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. A new version of the Fluorotron has been designed for this purpose. The instrument automatically focuses on the cornea and measures 100 fluorescence ratios, applying computer algorithms for blink and movement correction. Measurement time is approximately 15 seconds.