How dark should the darkroom be?

favalora · Post by **favalora** » Mon Jul 02, 2012 8:42 pm

Hello -

I have an utter newbie question. Hypothetically speaking, one of the potential future applications of the plumbing in my new house's basement is film-developing. If, one day, I lose my mind and decide to try making a hologram, how dark must that lab be?

I recall reading "not that dark, actually, and certainly not as pitch-black as regular photography, because the efficiency of holographic plates is so low."

Naively,
Gregg

Johnfp · Post by **Johnfp** » Tue Jul 03, 2012 6:46 am

OK, so let's look at the time your film will be exposed to the "dark room" which includes your table and processing area.
Take film out of container and put it in your film holder - 15 seconds
Allow film to settle in film holder - 15 minutes
Do exposure
Take film out of film holder and process until it goes into fixer or bleach - 2 minutes

So that give approximately 17 minutes

Now take a piece of film and just lay it in your "dark room" for 17 minutes without actually exposing the film holographically with the laser, then process it
If it turns dark in the developer then your dark room is too bright.
You want the piece of film to remain clear in the developer without exposure to light, then your dark room is dark enough.

DCG is w whole different story.

Dinesh · Post by **Dinesh** » Wed Jul 04, 2012 1:11 pm

It really depends on the kind of film you're intending to use. A (very) rough rule of thumb for silver is that you turn off all safelights and wave your hand in front of your face. If you can barely detect motion, then it's dark enough. If you can clearly see your hand, there's too much light. As John mentions, it's not the absolute darkness, it's the amount of time that the film is sitting there passively. As it sits there, it's absorbing light energy; there's a threshold under which there's no action, once past that threshold, the silver grains begin to "pop" and, enough of them "pop", the film gets fogged. The lower the light present, the longer it takes to reach that threshold. Also, the smaller the grain size, the longer it takes to reach that threshold (small grains = long exposure), so modern grains, being very small, don't require as much darkness as, say, the old Agfa. If memory serves, the Agfa grain size was about 40nm and Slavich claims 10's of nm. Another test is to place a coin on some pieces of film and leave them for varying periods in the "dark". If you now develop, the variation of the fogged parts show up quite clearly against the unexposed parts covered by the coin. If you try this with three or four pieces of film varying each one by about a minute, it gives a good indication of how dark your environment is. If you use this test remember to develop all the films for the same time - approx 3 minutes, ie don't wait till the film goes dark. If you develop for long enough all films will go dark.

For dcg, resist and polymer, the situation is quite different. These materials require almost two orders of magnitude more exposure, so you can almost expose them to white light for a few seconds with no adverse affects. As a matter of fact, a technique we used at Banknote to determine whether we'd etched a resist plate enough was to look at it at an angle in white light. If you saw interference rings, you'd gone too far, bit if you didn't, you went back into the dark and continued etching. In a lot of resist and dcg labs, you see fairly bright and not so narrow band yellow and orange light permanently on all over the lab.

favalora · Post by **favalora** » Wed Jul 04, 2012 4:20 pm

Thank you, Johnfp and Dinesh!

-Gregg

John Sonley · Post by **John Sonley** » Fri Sep 07, 2012 12:43 pm

Did a test recently with a new 490nm narrow band filter for use with red Agfa 8E75 T3 HD film - its blue rather than the greenj safelight I've used for the past years
I set the light source/filter 120 cm from the film and exposed it for 2, 5, 7 and 10 minutes using two examples of the film I have, both 5" X 4" pieces and then developing in classical Pyrochrome developer at 20 deg for 2 minutes and then fixing in a commercial fixer.
The light was bright nenough to see fairly clearly. In practice my preparation table is 250 cm and my plateholder is 150 cm from the safelight
Quite pleased with the results

Jeffrey Weil · Post by **Jeffrey Weil** » Fri Sep 07, 2012 4:26 pm

Johnfp has it right. Put the film out in the light longer than you'll ever do in real life and then develop it longer than you ordinarily would.

I'd add only one thing to his recommendation. Put a coin or something else opaque over part of the film when you do the test. If it's only slightly fogged you might not see it, with that coin giving you a side by side comparison between unexposed and exposed you'll see even the slightest exposure.

Jeff W

Dinesh · Post by **Dinesh** » Sat Sep 08, 2012 8:34 am

Jeffrey Weil wrote:Put a coin or something else opaque over part of the film when you do the test. If it's only slightly fogged you might not see it, with that coin giving you a side by side comparison between unexposed and exposed you'll see even the slightest exposure.

Right. This was something Peter Miller taught me.

Dinesh wrote:Another test is to place a coin on some pieces of film and leave them for varying periods in the "dark". If you now develop, the variation of the fogged parts show up quite clearly against the unexposed parts covered by the coin.

Jeffrey Weil · Post by **Jeffrey Weil** » Sat Sep 08, 2012 8:42 am

Dinesh, we both forgot a part!

After you develop place the film on a white piece of paper and then look at it reflectively, don't just hold it up to the light.

This way the light will have to pass through the film twice and that will also increase the visual differential between exposed and unexposed.

Jeff W