Metamerism exists if a sample looks different under two different lighting conditions.

The metamerism index (MI) indicates the probability that two samples (reference and measured sample) will show the same color deviation under two different illuminants. If the metamerism index is zero, the two samples have the same degree of metamerism, i.e. one sample looks as much similar or different under the two illuminants as the other.

The MI is a delta E value calculated from the difference between the following:

• CIE Lab deviation between reference and measured sample viewed under a reference illuminant (e.g. D50)

• CIE Lab deviation between reference and measured sample viewed under a test illuminant (e.g. A, C or F11)