tristimulus

(Pronounced TRISS-stem-mule-less. Although thinking about it as "tri" meaning three, and "stimulus" to excite, helps to define the word.) A method involved in the measuring the color of light, based on how it is perceived by the human eye and processed by the brain, by determining the amount of Red, Blue, and Green (hence the "tri") transmitted.

The Total Transmission percentages listed on color swatchbook backsheets is usually the Tristimulus Y value.

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For more, see Color Perception.