Previously referred to as extinction coefficient ελ.
Quotient of the spectral absorption coefficient a(λ) of a solution and the concentration c of the substance dissolved therein:
$\kappa \left( \frac{a ( \lambda )}{c} \right)$
Thus κ is the substance constant that makes it possible to calculate the concentration of the dissolved substance from the spectral absorbance A according to the Lambert-Beer Law. It is a function of the wavelength and is determined by absorption spectroscopy.