As described in the following section, many FPs can be excited at a wavelength range of 250 to 300 nm. To access this deep UV (DUV) range for simultaneous multicolor imaging, we propose the use of a visible, in particular green, wavelength laser light and induce two-photon excited fluorescence from the FPs. As shown in Fig. 1(a), the simultaneous absorption of two photons in the visible range can excite a fluorophore in a manner equivalent to that of a DUV wavelength. The use of fluorescence excitation in the UV region has been applied in autofluorescence imaging of intrinsic fluorophores, such as amino acids, biotin, nicotinamide adenine dinucleotide (NADH), and serotonin by single-photon excitation,19 multiphoton excitation with a visible light20,21 and an NIR light.6,22–24 Two-photon fluorescence imaging of exogenous fluorescence dye 4',6-diamidino-2-phenylindole, dihydrochloride using visible light has also been reported recently.25 For imaging FPs, a similar concept, which utilizes the state for multicolor imaging, has been demonstrated; however, in this case, FPs were excited by two-photon absorption at the NIR region.10 In particular, two-photon excitation where the effective absorption wavelength is in the DUV range () energy level has been demonstrated with the observations of amino acids,26,27 serotonin,21 and a glass substrate,28 but no demonstration of imaging biological cells using such visible-wavelength two-photon excitation of FPs or exogenous fluorescent probes has been reported.