Metamerism exists when two materials match under some lighting conditions but not under other lighting conditions. This can be a serious problem for manufacturers who combine various parts of different materials or materials from different sources into one product. A customer expects all the parts of a car that are the same color to match whether it is a sunny or cloudy day. Someone who buys a coordinated outfit expects the jacket and slacks to match in daylight as well as under the fluorescent
lights of the department store.
Metamerism index is a single-number index which indicates how well two materials that match under one illuminant will match under another illuminant. Typically, CIE Illuminant D65 is the reference illuminant and the test illuminant is one chosen depending on the end use of the material. For example, CIE Illuminant F2 may be chosen as the test illuminant for cloth going into garments that will be
displayed in department stores lit with fluorescent lights. This index is described in CIE Publication 15.2 (1986), Section 5.2.
For this index to be meaningful, the materials should be a good match under the reference illuminant. Using HunterLab instruments, any two illuminants may be chosen for calculating metamerism index. The metamerism index calculation is based on Hunter L, a, b values.
Conditions for Measurement
Instrumental: Any HunterLab spectrophotometer
Standard Observer Function: 2 or 10 degree
Transmittance and/or Reflectance: Either.
This index can be used to measure any material for any industry in which non-metameric matches are important.
(See attached pdf file for the complete article with formulas)