<sect2 id="editor-cm-pcs">
    <title>The Profiles Point to Real Colors in the Real World</title>

    <sect3>
        <title>
            Camera, scanner, working space, monitor, printer - what do all these color profiles really do?
        </title>

        <para>
            A color profile describes the color gamut of the device or space to which it belongs by specifying what real color in the real world corresponds to each trio of RGB values in the color space of the device (camera, monitor, printer) or working space.
        </para>

        <para>
            The camera profile essentially says, "for every RGB trio of values associated with every pixel in the image file produced from the raw file by the raw processing software, "this RGB image file trio" corresponds to "that real color as seen by a real observer in the real world" (or rather, as displayed on the IT8 target if you produced your own camera profile, but it amounts to the same thing - the goal of profiling your camera is to make the picture of the target look like the target).
        </para>

        <para>
            You cannot see an image by looking at its RGB values. Rather you see an image by displaying it on a monitor or by printing it. When you profile your monitor, you produce a monitor profile that says "this RGB trio of values that the graphics card sends to the screen" will produce on the screen "that real color as seen by a real observer in the real world".
        </para>

        <para>
            What the monitor profile and the camera profile have in common is the part (in italics above) about "that real color as seen by a real observer in the real world."  Different trios of RGB numbers in, respectively, the monitor and camera color spaces point to the same real, visible color in the real world. Real colors in the real world provide the reference point for translating between all the color profiles your image will ever encounter on its way from camera to screen to editing program to print or the web.
        </para>

    </sect3>

    <sect3>
        <title>How can a color profile point to a real color in the real world?</title>

        <para>
            Real people don't even see the same colors when they look at the world, do they?
        </para>

        <para>
            A long time ago (1931, although refinements continue to be made), the International Color Consortium decided to map out and mathematically describe all the colors visible to real people in the real world. So they showed a whole bunch of people a whole bunch of colors and asked them to say when "this" color matched "that" color, where the two visually matching colors were in fact produced by differing combinations of wavelengths. What was the value of such a strange procedure?  Human color perception depends on the fact that we have three types of cone receptors with peak sensitivity to light at wavelengths of approximately 430, 540, and 570 nm, but with considerable overlap in sensitivity between the different cone types. One consequence of how we see color is that many different combinations of differing wavelengths of light will look like "the same color".
        </para>

        <para>
            After extensive testing, the ICC produced the CIE-XYZ color space which mathematically describes and models all the colors visible to an ideal human observer ("ideal" in the sense of modeling the tested responses of lots of individual humans). This color space is NOT a color profile in the normal sense of the word. Rather it provides an absolute "Profile Connecting Space" (PCS) for translating color RGB values from one color space to another. (See <ulink url="https://en.wikipedia.org/wiki/CIE_1931_color_space#Tristimulus_values">here</ulink> and <ulink url="https://en.wikipedia.org/wiki/Color_vision">here</ulink>.)
        </para>

        <para>
            CIE-XYZ is not the only PCS. Another commonly used PCS is CIE-Lab, which is mathematically derived from the CIE-XYZ space. CIE-Lab is intended to be "perceptually uniform", meaning "a change of the same amount in a color value should produce a change of about the same visual importance" (cited from <ulink url="https://en.wikipedia.org/wiki/Lab_color_space">Wikipedia article</ulink>). Wikipedia says "The three coordinates of CIELAB represent the lightness of the color (L* = 0 yields black and L* = 100 indicates diffuse white; specular white may be higher), its position between red/magenta and green (a*, negative values indicate green while positive values indicate magenta) and its position between yellow and blue (b*, negative values indicate blue and positive values indicate yellow)"  (cited from <ulink url="https://en.wikipedia.org/wiki/Lab_color_space">Wikipedia article</ulink>).
        </para>

        <para>
            To be useful, color profiles need to be coupled with software that performs the translation from one color space to another via the PCS. In the world of &Linux; open source software (and also many closed source, commercial softwares), translation from one color space to another usually is done by <ulink url="http://www.littlecms.com/">LCMS</ulink>, the "little color management software". For what it's worth, my own testing has shown that LCMS does more accurate color space conversions than Adobe's proprietary color conversion engine.
        </para>

    </sect3>

</sect2>

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