Fluorescein is used to strain the cornea. After the excess fluorescein is rinsed from the tear film, enough time it allowed for the fluorescein to come into a steady state condition between the cornea and the anterior chamber. Fluorotron scans are then performed to determine the cornea and anterior chamber concentrations over time. The Fluorotron’s computer software can then use these concentration values to calculate endothelial permeability. If this sounds just like aqueous turnover, that’s because it is. The same method results in values for both aqueous turnover and endothelial permeability.
Selected Reference:
van Best JA, Diestelhorst M, Leite E, Fantaguzzi S, Schalnus R. Corneal endothelial permeability and aqueous humor flow using a standard protocol. Graefes Arch Clin Exp Ophthalmol 233:582-91 (1995). PubMed
A more complete bibliography can be found here.