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  1. #21
    ic-racer's Avatar
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    The following is a superimposed curve of Ilford MG paper and a cyan dichroic filter. The black lines represent different threshold density on the paper plotted against light wavelength. The colored lines represent transmission at different angles on incidence. In enlargers the angle of incidence will be zero degrees, so look at the BLUE curve to compare with the paper curve. As you can see the cyan filter will have very little on the paper, even when it is fully in the light beam. The filter shown passes over 90% of the useful light energy to the paper.
    Last edited by ic-racer; 10-11-2011 at 07:23 PM. Click to view previous post history.

  2. #22
    Photo Engineer's Avatar
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    Generally, cyan filters have a considerable amount of absorption in the Green and Blue regions of the spectrum, thereby increasing effective exposure times. Even "pure" dichroic filters will suffer from this problem to some extent. Take a look at the spectral response of a common cyan filter someday if you get a chance. It is quite a surprise if you have never seen one before.

    After looking at your curves, I feel I should add that the angle of incidence due to the light box, is probably not zero degrees and that is why you might experience the problem that I describe. It is seen in your red or cyan curves where what I describe above is manifesting itself.

    PE
    Last edited by Photo Engineer; 10-11-2011 at 07:36 PM. Click to view previous post history.

  3. #23
    ic-racer's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    , I feel I should add that the angle of incidence due to the light box, is probably not zero degrees and that is why you might experience the problem that I describe. It is seen in your red or cyan curves where what I describe above is manifesting itself.

    PE
    Good point about the angle of incidence. Though, to eliminate said concerns, most enlargers that I am familiar with have the dichroic filters in the collimated light path before the light box, to get that straight-on shot from the lamp/reflector.

  4. #24
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    I would have to disassemble the boxes, but IIRC, there is a 45 degree beam splitter in the system. I'll take a look at one of mine.

    PE

  5. #25

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    if you are still interested there is a 35mm mixing box at eBay now #380376800642
    nothing to do with it, just found it
    have fun with the hunt.

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