Abstract: A method is provided of measuring in vivo of an endogenous fluorophore in a tissue site. A known excitation wavelength of the endogenous flurophore is selected within a range of wavelengths at which the endogenous flurophore undergoes fluorescence. The tissue site is irradiated with irradiated light having at least the selected excitation wavelength within the range of wavelengths. A fluorescence emission of the tissue site resulting from the irradiation thereof is detected. A relative or absolute concentration of the endogenous fluorophore is determined by multiplying it by a calibration factor that depends one at least one of, a known excitation and emission property of the endogenous fluorophore, an intensity of the irradiated light, optical properties of an excitation probe, and specific properties of the tissue. The relative or absolute concentration of the endogenous fluorphore is used to estimate at least one of a, in vivo cellular energy production status or state of end-organ tissue oxygenation.