Evaluation of Reflectance Remission Spectra

If light falls on to a white powder, a fraction of it is returned by regular reflection from the crystal surfaces, governed by the grain size distribution. The smaller the crystals are, the smaller is this regular reflection. The bulk of the incident radiation, however, penetrates into the layer of powder to a depth which depends on its wave length, returns to the surface after repeated scattering and is then emitted as a diffuse reflection (remission) in hemispherical form. 

A standard white , reflecting as uniformly as possible over the wave length range concerned, would naturally be best both as standard and as adsorbent. TLC is however restricted to particular adsorbents. Limits are thus set, resulting from the diminution in the degree of 

If light-absorbing substances are mixed with the standard white or if the substances are present as absorbing spots on the surface of such a standard, the intensity of remission will be weakened this weakening occurs in those wave length regions where absorption bands would occur in transmission measurements on solutions of the substances. The remission can be calculated quantitatively with the help of the Kubelka-Munk function. 

The degree of reflection is proportional to the concentration of substances adsorbed on a standard white, under certain suppositions which have been studied in detail by Kortum and co-workers [371, 373] for powders. The relation is  

The standard white can thus be used as a diluent for solids, analogous to the use of solvents when appljdng the Lambert-Beer law to determine concentrations in solution. 

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