Abstract: Methods and apparatus are disclosed which facilitate the rapid, noninvasive and quantitative measurement of the concentration of flavonoid compounds, as well as their isomers and metabolites, in biological tissue such as human skin. Low-intensity, visible-light illumination of intact tissue provides for high spatial resolution, and allows for precise quantification of the flavonoid levels in the tissue. The preferred embodiments make use of a previously unknown, low-oscillator strength, optical absorption transition of flavonoids. This makes it possible to optically excite flavonoids in living human tissue outside the absorption range of other, potentially confounding skin chromophores.