green safelight cuts
off fairly sharply on the blue end but appears to be visually
bright. However, it passes too much yellow for some VC papers.
For those you need the same kind of safelight used for
orthochromatic film, namely red or dark red. Yellow is more
comfortable to work under than red and is visually brighter but
red may be necessary depending on the results of the fog test on
the paper.
Note that the _visual_ color of the safelight may be very
misleading about its actual spectral output. A light appearing to
be red may, in fact, have substantial output in the blue or even
green region. The sodium lamp, should have output only at the
sodium D line, which is yellow and falls in the area that most
printing paper is not very sensitive to. I don't know about
LED's, they may need an additional filter. Again, the coin test
is the final answer.
On 10/8/2021 1:38 PM, thierry.arnaud@xxxxxxxxxxxx wrote:
Thanks for the answer.
I plan to use leds. That is why I would like to know how many
lumens can support a paper when the light has a certain
wavelenght (here with the Blood Red Filter, about 670nm).
Because there are now leds very powerfull and it is possible to
set many of them behind a large sheet of filter.
Apparently nobody knows this quantity of light and if there is
a limit.
My idea is the following : if the paper is not sensitive to a
certain wavelenght of the light it is possible to push the
intensity of the light (more lumens) of this particular
wavelenght without fogging the paper and BTW to work in a
pleasant lab.
I would prefer this setting because the sodium yellow light
makes too noise but the leds work silently.
Best,
Thierry ARNAUD
-------- Message original --------
De : John Stockdale <jstoc@xxxxxxxxxxxx>
Date : jeu. 7 oct. 2021 à 23:20
À : pure-silver@xxxxxxxxxxxxx
Objet : [pure-silver] Re: Light_in_the_Lab
I built a number of safelights using discreet LEDs.
Starting with graphs of luminous outputs at the various
wavelengths and the graphs of sensitivity of the paper to
various wavelengths I chose some orange LEDs, but they did
not pass a reasonable test. They caused fogging. I read
that one could inspect the colours of the light by
reflecting off the back of a CD (the "rainbow effect") and
these orange LEDs showed yellow through to a hint of aqua.
Then I went to red and then to very red LEDs, and these are
safe but of course it's a more difficult light to work
under. So I made up a hand held light with the orange LEDs
that I use for brief inspections. The difference is
astronomical.
If you use incandescent globes a dimmer can be useful.
Dimming not only dims but also alters the spectrum usefully.
I envy people who have the sodium vapour safelights because
yellow is a lot easier on the eyes.
Let us know how it goes,
John
==================================
On 8/10/2021 5:15 am, Tim Daneliuk (tundra) wrote:
On 10/7/21 11:01 AM,thierry.arnaud@xxxxxxxxxxxx wrote:
Interesting!
Thank you Dana!
But it gives no idea of the maximum quantity of lumens of a light
which is behind my red filter.
It's not that simple. The acceptable amount of light depends upon
multiple
factors:
- The intensity of the light source in the relevant parts of the
spectrum
- The spectral response of the filter
- The attenuation of the filter
- The distance of the filtered light source to the paper, worst case
(shortest distance)
- The highest sensitivity of paper you plan to use
In theory, you could figure this all out analytically, but why bother?
Build/buy a light source with the ability to replace bulbs and filter.
Take a shot at it and do a coin test.
As an aside, it is almost always preferable to have multiple, lower
output
safelights in various parts of the darkroom, than one hanging from the
ceiling.
In my case, I use one smaller Kodak "Bullet" light over the sink, and
two 8x10
wall mounted ones at the far end of the darkroom. This allows
brighter illumination
far away from the most light sensitive end of things (unexposed paper
by the enlarger
and paper in development) while still illuminating the room nicely. I
think I ended up
using deep red X-Ray filters with a 25W bulb in the bullet and 40W in
the 8x10s.
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