What happens to the light that is shined on a sample, as from the light source of a spectrophotometer? Some is reflected back to the instrument detector, some is absorbed by the colorants in the sample, and some of it is scattered in all directions within the sample.

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For samples with opacities greater than 75%, the Kubelka-Monk equation (established in 1931) defines a relationship between spectral reflectance (R in %) of the sample and its absorption (K) and scattering (S) characteristics, as follows:

K/S values for the sample measured are calculated separately for each wavelength read. K/S may only be calculated for measurements made in a reflectance mode. K/S is a spectral data type, meaning it is calculated and displayed for each measurement wavelength in the Spectral Data Table of your EasyMatch QC or Universal software. K/S grows to infinity as reflectance decreases to zero, so the software may not display a K/S value for a particular wavelength or wavelengths if the reflectance is very low (i.e., less than 0.05%).

The Kubelka-Monk equation is useful when formulating colors for industries such as textiles, paper, and coatings. For these applications, it is assumed that the scattering (S) of a dye or pigment depends on the properties of the substrate or opacifier, while the absorption (K) of light depends on the properties of the colorant. The Kubelka-Monk equation is roughly linear with respect to colorant concentration, as follows:

K/S = kC

where

C = concentration of the colorant k = a constant.

(See attached pdf file for the complete article with Diagrams and Formulas)

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