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 malce use of a previously unknown, low-oscillator strength, optical absorption transition of flavonoids. This malces it possible to optically excite flavonoids in living human tissue outside the absorption range of other, potentially confounding skin chromophores. A system constructed in accordance with the invention includes a source of light for illuminating a localized region of tissue with light that overlaps the absorption bands of a flavonoid compound a device for detecting the fluorescence emitted by the flavonoid compound resulting from the illumination and a processor for determining the concentration level of the flavonoid compound based upon the detected fluorescence.
