Exposure time for a really dim light

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I would appreciate any advice on the selection of exposure times for a really dimly illuminated instrument. I like tinkering with old telephones and admire the engineering design behind the elegant and effective dial mechanism with its governor spinning in a small cup. This design originated about 100 years ago in America and was still in use when the first trimphones appeared. I have recently acquired such a phone and was surprised to see that it still retains its tritium illuminated dial surround. BT's health and safety police tried to recall and dispose of these in the 90s but this one escaped their attention. Tritium isn't dangerous unless you eat it, so I have a couple of compasses illuminated with tritium that are only just through one half life and are still available, and legal, to buy. Ex-army luminous compasses - now they are dangerous, being lit by radium, which killed Marie Curie, if I remember correctly.

What a nanny-state we live in now; people of my age will still remember the time when as children in the 60s, we could go into a shoe shop to x-ray our feet. Petrol was much safer too; to make cars go faster you could buy blends that was so good, they gave it stars as a mark of quality.

My phone's weak beta emitter has run through about 4 half-lives and the illumination is so weak that I have to wait for my eyes to get their night vision and it catches my peripheral vision best. Centre vision is dimmer (something to do with the placement of rods and cones in the retina, I believe)

I would like to get a photo of the phone illuminating the numbers on the dial back plate. I have a tripod and cable release. The choice of camera and lens is probably irrelevant but the film might make a difference. I'd prefer to waste one of my rolls of Poundland stuff, pushed and stand developed in Rodinal, rather thanuse Tri-X, if possible. How many minutes/hours would members recommend?
 
To me, the obvious thing would be to select a camera/lens combination that offered a fast aperture (f2 or better) and then do a series of bracketed exposures, starting at 30sec, then 2min, then 10min etc. treat it like astrophotography where the light source is dim, but in this case also stationary.
 
I would like to get a photo of the phone illuminating the numbers on the dial back plate. I have a tripod and cable release. The choice of camera and lens is probably irrelevant but the film might make a difference. I'd prefer to waste one of my rolls of Poundland stuff, pushed and stand developed in Rodinal, rather thanuse Tri-X, if possible. How many minutes/hours would members recommend?
I'd start by researching reciprocity failure, since that will be a deciding factor for film exposure time. Fuji Acros is the star for reciprocity failure, but it's now discontinued. As far as colour film is concerned, bracketing widely should get you some kind of result.
 
Well if you don't want to use a digi camera o_O Any top of the range (and some cheap ones too) manual focus film camera and cheap AF film cameras can measure low light up to 30-45 secs....so it depends what cameras you have.
If you have a good camera, you could use something equivalent of a Kodak grey card (along side the subject) then take a reading from that and that should give a good starting point.
 
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