Calcein, also known under the name Fluorexon, is a fluorescein derivative and a fluorescent dye with an excitation maximum at 494 nm (excitation) and a radiation maximum (emission maximum) of 517 nm (at a pH of 9.0).
Calcein is useful in fitting soft and hybrid contact lenses, along with its evaluation. It has a molecular weight of 710kDa and is generally preferred over fluorescein. Calcein is known to stain devitalized tissue. It is not preferred for highly hydrated soft lenses.