Fluorochrome (eg, fluorescein) conjugated directly to an antibody binds to a cell. Cells stained with fluorescein are stimulated by a laser-emitting light of a wavelength significant to stimulate the fluorescein molecule. Fluorescence occurs when the molecule excited by light of a particular wavelength loses its energy by emitting light of a longer wavelength. The fluorescence spectra shown above demonstrates the excitation spectra of a fluorochrome (left), the wavelength of a typical laser source, the emission spectra of a fluorochrome (right), and the wavelength range detected by a typical band pass filter.
