Yellowness index is a calculated number used to track a shift toward yellow in materials intended to be white, clear or nearly colorless. It is particularly useful for ageing, heat history, UV exposure, contamination and batch consistency—but only when every result uses the same formula and sample method.
What the index measures
The instrument measures a reflectance or transmittance spectrum, calculates the required tristimulus values and applies the selected yellowness equation. A higher positive value commonly indicates a stronger yellow contribution under that method. The number is application-specific; it is not a universal description of all yellow colors and it does not identify the chemical cause.
Name the yellowness equation
ASTM E313 is the current widely used practice and supports specified illuminant/observer conditions; fixed C/2° and D65/2° reports are common in instrument software. ASTM D1925 was withdrawn in 1995 but still appears in legacy plastics specifications. E313 and D1925 values are not interchangeable, so supplier documents should state the exact index and calculation condition.
Where yellowness index is useful
Plastics
Track resin, film or moulding changes after processing, heat or weathering.
Textiles and consumer goods
Monitor storage, washing, exposure or material degradation in pale products.
Coatings and building materials
Quantify discoloration in clear coats, sealants and nominally white surfaces.
Control the method before trending results
| Variable | Why it matters |
|---|---|
| Formula and observer | Different equations and calculation conditions produce different scales. |
| Reflectance or transmission | The modes answer different optical questions and cannot be mixed in one trend. |
| Thickness or backing | Path length and background can change a pale material’s apparent yellow contribution. |
| Surface and orientation | Texture, gloss, directionality and contamination can move the result. |
Pair the index with diagnostic data
When a result fails, review the spectrum and L*, a*, b* values. The index shows that a change occurred; the additional data helps reveal its direction and likely process relationship.
Set a product-specific limit
Create the limit from approved material, ageing studies and measurement-system capability. State the formula, geometry, illuminant, observer, sample thickness or backing, conditioning and sampling plan. Include a retest and escalation rule for borderline values. Use the tolerance-setting process, choose the optical mode and retain spectral data for diagnosis.
Last reviewed 21/09/2026 · Level: Intermediate
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Applications
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