effectively infinite thickness

in flame spectroscopy
https://doi.org/10.1351/goldbook.E01895
Solid or liquid samples of one @G02680@ or more are generally used in @X06718@. 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 @B00756@ of effectively infinite thickness.
Source:
Orange Book, 2nd ed., p. 182 [Terms]