Computed from tristimulus values, most often with the ASTM E313 formula, it reads zero for a neutral white and climbs as blue reflectance falls relative to green and red; negative values mean a bluish sample. Mills use it to follow light and heat yellowing of mechanical and recycled grades, and to check whether a shade change has pushed a white toward cream.
Yellowness index on the groundwood grade went from 9 to 14 after the light-box exposure, so the complaint checks out.
Also heard as
- YI
- E313 yellowness
Related terms
Brightness reversion
Loss of brightness as pulp or paper yellows with age, heat or light exposure.
b*
Yellow-blue coordinate in CIELAB, positive toward yellow and negative toward blue, watched closely on white papers.
Whiteness
How white a surface looks to the eye, judged across the whole visible spectrum rather than one blue band.
CIELAB
Color space describing a color by lightness L* plus a* for red-green and b* for yellow-blue.