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  1. #31
    Ole
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    Quote Originally Posted by jjstafford
    Please post the source for this red light and be our hero!

    Sincerely,
    John
    I use an old-fashioned red bulb, 25W I think, at the other end of the darkroom. It is a lot brighter than the near-invisible green light I use otherwise - but terms like "strong" and "bright" are relative.

    It is surprising to many how much light it really takes to fog film, human eyes are certainly far more sensitive to low light levels than 25 ISO film!

    No simple light filter is perfect, an certainly not HAMA's coloured light bulbs. But with a little forethought it is very easy to avoid fogging: Remove film from holder in darkness, start developing in darkness, do not turn on light until you are getting close to the expected end of development. In most cases you will find that the colour of the light matters not at all - it might just as well be white as long as it's weak enough. But when some films have reduced sensitivity in some part of the spectrum, I consider it a good idea to exploit this by using a light sorce which is centered around this part of the spectrum. So whether or not I can see farther into the red than the EFKE PL25 film can is irrelevant, since neither I nor the film can see the longer wavelengths that the lightbulb gives off. Good enough reason for me to use red light when developing these films by inspection.

    the young, healthy human eye cannot see above about 640nm, and the aged eye even less
    Where did you find that information? I couldn't find it? Most sources seem to agree that the visible spectrum extends to just short of 700nm, with people operated for cataracts seeing as far as 780nm. No, I haven't had cataracts, so I don't expect to see into the IR.
    -- Ole Tjugen, Luddite Elitist
    Norway

  2. #32

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    Thanks for the reply, Ole. John of J&C replied privately to me to clarify. I am quoting it here in order to be fair to the facts so we can all benefit. '"I read the thread through this morning. I thought your question was "could the film be handled in the darkroom under a red safelight". To which my answer applies. If your talking development by inspection then after several minutes of development in the dark the films sensitivity decreases and visible light can be used to inspect the film for short periods of time without fogging."'

    John's clarification makes perfect sense and is in accord with your experience. I'm not at all unhappy to be corrected regarding the red sensitivity. It is important to have the facts out there, challenged and reaffirmed. Thanks for your persistence, Ole.

    Now, if I could get the straight poop on compartive green and blue sensitivity I would be happy, since that is my main concern.

    My figures regarding the sensitivity of the human eye are taken from the recently published studies by Margaret Livingstone.

  3. #33
    Ole
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    I did find a nice page with curves of the sensitivities of the various receptors in the eye just before my PC crashed...

    Those curves explain why we normally use green light for film (max. sensitivity of the eye), and why astronomers like red light (rods are not sensitive, so dark-adaptation is unaffected), and why I get away with using a relatively bright red light with orthopanchromatic film: The sensitivity of the eye is higher than that of the film, even if the sensitivity of the eye has dropped to about 10% of max. The film's sensitivity has dropped even more. Doing other films with the same red light might be a bad idea as the total intensity must be increased so much due to the reduced sensitivity that the film would be more easily fogged than with a green light of the same perceived brightness!

    I must admit that much of the time I use white light for DBI; my darkroom isn't totally lightproof and after about five minutes I can generally see what's going on, after ten minutes I can see if the negative is done or not. Since no lightmeter I have tried shows the slightest sign of a twitch of the needle, I assume that the light level is nevertheless too low to register on film.
    "Safelight" for film development is something I use only at night or in the winter.
    -- Ole Tjugen, Luddite Elitist
    Norway

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