In 1987, Bonanno and Polse first described the use of a non-invasive fluorometric technique for measuring human in vivo stromal pH. Briefly, the theory of the measurement is based on the principle that the fluorescein molecule contains pH-sensitive charged groups. Two different fluorescence intensity (FI) values are obtained when fluorescein is excited with wavelengths of blue light at 490 nm (peak fluorescence) and 450 nm (relatively independent of pH), and the ratio of these intensities (e.g., 490/450 nm) is pH sensitive. Bonanno J, Polse K. Measurement of in vivo human corneal pH: Open and closed eyes. Invest Ophthalmol Vis Sci 1987;28:522-530. Bonanno J, Polse K. Corneal acidosis during contact lens wear: Effects of hypoxia and CO2. Invest Ophthalmol Vis Sci 1987;28:1514. Rivera R, Gan C, Polse K, Bonanno J, Fatt I. Contact lenses affect corneal stromal pH. Optom & Vis Sci 1993;70:991-997.