Carotenoid pigments within the skin and retina are measured using resonance Raman spectroscopy (RS). These RS instruments are unique in that they have been designed to obtain vibrational spectra in normal and diseased subjects using noninvasive procedures. Raman spectra have traditionally been used as a means of identifying a given chemical within some substrate. The new generation of RS instruments, however, has been designed to quantify the amount of carotenoids within the retina and skin. These amounts are typically reported in nonstandardized units called Raman counts (RC). These RCs are dependent on many factors intrinsic to their measurement, such as the specific optics used for stimulation and acquisition. The question of whether RCs can be used to derive valid quantitative measures of the carotenoid pigments in vivo is discussed.
Billy R. Hammond,
"Resonance Raman spectroscopic measurement of carotenoids in the skin and retina," Journal of Biomedical Optics 10(5), 054002 (1 September 2005). http://dx.doi.org/10.1117/1.2116767