For live-cell imaging, two-photon excitation microscopy (TPEM) is proving to be a significant technological advancement. The unique features offered by TPEM are the ability to image thick sections, excellent optical sectioning capabilities, low damage to living cells, and less out of focus fluorescence and out of focus photobleaching. of these features, the most useful for the biological microscopist, is optical sectioning. Optical sectioning is an intrinsic property of the two-photon process, whereby, two infrared (IR) photons are absorbed quickly to excite a single UV/blue transition. The probability for exciting a two photon transition is proportional to the instantaneous excitation intensity squared. Therefore, for a focused laser beam, only light at the focal point of the excitation beam excites a fluorescent transition. Thus, the need for confocal apertures and time consuming deconvolution algorithms are, for the most part, eliminated.

We have continued to develop and enhance our ability to perform high-speed, two-photon excitation fluorescence microscopy. in 1998, we successfully deployed a prototype, video-rate twophoton laser scanning system (30 frames/sec or faster at reduced scan width) developed with support from Nikon Corporation. That system was built upon a Nikon RCM 8000 confocal microscope.