effectively infinite thickness

in flame spectroscopy
Solid or liquid samples of one gram or more are generally used in X-ray spectroscopy. If the sample is thick enough so that the intensity of characteristic fluorescent radiation is not significantly modified by an increase in thickness, the sample is described as a bulk sample of effectively infinite thickness.
Orange Book, 2nd ed., p. 182 (http://media.iupac.org/publications/analytical_compendium/)