Fluorescein is a highly fluorescent substance with an absorption maximum at 494 nm and emission maximum of 512 nm in water. 

Fluorescein and its derivatives are commonly used in the following applications:  

• Microscopy
• Analytical chemistry as an indicator in silver nitrate titrations
• Forensics and serology to detect blood traces
• A specific type of dye laser as the gain medium
• Dye tracing
• Environmental testing simulations 

The sodium salt of fluorescein, fluorescein sodium, is used extensively in ophthalmology and optometry as a diagnostic tool.