spectrophotometry

 The art of comparing, photometrically, the brightness of two spectra, wave length by wave length; the use of the spectrophotometer.  [Webster 1913 Suppl.]

 the art or process of measuring the degree of absorption of light at different wavelengths by a chemical substance, by means of a spectrometer or spectrophotometer. It is a technique for chemical analysis.  [PJC]