An optical property of a substrate that describes the degree to which a substrate diffusely reflects light of all wavelengths (i.e., colors) of the visible spectrum. The ideal white standard (a powder of barium sulfate) reflects 100% of all the light falling on it. Whiteness is described in terms of the amount and uniformity of light reflectance over the visible spectrum (from 400 to 700 nanometers, or from the violet to the red end of the spectrum) as compared to the light reflectance of the ideal white standard. The comparative whiteness of various papers is determined by comparing their spectral reflectance curves.
Whiteness is an issue in paper production due to its relationship to print quality. There is more of a contrast between the paper and a colored or black ink the whiter the paper is. The greater the whiteness of the paper, the greater the contrast—and the readability—of the ink.
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