Densitometry - blue filters

Discussion in 'B&W: Film, Paper, Chemistry' started by michael_r, Jul 22, 2012.

  1. michael_r

    michael_r Subscriber

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    Assuming the use of graded silver papers, reading stained negatives using the blue channel on a color densitometer is said to give reasonably useful readings. My understanding is that the blue channel utilizes a filter corresponding to wratten #94. Anyone know what the transmittance curve (ie transmittance % by wavelength) looks like for a wratten 94? I don't have one (nor do I own a color densitometer) so I was planning on trying my black and white densitometer with a #47 as many people have done. I know what the transmittance curve is for the #47, and I assume the #94 passes a narrower band, so my thinking is the #47 should give results at least as useful as color densitometer would. Opinions?
     
  2. Mr Bill

    Mr Bill Member

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    Hi, I don't find Wratten 94 listed in Kodak's filter handbook (1990), so it's questionable if you'll find any curves for it.

    You mentioned the blue channel in color densitometers. There are two main responses, called status A and status M. The status A blue response is centered on roughly 440 nm. The status M blue response is centered on roughly 450 nm. Both have a fairly narrow response, roughly on the order of 40 nm spectral width.

    Offhand, I don't have any idea which would be better for your purposes.
     
  3. Mr Bill

    Mr Bill Member

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    Well I'll be... An older handbook lists the #94, so it seems to have been discontinued sometime before 1990.

    Wratten 94: here is wavelength, nm, followed by % trasmittance:
    wave trans
    420 ---
    430 1.2
    440 4.9
    450 8.5
    460 8.3
    470 4.7
    480 1.4
    490 0.3
    500 ---
     
  4. E76

    E76 Member

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    Looks to be pretty close to status M, in which case results would be similar to a real color densitometer. I'm not sure it matters too much in this case, but usually you want to use status M filters for negatives, and status A filters for transparencies.
     
  5. michael_r

    michael_r Subscriber

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    Seems like if we "think like the enlarging paper", a #47 should be fine. It peaks around the same wavelength as the #94 based on the numbers Mr Bill provided, although with a #47 the transmittance figures are much higher, peaking at 50%. The cuttoff around 490nm is about the same as a #47. The difference is the #47 cuts off at shorter wavelengths in the 380nm range. For the purposes of black and white enlarging paper it looks like the #47 is a good bet.