Photo Photoluminescence and the darkroom

dswiger · Jun 1, 2020

I have an old model 168 Gra Lab darkroom timer.
The hands are in need of repainting.
I have acquired some sample, non-radioactive, powders from these guys
https://unitednuclear.com/index.php...ucts_id=1112&zenid=43ggip66e8blcghpr5q7i25u96

I have both a conventional green color a reddish-orange color.
I plan on running some experiments first but was wondering if anybody here has knowledge or experience with this

Donald Qualls · Jun 1, 2020

The greenish powder is almost certainly mostly sphalerite, aka zinc sulfide. It stores up energy from exposure to light, especially UV, and releases it over time to produce the glow. This is probably the base for the original luminous paint on darkroom timers made since the 1960s (radium paint was phased out, completely gone before 1970). The orange powder is probably a different mineral with similar properties.

There are a lot of phosphor compounds around, and they vary wildly over how rapidly the "shut off" after an excitation is discontinued. Sphalerite is one of the longer-persistence ones. Old CRT monitors and TVs use short-persistence phosphors to void image blurring when the displayed image moved between scans.

dswiger · Jun 2, 2020

Donald Qualls said:
The greenish powder is almost certainly mostly sphalerite, aka zinc sulfide. It stores up energy from exposure to light, especially UV, and releases it over time to produce the glow. This is probably the base for the original luminous paint on darkroom timers made since the 1960s (radium paint was phased out, completely gone before 1970). The orange powder is probably a different mineral with similar properties.

There are a lot of phosphor compounds around, and they vary wildly over how rapidly the "shut off" after an excitation is discontinued. Sphalerite is one of the longer-persistence ones. Old CRT monitors and TVs use short-persistence phosphors to void image blurring when the displayed image moved between scans.

Thanks for the info. I'll probably do some testing to make sure it's OK for the darkroom

Donald Qualls · Jun 2, 2020

One of the big advantages of the yellow-green glow from sphalerite is that it's very close to the sensitivity peak of the human eye -- so it can be exceedingly dim and still be visible. This makes it the preferred choice for darkroom luminescent paints. I have a large enlarging timer that has green GITD hands and numbers -- enough total emission after spending all day in a lit room that I can see my hands in front of a whitish wall after fifteen minutes or so in the darkroom -- but it has never fogged my film to a detectable level, even when doing ten minute tray development.

DREW WILEY · Jun 6, 2020

Depends on the film. I absolutely have had those little Gralab hands fog film being tray developed, even six feet away. Now I never ever use any kind of timer except on a shelf below sink level where the film never "sees" any kind of luminosity.

Mal Paso · Jun 6, 2020

My Gralab is on the wall above the open top developing tanks but there is an opaque "flag" between the timer and the tanks. No timer light can directly hit film. I've never had a problem with the Gralab green as long as there is no direct exposure.

DREW WILEY · Jun 13, 2020

I gave up on the ole Gralab clock hand timer long ago, though I keep it in storage as a hypothetical emergency backup unit. What I do use is a Zone VI compensating dev timer; but even the red LED readout portion of it sits below the sink.

Photo Photoluminescence and the darkroom

dswiger

Donald Qualls

dswiger

Donald Qualls

DREW WILEY

Mal Paso

DREW WILEY

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