Bacteriorhodopsin molecule is purple and is most efficient at absorbing green light (in the wavelength range 500-650 nm). In the native membrane, the protein has a maximum absorbance at 553 nm, however addition of detergent disrupts the trimeric form, leading a loss of exciton coupling between the chromophores, and the monomeric form consequently has an absorption maximum of 568 nm.
Bacteriorhodopsin has a broad excitation spectrum. For a detection wavelength between 700 and 800 nm, it has an appreciable detected emission for excitation wavelengths between 470 nm and 650 nm (with a peak at 570 nm).
When pumped at 633 nm, the emission spectrum has appreciable intensity between 650 nm and 850 nm.