Multiphoton excitation of a fluorophore in a specimen occurs when two or more photos of excitation light are absorbed at the same time. Because the photon energy is inversely proportional to wavelength, longer wavelength light (approximately two times as long) may be used for multiphoton excitation compared with one-photon excitation. A fluorophore, for example, that normally absorbs ultraviolet (UV) light of 350nm can be excited by two photons of near-infrared (IR) light of approximately 700 nm. Because excitation occurs only where photons coincide, the excitation light is not attenuated by fluorphore absorption above and below the point of focus (enabling greater depth penetration). Phototoxicity and photobleaching are also greatly reduced. Multiphoton imaging is generally achieved not only by focusing photons spatially, but also by focusing them in time using a pulsed laser. IR imaging also requires objectives with high IR transmission rates.