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RGB to wavelength related problem

Discussion in 'Lighting' started by Lord K., Nov 29, 2009.

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  1. Lord K.

    Lord K. Guest


    I am aware that the "RGB to wavelength" question is widely covered on the
    net, and I know that there is no way to perform such a conversion. But my
    real need is very specific; maybe some good model can be used for
    performing what I want.

    My first goal was to perform some "black and white conversion" from a
    color picture (I know that I am reinventing the wheel...). To be more
    precise, I wanted to simulate some old films. I am aware of the
    documentation that can be found on this topic, but here is what I did. I
    had a look at the technical specifications of these old films. For
    instance, you can find the well-known Tri-X at:
    and the spectral sensitivity curve at:

    At that point I searched on the net and discovered that the "rgb to
    wavelength" conversion can't be achieved. I found some links where people
    describe some more-or-less good "models", but of course it was too much
    complicated for my real need.

    Suddenly I noticed that my favorite film has a very specific curve:
    It looks like the curve is made of two straight lines; we could even say
    it is made of a single straight line over the visible spectrum.

    For that reason, the classical way of converting the RGB color to black
    and light with the channel mixer (by giving some coefficient to the red,
    green, and blue) is probably not bad. But can't we do better?

    Do you see a good way (probably not perfect I know) of getting the gray-
    level corresponding to this "spectral sensitivity curve" from the RGB
    values? Would it be a good idea, for instance, to get the CYM (cyan,
    yellow, magenta) of the color in order to have SIX values (rather than 3)?

    Assuming the curve is a straight line, could something be done by
    converting the RGB to HSV and get the Hue which can be mapped to the

    I would be very happy to have some "scientifical" filter for this film?

    Best regards,

    Lord K.
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