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A white point question ...

Apr 30, 2013 3:45 PM

Most of the chromaticity diagrams for RGB color spaces show the D65 illuminant as a white point. White point is where all three primaries are equal in intensity, so it is a neutral color at every luminance level.

 

What about D65? ... is it an exactly neutral color illuminant?

 

Or, it means that, if we look at the chromaticity diagram under the illuminant D65, the white point of the color space takes the color of this illuminant and for this reason, white points are labeled as D65 in chromaticity diagrams?

 
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  • Currently Being Moderated
    Apr 30, 2013 3:52 PM   in reply to AttilaHan

    You may be confusing gray point or gray balance with white point. 

     

    …or I may be misconstruing your question. 

     

    Message was edited by: station_two

     
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    Apr 30, 2013 3:55 PM   in reply to AttilaHan

    This might help:

     

    http://www.techmind.org/colour/coltemp.html

     

     

    —ADDENDUM:  Please note I edited my first reply.  Edits are not seen by anyone accessing the forum through email.

     
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    Apr 30, 2013 3:57 PM   in reply to AttilaHan

    You typed:

     

    White point is where all three primaries are equal in intensity

    That is incorrect.

     
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    Apr 30, 2013 3:59 PM   in reply to AttilaHan

    Incidentally, this topic belongs in the Color Management topic, where the color gurus hang out.

     

    http://forums.adobe.com/community/design_development/color_management? view=discussions

     
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  • Currently Being Moderated
    Apr 30, 2013 4:08 PM   in reply to AttilaHan

    AttilaHan wrote:

     

    Thanks for the links ...

    station_two wrote:

     

    You typed:

     

    White point is where all three primaries are equal in intensity

    That is incorrect.

     

    I think that it is correct ))

     

     

    I don't think so. 

     

    According to that statement, as phrased, RGB = 0,0,0  or even RGB = 1,1,1 would qualify as "white points"…

     
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    Apr 30, 2013 4:15 PM   in reply to AttilaHan

    AttilaHan wrote:

     

    White point is a general term used for all neutral colors ... for white, for black and for all the shades of gray. It is called white point, but not necessarily white in color. All the chromaticity diagrams show a shade of gray as a white point…

     

    That's not what your OP says, but I'm out of this thread now.

     
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    Apr 30, 2013 4:19 PM   in reply to AttilaHan

    AttilaHan wrote:

     

    Ok, my question was ... D65 illuminant is really a neutral color?…

     

    Of course not—by definition:

     

    CIE Standard Illuminant D65 (sometimes written D65[1][2]) is a commonly-usedstandard illuminant defined by the International Commission on Illumination (CIE).[3]It is part of the D series of illuminants that try to portray standard illumination conditions at open-air in different parts of the world.

    D65 corresponds roughly to a midday sun in Western Europe / Northern Europe, hence it is also called a daylight illuminant. As any standard illuminant is represented as a table of averaged spectrophotometric data, any light source which statistically has the same relative spectral power distribution (SPD) can be considered a D65 light source. There are no actual D65 light sources, only simulators. The quality of a simulator can be assessed with the CIE Metamerism Index.[4][5]

    The CIE positions D65 as the standard daylight illuminant:

    [D65] is intended to represent average daylight and has a correlated colour temperature of approximately 6500 K. CIE standard illuminant D65 should be used in all colorimetric calculations requiring representative daylight, unless there are specific reasons for using a different illuminant. Variations in the relative spectral power distribution of daylight are known to occur, particularly in the ultraviolet spectral region, as a function of season, time of day, and geographic location.

    —ISO 10526:1999/CIE S005/E-1998, CIE Standard Illuminants for Colorimetry

     

    http://en.wikipedia.org/wiki/Illuminant_D65

     
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    Apr 30, 2013 4:24 PM   in reply to station_two

    256px-SPD_D65.png

     
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    Apr 30, 2013 6:19 PM   in reply to station_two

    There are two things getting mixed up, here:  the surface reflecting light having neutral reflectivity and the color of the light (or more precisely, the spectral characteristics of the light) illuminating the surface, D65, D50, sunlight, fluorescent light, etc.

     

    D65 is a color of light with a particular power spectrum of different wavelengths that is approximately equal to European-mid-day sunlight.

     

    If a surface has neutral reflectivity then it reflects back the same intensity of the RGB colors as the light itself has and so the color of the light-source can be reverse-engineered by using the WB-eyedropper on the neutral surface to compute the color of the light and make other colors of the scene correct as well, with correct meaning as the human eye would see it on earth viewed with whatever light-source is illuminating the scene.

     

    The RGB colors are what our eyes detect with three different kinds of photosensitive cells our retina, so neutral has something to do with human perception, as well as the color of light coming from or closest star filtered through the earth’s atmosphere, not just pure spectral numbers or photon counts on a sensor.  These photosite photon counts must be converted to human-perception-on-earth-with-daylight numbers by the camera profile numbers and formulas.

     

    The complication of metamerism is evident when even though two light sources both appear to be neutral when reflected off a neutral surface, they don’t show the same colors when reflecting off a non-neutral-colored surface—white looks white but a blue flower looks different in fluorescent light compared to sunlight even though the photos are WBed for fluorescent and sunlight respectively by eyedroppering a neutral surface. 

     

    This is why camera profiles are not just about white-balance but also clamping other colors to the right value—why a color-checker has different colored patches and not just a series of grays. 

     

    AttilaHan, what were you wondering about, or trying to understand, when you asked your initial question about D65 being neutral?

     
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  • Currently Being Moderated
    May 1, 2013 4:40 AM   in reply to AttilaHan

    White point is a general term used for all neutral colors ... for white, for black and for all the shades of gray.

    Where did you get that definition?

    http://en.wikipedia.org/wiki/White_point

     
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