Pros and Cons of RGB versus Tristimulus Color Scales

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FAQ: "We are currently using a HunterLab Colorimeter and EasyMatch QC software to collect data on some of our newer polymers. We are reporting color using the Hunter L, a, b; XYZ, and L*a*b* scales. One of our colleagues is suggesting that we collect and report using the RGB scale. I think we are just fine and will not change the technique, but could you please provide me with a quick and simple explanation as to why RGB would not be a good choice for our reflectance work? I have a quick converter to provide RGB numbers, but I really don't want to go down that path."

Color is inherently three dimensional such that you if you have 3 numbers in one color scale, it is basically a 3 x 3 matrix to mathematically convert to any other 3 dimensional color scale. The sRGB conversion of RGB from CIE tristimulus values is best defined by IEC 61966-2-1.

Reference:

IEC 61966-2-1 Multimedia systems and equipment - Colour measurement and management - Part 2-1: Colour management - Default RGB colour space - sRGB

IEC - International Electrotechnical Commission, Geneva, Switzerland www.iec.ch

IEC 61966-2-1:1999 provides the definition of the sRGB conversion and provides implementations details in a multi-part series under IEC 61996.

  • The origins and purposes of the tristimulus and RGB scales are different. RGB is an additive color scale used for representing colors digitally in multimedia digital devices and is based on their requirements. CIE XYZ is a colorimetric scale developed to quantify color as perceived by humans.
  • For a typical 8 bit digital system, the RGB range used to represent digital colors is from 0 to 256 levels in red, green and blue channels. CIE XYZ levels to the second decimal place can represent far more colors and has a higher level of precision. The conversion from the more precise X, Y, Z to less precise RGB for reporting purposes reasonable but not the inverse RGB-to-XYZ conversion, which is mathematically correct but is at best, an approximation of the true color of an object.
  • RGB is used as a flat, solid color representation of the average color of the object on a screen. XYZ is the measured color of the true product color, not a rendition.
  • RGB is typically only calculated for D65/2 conditions although it has been expanded to D65/10.

Both scales have their merits but XYZ, and its derivative L, a, b-type scales, is the better of the two for color measurement. It measures the true color of the product; RGB is useful for display of XYZ on a digital display.

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