A two-photon microscope is also a laser-scanning microscope, but instead of UV, blue or green laser light, a pulsed infrared laser is used for excitation. Only in the tiny focus of the laser is the intensity high enough to generate fluorescence by two-photon excitation, which means that no out-of-focus fluorescence is generated, and no pinhole is necessary to clean up the image. This allows imaging deep in scattering tissue, where a confocal microscope would not be able to collect photons efficiently. Two-photon microscopes with wide-field detection are frequently used for functional imaging, e.g. calcium imaging, in brain tissue. They are marketed as Multiphoton microscopes by several companies, although the gains of using 3-photon instead of 2-photon excitation are marginal.