Why does absorbance data relate more conveniently to concentration than transmission data

For quantitative work the absorbance is more useful than the transmittance because it is directly proportional to concentration. The relationship is known as the Beer-Lambert Law and is:

A = abc

where a (molar absorptivity or extinction coefficient) is a constant for a given absorbing chemical species, b is the radiation path length through the sample (measured in cm), and c is the stoichiometric molar concentration (mol/L) of the absorbing species.