metamerism

Metamerism is a form of structural isomerism in organic chemistry. Two metamers have the same molecular formula and the same sequence of atoms, but differ in how substituent groups are distributed around a functional group. A classic example is the family of ethers with formula C4H10O: methyl propyl ether (CH3–O–C3H7) and ethyl ethyl ether (CH3CH2–O–CH2CH3). These compounds share formula and carbon skeleton but differ in the R1 and R2 groups attached to the oxygen atom, yielding different physical properties and chemical behavior.

In color science, metamerism describes a different phenomenon: two color samples may look identical under one light source but appear different under another. This occurs when the spectral power distributions of the samples differ, and the observer’s color matching results converge under one illuminant but diverge under another. Metamerism is a common concern in textiles, paints, and printing, where pigments or dyes designed to match under showroom lighting may diverge under daylight or fluorescent lighting. It is often addressed by testing color matches under several standard illuminants (for example, D65 and incandescent A) and by selecting pigments with complementary spectral properties.

Metamerism is distinct from other forms of isomerism. It involves the same molecular formula and carbon skeleton but a different distribution of substituents around a functional group, whereas constitutional isomerism refers to different connectivity elsewhere in the molecule and stereoisomerism to spatial arrangement in chiral or geometrical isomers. The concept highlights how similar molecules or colors can behave or appear differently under varying conditions.