Metamerism occurs when two samples appear identical under one set of viewing conditions, but not under another set of conditions. When this occurs, we call the two samples a metameric pair. Metamerism occurs very frequently in plastics.

1. Colorimetric Metamerism

The spectrophotometer curve is the best indicator of colorimetric metamerism. Two spectrophotometer curves that show at least three or more crossings are a clear indication that metamerism is present, indicating the likelihood of a visual observation as shown. A metameric pair will match closely and have nearly identical tristimulus values under one illuminant, but will not match visually and will exhibit a significant instrumental color difference under other illuminants. Most commonly, metamerism occurs because each sample is a different polymer, has a different colorant formulation or both. Because metamerism is common, SABIC Innovative Plastics customers should be careful to give ColorXpress* Services personnel the information on the various light sources that the application will see.

2. Observer Metamerism

This form of metamerism is more difficult to recognize. No two individuals see exactly alike. Two individuals may disagree when evaluating a pair of colored samples or on the color of a single sample. All ColorXpress* Services colorists undergo periodic color and hue vision testing to promote consistent visual assessment.

3. Geometric Metamerism

This subtle form of metamerism is sometimes called “flop” and occurs when samples appear to match at one angle of illumination but do not match if the angle of illumination or angle of viewing is changed. Geometric metamerism may develop from factors such as the orientation of colorants and additives in samples during processing. Geometric metamerism most often is an issue with pearlescent and special effects plastics, but can also be an issue with highly impact modified plastics, due to the orientation of the rubber particles in the matrix.