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 E313-15 (approved 2015), 'Standard Practice for Calculating Yellowness and Whiteness Indices from Instrumentally Measured Color Coordinates.'
ASTM Designation D1925-70, 'Standard Test Method for
Yellowness Index of Plastics.'
ASTM D1925 has been withdrawn, but its yellowness index is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:
