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Yellowness index

Yellowness is a property important in many industries, for several reasons. First, processing of various materials may cause yellowing. Next, the purity of some products may be determined based on the amount of yellowness present. Also, some products degrade and yellow with exposure to sunlight, temperature, or other environmental factors during use. Thus, yellowness has become an important variable to measure in industries such as textiles, paints, and plastics.

There are different types of yellowness indices available, depending on the type of product being measured. Two of the most common are

  • ASTM Designation E313-73 (Reapproved 1993), “Standard Test Method for Indexes of Whiteness and Yellowness of Near-White, Opaque Materials”

  • ASTM Designation D1925-70, “Standard Test Method for Yellowness Index of Plastics.”

ASTM D1925 has recently been withdrawn, but its yellowness index is still used in many industries. Samples measured by method E313 must be nearly white and opaque. When items are being compared using YI E313, they must be similar in gloss, texture, translucency, and other physical attributes. If these criteria are not met, the yellowness values will not be meaningful. Products such as paints, textiles, and plastics are often measured by this method.

The D1925 method requires samples that are nearly colorless transparent plastics or nearly white translucent or opaque plastics. When items are being compared using YI D1925, they must be similar in transparency, translucency, opacity, thickness, shape, and other physical attributes. As with method E313, if these criteria are not met, the yellowness index values are not meaningful.

The yellowness index YI is calculated from the following equations:

for E313-73 yellowness index

for D1925 yellowness index

where X, Y and Z are the tristimulus values.

The yellowness index is just defined for illuminant C!