Re: How to capture exact RBG or CMYK color numbers?

Discussion in 'Digital Photography' started by Wolfgang Weisselberg, Apr 5, 2013.

  1. Jennifer Murphy <> wrote:
    > On Thu, 04 Apr 2013 09:48:48 +0100, bugbear
    >>Jennifer Murphy wrote:



    > Aha. If I shine a red light on it, it will look different than if I
    > shine a green or pure white light on it. So the only way to get the
    > "true" colors of the object is to take it to a carefully controlled
    > environment where it is illuminated by a perfectly balanced light
    > source. This business is maddeningly complex and nuanced.


    Welcome to the mad world of colour management.


    >>You can quite readily calibrate your shot
    >>by including a known target in the photograph,
    >>or shooting the target under the same conditions
    >>as your photograph.


    >>Proper targets aren't super-cheap.


    >>Here's a handy page about this:


    >>http://www.aff.org.au/AFF2_Eckert_colour_final_summary.htm


    > What if I lower my expectations or, at least, make them soemwhat more
    > realitic.


    Chicken. :)

    > Instead of saying I want to get the "exact" RGB numbers,
    > suppose I say that I want to get the RGB numbers for the painting as it
    > looks hanging in my living room under midday ambient light?


    You need to specify what RGB you're using. sRGB, AdobeRGB,
    ProPhotoRGB ...

    Basically you need to define what colour pure red, green
    and blue is in your RGB values (and how large one step is,
    etc.) --- and to span all of the visible range (which isn't
    a triangle but a "horseshoe") you need to place these pure
    colours outside the visible range.


    What you probably want is the spectrum of the colour under a
    well known light. Using that info you (or rather a software)
    can (as I understand it, usually) calculate how it looks
    under a different light. (That's what's done to profile
    printer-ink-paper combinations.)


    > Could I then get a color wheel and match up 4-5 spots on the portrait to
    > spots on the wheel and record the RGB numbers? Or it there some sort of
    > handheld scanner that will read out RGB numbers?


    You'll want patches that are large enough and homogenous over
    the patch in every way.


    > I couldn't find anything, but I didn't know what to search for. I did a
    > search for "rgb scanner" and came up with some sort or hot car
    > accessory.


    Nope. You want a something like a spectrophotometer (and
    you'll probably have to place it directly on the patches, too,
    so there's no external light on it).

    -Wolfgang
    Wolfgang Weisselberg, Apr 5, 2013
    #1
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  2. Jennifer Murphy <> wrote:

    > I'm going to get a color checker chart. I'll give it to my friend when
    > the project is over. The only question is which one. I posted a list in
    > another reply. I'd love to have your opinion. ;-)


    Choose one that your RAW converter supports, if at all possible.
    Software that can at least work with 16 bit TIFF might be a
    secondary solution.

    You may have to replace the color card every few years if
    you need to be colour exact: the colours on the card age,
    and age differently from each other. So if you don't use
    them very often you may want something not too expensive


    Oh, do remember: if the light changes, you need another shot
    where the colour card is in the (test)image.

    -Wolfgang

    PS: As I understand it, you can use colour cards with scanners
    or cameras with the free[1] colour management system
    Argyll (http://www.argyllcms.com/) to create an ICC for
    that specific device (and light, with cameras) using the
    overview at
    http://www.argyllcms.com/doc/Scenarios.html#PS1
    (there's a list of targets and sources for them that work
    more-or-less out of the box with Argyll CMS. You
    probably want a camera target, not a scanner target.)

    Unfortunately I don't have any personal experience with
    that specific workflow.

    [1] as in speech and as in beer
    Wolfgang Weisselberg, Apr 10, 2013
    #2
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