1. A method for determining the state of health of a tissue in vivo, the method comprising the steps of: obtaining from a tissue a fluorescent emission having a first intensity at a first wavelength and a second intensity at a second wavelength; normalizing said first and second intensities with respect to an intensity at a wavelength that corresponds substantially with an isosbestic point thereby to produce first and second normalized intensities; and determining a state of health of said tissue based upon a comparison of said first and second normalized intensities.