The spectrometric measurement of the absorbance of a 50 % caprolactam water solution is carried out at a wavelength of 290 nm. The absorbance, expressed in relation to an optical path length of 1 m, is given by a specified formula in which A_O is the correction for the difference in absorbances of the cells; A₁ is the absorbance of the test solution, and 1 is the optical path length, in centimetres, of the cell.