Measurement Reference Standard
A sample with a known spectral reflectance at each wavelength, usually a white material, against which all percent reflectances (%R) are expressed. Most of the spectrophotometer manufacturers use a numerical reference standard which is called NBS Absolute White.

Metamerism
A phenomenon exhibited by a pair of colors which match under one illuminant, but not match under another illuminant. A metaneric pair of samples exhibit the same tristimulus values for a specific set of viewing conditions but have different spectral distributions. This results in different tristimulus values for other viewing conditions.

Metamerism Index
A calculation representing hte color difference of a pair of samples calculated for two difference illuminants.

Munsell Notation
The tri-coordinate dimensions of Munsell Color Space defined a value, hue and chroma.

Nanometer (nm)
A unit a length used to describe wavelength.

Object
One of the three items necessary for the production of color.

Opacity
A general term used to describe the degree to which a material obscures a substrate. Opacity is sometimes described in term of contrast ratio.

Opaque
A sample sufficiently thick enough that any furture increase in thickness would not change its reflectivity as measured over samples of contrasting backgrounds.

Percent Reflectance (%R)
Reflectance of a sample at each wavelength expressed as a percentage of the reflectance which would be obtained from the measurement reference standard.

Purity
The percentage of color comparing to the percentage of white in any color.

%R
See Percent Reflectance above.