The Collaborative International Dictionary of English v.0.48:

Spectrophotometry \Spec`tro*pho*tom"e*try\, n.
   1. The art of comparing, photometrically, the brightness of
      two spectra, wave length by wave length; the use of the
      spectrophotometer.
      [Webster 1913 Suppl.]

   2. (Chem., Physics) 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] -- Spec`tro*pho`to*met"ric, a.
      [Webster 1913 Suppl.]