Measuring White: Whiteness and Yellowness Indices

White should be the simplest color - it's just "all of it," a surface reflecting every wavelength equally. In practice it's the fussiest thing to specify. Most white materials aren't neutral: paper pulp, cotton, and plastics carry a faint yellow cast, and the eye is exquisitely sensitive to it. Two whites that measure nearly identical in ordinary color terms can look markedly different in cleanliness and quality.

Worse, whiteness is preference-laden. A perfectly neutral white can look dull and creamy, while a white nudged slightly toward blue reads as brighter, fresher, cleaner. So the industries that sell "white" - paper mills, detergent makers, dentists, plastics producers - need numbers that track not just neutrality but the specific, slightly-blue direction people call "whiter." Those numbers are the Whiteness and Yellowness indices.

The CIE Whiteness formula turns a near-white's tristimulus values into one score: a perfect diffuse white under the reference light is exactly 100, yellower samples fall below, and slightly blue ones rise above. A companion Tint value (Tw) says whether the white leans green (positive) or pink (negative) - because a "white" can be off in that direction too. Nudge a sample and watch both numbers move.

Where whiteness measures how clean a white is, the Yellowness Index (YI) measures how far it has drifted toward yellow - and it's the number of choice for tracking degradation. Plastics yellow under heat and UV; paper and adhesives yellow with age; coatings amber over time. YI (ASTM E313) reads zero for neutral and climbs as the sample yellows, so a rising YI is a direct readout of a material going off. Age a sample and watch it climb.

Here's the counterintuitive heart of it. Add a whisper of blue to a slightly yellowish white and it measures - and looks - whiter. The blue cancels the yellow cast, and because the whiteness formula rewards the blue direction, the score can climb past 100. This is bluing: laundry "blue" and the blue tint in copy paper are centuries-old versions of the trick. But overdo it and the sample stops reading as white and starts looking blue. Find the sweet spot.

Modern whites don't just add blue pigment - they add optical brightening agents (OBAs), also called fluorescent whitening agents. These absorb invisible ultraviolet light and re-emit it as blue, effectively creating extra blue light out of nothing the eye could otherwise see. Under a UV-rich light (daylight, a nightclub blacklight) an OBA-treated white glows and its whiteness leaps. Under UV-free light, the trick vanishes. Toggle the UV.

A single "how white?" number drives quality control across a surprising range of industries.

Six questions on whiteness, yellowness, bluing, and optical brighteners. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Measuring white draws on fluorescence, tristimulus values, the observer, and the instruments that read them - here's where to go next.