Multiphoton microscopy uses pulsed long-wavelength light to excite fluorophores within the specimen being observed. The fluorophore absorbs the energy from two long-wavelength photons which must arrive simultaneously in order to excite an electron into a higher energy state, from which it can decay, emitting a fluorescence signal. It differs from traditional fluorescence microscopy in which the excitation wavelength is shorter than the emission wavelength, as the summed energies of two long-wavelength exciting photons will produce an emission wavelength shorter than the excitation wavelength. Multiphoton fluorescence microscopy has similarities to confocal LSM. Both use focused laser beams scanned in a raster pattern to generate images, and both have an optical sectioning effect. Unlike confocal microscopes, multiphoton microscopes do not contain pinhole apertures, which give confocal microscopes their optical sectioning quality.
The longer wavelength, low energy (typically infra-red) excitation
lasers of multiphoton microscopes are well-suited to use in imaging live
cells as they cause less damage than short-wavelength lasers, so cells
may be observed for longer periods with fewer toxic effects.