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Chromaticity diagram

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  • Chromaticity diagram

    Hi all,

    I hope it is the good place to post this message otherwise just let me know and I will change the place.

    I am writing an article about colors and I need to understand how the Chromaticity diagram works.
    There is one here. We can take it as reference.

    Actually, I understand why the CEI made one, but I don't understand very well 2 things.

    The first thing is the coordinates:
    The coordinates (x and y), even if I know they are based on the M-cone (Y = 1).

    But I don't understand the paragraph "color matching function" on the page
    I don't understand how they find X, Y and Z.
    And I can't figure out waht is x and y exactly.

    The second thing and the most important:
    How can you explain that some color spaces like ProPhoto RGB can have a dot outside the nanometer edge.
    What does that mean?

    I understand that the human eye can see it, but the dot is above the nanometer edge.... Is it another dimension? lol
    Because we can't see below 380nm and above 750 nm.... but between the both edges, we can see the electromagnetic waves.

    If a very good soul can help me plz?

    PS: Sorry for my english. I am learning ^^

  • #2
    Who is the target audience of your color article? Designers? Printers? Physicists?
    To what are you trying to relate this information? Web, print, laser science?

    If this has anything to do with Graphic Designers, the function of color as relates to that CIE chart is fairly simple.
    If it relates to color matching on a spectrophotometric level in regards to machine/media profiling, it gets more complicated.
    If it relates to what color space one should be using in either regard, that can get a little weird.

    About the ProPhoto, the dots outside the indicated color gamut are outside human visible perception. The light exists, and will register on a spectrophotometer, you just can't see it. ProPhoto contains the largest amount of color data. If you have optical scanners (camera or flatbed) that can scan into the ProPhoto gamut, use it. The more color information you start with, the better off your imagery will be when you have to interpolate it to fit into one of the smaller gamuts.
    Last edited by PrintDriver; 02-12-2017, 07:20 AM.


    • #3
      re XYZ, the article you linked to explains it well.
      The XYZ tristimulus values are thus analogous to, but different from, the LMS cone responses of the human eye. Defining Y as luminance has the useful result that for any given Y value, the XZ plane will contain all possible chromaticities at that luminance. The unit of the tristimulus values X, Y, and Z is often arbitrarily chosen so that Y = 1 or Y = 100 is the brightest white that a color display supports. The corresponding whitepoint values for X and Z can then be inferred using the standard illuminants.
      You need to understand more about the tristimulus values and how they are derived. XYZ values are non-variable, as compared to LMS values which are variable. The idea is to have a non-variable model in order to apply spectro-measured color matching across various uses of color profiles and while keeping it visible to the human eye.


      • #4
        Hi Alex and welcome to GDF.

        We ask all new members to read very important links here and here. These explain the rules, how the forum runs and a few inside jokes. No, you haven't done anything wrong, we ask every new member to read them. Your first few posts will be moderated, so don't panic if they don't show up immediately. Enjoy your stay.
        Shop smart. Shop S-Mart.


        • #5
          First of all, thank for helping.

          I am writing this article for developers or/and designers who would like to get a better understanding of the color and how it is working.

          I won't post the address, I don't want to do an ad I just want to understand that, but it will be great if a pro designer could read it and could criticize it whether one of you have time I will send it in private message otherwise it does not really matter.

          I understand the dot of ProPhoto is outside the human field of view because our eye can react only between 380 and 750 nm.

          But if you watch the CIE graph, you can read this wavelength, and I don't understand why the graph has this shape.
          How would be the graph whether we can see from the 100nm to 10000nm?

          For sure I need to understand the calculation... but it looks a bit complicated. I will study it a bit more.
          If I get the answer more accurate I will post the answer anyway ^^.

          I have some questioning about color space, monitors and files but I prefer to read more documents before posting my question. lol



          • #6
            Just answer my own question. lol.

            The graphic will be exactly the same, the calculate is done to get a relative abscissa between 0 and 1 for any value.
            So betwee100nm and 10000nm you will get the same graph except the ProPhoto will be very small. lol

            (We can close the topic)






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