In biology, metamerism is defined as the property of having repeated structural segments in the body of an organism and is often studied in zoology and developmental biology
Metamerism is also a particular psychophysical phenomenon related to color and the perception of color.
Sample metamerism happens when two color samples look like they match if put under one type of light source but will look like they do not match if they are put in another type of light source. This phenomenon usually happens when the spectral reflectance distributions of the two samples have subtle differences and their plotted reflectance curves meet or cross in two regions at the very least. By lighting these samples with lights that have significantly different spectral power distributions the phenomenon can be witnessed and even exaggerated
Illuminant metamerism, on the other hand, happens when a number of samples that are matched spectrally will look different if each sample is viewed independently but simultaneously illuminated and viewed with lights that have different spectral power distributions. The differences are seen as very noticeable variations in color. This is a type of metamerism that is not often witnessed unless the person employs a light box that allows the user to see both lights using a divider and the two samples are lighted by the two different light sources.