THE MATHEMATICS OF COLOR DIFFERENCE.
- Experiments were done that were set up to test how an observer would judge just barely visual color differences. The device was similar to the devices that were used to test the human color matching function. In this case the red, green, blue filtered lights were focused to mix colored light to match the target color. The observer changed the balance of the light mixture until they just saw a change in the color of the match lights to the test light.
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- The data from this experiment showed that we see color more critically when a color is lighter than darker, and we judge color difference more critically in the blue range than in the red or green areas of color space.
- The mathematics resulting from these experiments is called color difference calculations. They can predict that one color will appear lighter or darker, redder or greener, yellower or bluer than another. We can also calculate if the color is shifting in lightness, changing in hue, or changing in chroma.
| Lighter | ||
| Greener |  |
Yellower |
| Bluer | Redder | |
| Darker | ||
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| In a practical way, we now have the tools to take measurement data from measuring several colored objects and calculate meaningful data that tells us what the color of each object is and the color difference between the objects. We express this data in the three dimensions of human color response. The mathematics is expressed as L, a, b factors defined as either Hunter L,a,b or CIE L,a,b: | ||
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| Lighter +L | L = lightness to darkness | Darker -L ................... |
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| a = +a redness to greenness -a |
| b = +b yellowness to blueness -b |
| (greener or less red the "a" factor becomes smaller) | (redder or less green the "a" factor is larger) |
| (yellower or less blue, the "b" factor is larger) | (bluer or less yellow, the "b" factor becomes smaller |
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Two other color difference equations are LCh or CMC (which all report in LCh):
L = lightness to darkness (+ L or - L)
C = chroma, from neutral to fully saturated color (+C or - C)
h = hue, is the hue angle of color change around the color wheel (+h or minus h)
Another color difference calculation often discussed is Delta E, called total color difference. It is a color difference index calculated as the square root of the sum of the squares of the color difference of the three dimensions of color space. It is useful when you need a single number indicator of overall color change.
The practical use of color difference calculations is covered in the section "Color Quality Control Testing"
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