Fluorescein

The excitation spectrum and emission spectra (bold line) are shown for Fluorescein (FITC). The red-shaded region represents the bandpass filter commonly used to measure FITC fluorescence. On most flow cytometers, FITC is excited by an argon laser (tuned to 488 nm)
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Compare the FITC fluorescence spectra to spectra for several FACS immunofluorescence dyes.