spec·tro·pho·tom·e·ter 
(spĕk′trō-fō-tŏmĭ-tər)
(spĕk′trō-fō-tŏmĭ-tər)n.
An instrument used to determine the relative intensity of various wavelengths in a spectrum of light.
spec′tro·pho′to·metric (-fō′tə-mĕtrĭk) adj.
spec′tro·pho·tome·try n.